Saturday, May 28, 2022

Sport vs Physics

Right to introduce myself uh my name is steve hake i'm from the university of sheffield i'm a lecturer in sports engineering this lecture today is the first of a national tour sponsored by the institute of physics and the engineering technology board .

With their engineering and the olympics campaign and we're going to talk about today is physics versus sports so we'll ask ourselves a few questions how does physics influence sport how does science and technology have an effect on sport can we actually enhance performance .

Using engineering technology is it possible we always talk as if we can are there any statistics are there any results out there that actually say yes we can and lastly which is dominant is it sport or technology if it's technology again you get the press say no it's just cheating .

Okay and that's very topical at the moment so let's go straight into the olympics and this is the latin motto of the olympics sitius altius fortius means faster higher stronger and what we'll do is we'll take one particular event for me that .

Represents each one of those words so if we look at the hundred meter sprints the pole vault and the javelin we'll go straight into the 100 meter sprint this lady is called was called flojo um she is the current uh holder of the hundred meters and 200 meters uh sprint unfortunately she's .

No longer alive she is winning there by a very very large margin now when you think about the technology that goes into sprinting there isn't really a lot that you can manipulate apart from the body and you can see she's worked out .

Very extensively what you can do and what people have started to do is to wear some of these special running suits they're very very tight-fitting i was told that i could actually get into that but um i can't quite believe that but it's a lycra suit it's got special .

Fabric on it so people have started to wear things like this they wear shoes where the soles are very very stiff so you can transmit the force from your body into the surface and there are other technologies which are kind of represented by this slide here if you look at this .

Slide okay you've got some muscular legs there you've got some shoes they sometimes have some shock absorbing properties but really the 100 meter sprint you don't want to worry about shock absorbency you've only got 10 seconds to think about it you want to get the force transmitted .

From your feet into the surface as quickly as possible so you can end up with spikes on the bottom there and you also have these um these starting blocks now in these starting blocks they're electronically activated from the the starters pistol and if you set off too early it's .

Considered a false start and that was one of the major parts of bits of technology that was introduced to the 100 100-meter sprint and initially actually people thought that starter blocks themselves were a form of cheating and a lot of people didn't like it now this is the 1992 barcelona olympics .

And linford christie who you might have heard of was 100 meter sprinter in this race for great britain right let's just see what happens okay you've got a very tense start and the athletes are very jumpy you can see who goes first now actually linford christie here was the one that .

Got called to the false start but actually it was the guy next to him i think possibly a little bit of mind games going on where he's back there and he just nodded his head slightly and that made linford christie go forward but i think linford christie was called for that .

False start now you can see these uh starting blocks in here and these are wired up so as soon as the bang goes okay a timer starts and if you set off within point one of a second it's considered a false a false start because no one in theory can react .

Faster than 0.1 of a second now let's see what actually happened in the race so linford christie is that guy there hey now right so olympic christie is one of the most successful british athletes .

Of all time he won 23 major medals and won that olympic goal there in 92. okay let's look at the statistics here we've got some statistics along the side here we've got the winning time in 100 meter sprint and here we've got the dates uh for the olympic uh .

The olympics going on the bottom here and what you see you see the thing going down now this one here 12 seconds is slightly higher than the rest i've tried to figure out why that is so much higher than all the others and actually what you find is in these early days the technology of the clocks was not .

Particularly sophisticated and they could only measure to plus or minus half a second so if you think of that one that one could have been either there or it could have been there so the error in that point is really really quite large so it could have been down here but because they could only measure to .

Half a second they got to around here they can measure to 0.1 of a second now they can measure to a thousandths of a second and they use photographic uh evidence as well but what you see is you see the times coming down and that shows you how athletes performance is improving without really any technologies going into it .

The shoes haven't changed that much really the these speed suits don't have that much of an effect you don't see any particular dips or jumps apart from after world wars after world wars you see strange things going on this one here is the current world record by tim montgomery 9.78 .

Seconds so what we learned from that is technology doesn't really have an influence 100 meters sprint but times nonetheless times are going down so let's go on to the next one let's look at the pole vault now this guy is daley thompson um british decathlete and let's see what .

He does in the pole vault fast run up up he goes and over he grabs the pole and he starts sprinting as fast as he can now if you start strapping weights to you you're not going to become a sprinter okay that's not you you're not going to .

Sprint particularly fast so you want that pole to be as light as possible that's the first thing you want so then you can run faster so you run along you're holding this pole up again you want it to be light because if you're carrying something a long pole like that it can be quite hard to do .

So you sprint as fast as you can now you might be able to see that the pole wobbles a little bit as he's running along and as he comes down he plants it in a hole a box just before the uh the pads that he's going to fall onto now the pole bends slightly downwards so .

It's bending that way when actually what's going to happen now it's going to bend the other way so what he's doing now he's converting his the energy of the run which is kinetic energy into potential energy potential energy being energy due to height and he's going up in the air and he's .

Going to turn himself upside down now what happens is as he gets to here the pole has bent an awful lot and there's energy being stored in that pole as spring energy it's called strain energy but spring energy is good enough as well so you store some energy in there it then gets returned to the athlete .

The athlete turns upside down points the feet upwards and pushes off the end of the bar and then just gives the bar a little bit of a push to make sure it doesn't hit hit so push the pole so it doesn't hit the bar and then over he goes away there he goes now let's take him back again .

Okay you get to there now look at the shape of his body what's interesting with the shape of the body if you think of where a center of mass of someone is now the center of mass if i was to condense all my mass into one point it would be about here that's where my center of mass is and it's the same with daley thompson .

Now if i bend over my center of mass is in a slightly different place and it's kind of around here so what daley can do is he can actually go over the bar and if he bends his body the center of mass is just slightly outside of his body so although his body goes over the bar .

The center of mass might just clip the bar or could even go underneath it so that's one technique that they use and and high jumpers use that technique uh as well so that's steady so daley was very very successful he held the world commonwealth european world .

Championship he held everything all at the same time and it's not been replicated by anybody so daily was very very successful right let's look at the statistics of the pole vault now we've got the heights over here starting 2.5 meters up to 6.5 meters and we've got the dates along the bottom there again .

Okay the first event was 1896 and the height jumped was a little bit over three meters the height started to go up um and it was only about here actually they started to use mats and sand pits so i think that would have been something that stopped the heights going up any further because this is quite high to jump from to fall from shall we .

Say now in the early days how the athletes used to do it used to have a a big pole it was relatively heavy it was made out of wood it wasn't very flexible it wasn't very bendy so when you jumped over you would jump over and you kind of just have to lift your feet out the way and .

You go with your head up and your feet down then what happened around here this is when the carbon fiber glass fiber composites the plastic materials came in which were a lot lot bendier and allowed the athletes to turn upside down so it bent so much .

That the athlete could then twist and put their feet up first so here this is when the new materials came in and suddenly you can see this kind of discontinuity in the graph and the point start to go up again but it's leveling off again now and we are really reaching the limits on which athletes can can physically .

Jump unless someone brings in some new technology some new material which has some kind of advantage so if you want to make yourself a little bit of money go back to your classes come up with a new material a new technique for using that pole because it's ripe .

For some advancement there and the rules allow it and that's interesting the rules in pole vaulting allow you to use anything any material any length of pole any design you want there's a few kind of stipulations about the ends of the pole and what you're allowed to do .

But you're allowed to use anything and because anything was allowed it did actually happen and the rule makers like that because it means the heights go up and you get world records and so on and that's the current world record there that's a modern pole vault design what you have is you have a hollow interior .

You have some rings of glass fiber that gives you some torsional rigidity so that if it twists it doesn't just buckle and fall to bits so that means that when you go up it doesn't just fall over then on the outside you've got the the strongest material on the outside which is the carbon fibers .

The carbon fibers are very strong that's where all the stresses are concentrated so when you bend this pole around to a 90 degree arc okay the stresses are all around the outside and that's where you want the carbon fibers and that's where you find them so just watch uh daley do it again up he goes convince himself to potential .

Energy and down again and he gets himself a gold medal you can see the bar and you can also see the bar vibrating afterwards you can see it wobbling like this what you want to do is you want to minimize any energy losses you want all the energy to go into daily getting over that bar .

So you don't want this thing vibrating so you want this pole to be as stiff as it can be so it doesn't have any vibrations in it and all energy goes back into daley thompson or whichever athlete okay let's move on to the javelin now there's a a sky view of the sydney uh olympics there's the .

Run-up for the javelin and you can see that the fields the javelin field there there's kind of an arc there and then you have these arcs here and you measure from that arc to wherever wherever the thing lands of course you've got a 100 meter sprint which is kind of down there .

And you've got the athlete having to run across the track there let's have a look see what happens this is steve bakley again and brits right so this is the uh that's steve bakley he's the brit the other guy is jan celesney his main rival so you see him throw that javelin as .

Fast as he can you might just be able to see the tip wobbling like that at the back and the thing comes down lands at about 30 45 degrees something uh some something around there and that was a throw of about 90 meters .

Now if we look at what happens with a javelin you throw the javelin in the air and if you take the center of mass again if you took all the mass and concentrated it in one point it would be there that would be the balance point and you throw it and it flies like that a bit like a wing if you .

Imagine a cross section of a wing so the javelin's flying lap is flying in that direction like that at a slight angle now what that does is that gives you some drag okay you throw anything through the air and you get some drag you also get some lift though like an .

Airplane wing you get some lift and they act somewhere in the region there you also get a thing called a pitching moment because what happens is you've got that centre of mass but these other forces they kind of wander around a bit and so you get this javelin .

I couldn't find a javelin to bring unfortunately a bit too dangerous to bring on the train so we'll have to make do with this so what happens that javelin kind of opens up the angle and rotates like this as it flies through the air and that's really quite important .

Because what happened was you saw how big that stadium was about 1983 distances thrown were getting up to about 100 meters and also the trajectories looked something like this the javelins opened up in the second half of the flight so that .

They're flying through the air going over opening up like that and then coming down and just landing flat like that and the rule is that the tips got to hit first now what was happening about 20 of them the judges who've got to stand underneath this javelin coming towards them we've got to try and get .

It get close enough but not too close and decide uh whether it's landed tip first or not and about 20 at least 20 were just kind of outlawed or or ruled illegal because they couldn't figure out whether it landed hit first or not so that was the first thing the ruling .

Bodies realized they didn't like that that they wanted to change it so it landed tip first but also they were hitting about 100 meters and if you think about the size of the stadium they're in danger of actually getting an athlete perhaps as they were running past or someone .

In the audience which was not going to be good pr so what they said was okay we want to make the thing uh land a little bit shorter somewhere in here rather than here in the pits or in that in that um that zone there down there where the athletes are so what they did .

Is they actually just move the center of mass forward they just change the geometry of the javelin so that that centre of mass move forward only by four centimeters not very far but it was enough it's like putting a big lump of um plasticine on the front what it meant .

Is it meant that it now lands tip first so you have a picture moments in that direction and it now lands like that which is what you've just seen steve backley do so if you look at the performance statistics you can kind of see what goes on there so started slightly late in the olympic calendar .

About 1908 distances started out about 50 meters distances rapidly went up to just around the wall up to about 70 meters and then an american decided to make a hollow javelin and make it slightly larger in diameter so it was the same .

Weight but made it hollow and made it larger in diameter and that gave more lift and you can see suddenly these distances start to go and actually it was that that made the whole thing land a bit flat because you're getting too much lift in that second half of the flight .

So this is when the ruling bodies that the guys said okay right enough enough let's change the rules let's put the center of mass forward by four centimeters and lo and behold as they did that this guy threw 104 meters in east german through 104 meters and that really scared them because then they were .

Really in danger of killing someone if they weren't careful so you see the distances suddenly dropped down again but they've started to go up as the athletes adapt to what is going on athletes are very very good at adapting um to the new technologies uh that was .

The old world record and that's the new world record this guy called yanza lesni which is steve bakley's arch rival um never quite beats him at 90 meters the dis the difference between these two guys is no more the length of a ruler okay so for 90 meters that's the distance between a gold .

And a silver so what's interesting in this one in this one the rules were changed and they used physics and technology to downgrade the performance so instead of allowing it just to keep going up they said no enough enough and they downgraded performance here so that that's kind of .

Interesting there right we'll change tax slightly now and we were asked a while ago to see if we could help dave holding here with his hunt for gold medals so you can see that dave is an extremely strong looking guy very very big and what we're doing there .

Is we're just timing him using some timing gates you can see him go past some time he gets there and just timing him to see what his acceleration is like over that first 30 meters what's also interesting if you can see the front wheel just just tipping off the floor .

One of the things you want to do is you want to try and get the set his center of mass on the back wheels if you've ever tried different bikes you get the lowest rolling resistances the rolling resistance the force stopping you rolling along the ground if you have large wheels large thin wheels .

So you don't really want that front wheel touching the ground at all except to trying to see around the corners and so you can see him just bobbing up and down on the ground there so okay they said please can enhance his performance how are we going to do it okay propulsive force okay you can't .

Stick any engines on you can't put a rocket motor on anything you just got to say dave do your best you go off and do your coaching go and do your nutrition you do your training so that's that's dave doing that bit then the rolling resistance aerodynamic drag are the things trying to slow him down so rolling .

Resistance is this force on the bottom of the wheels and you can think about it well if i roll something along the ground okay at some point it will come to a stop so you've got a force stop at some point it'll come to stop eventually as it goes out on the door .

Okay it will stop and that's a runners it's quite a small force aerodynamic force is much much larger so you know if you go down uh try and freewheel down a hill on a bike you will hit the terminal velocity and it's the aerodynamic force from the air that will slow you down there and then .

You've got uh the wheelchair athlete and weight as you're going up and down the hill you've got that weight acting on you and that acts to either accelerate you or decelerate you in some way so in terms of looking at the aerodynamics what we did is we created a virtual dave .

So this is a computer model from a scan of his body we created this computer model you can see these kind of they look like sinews but there's actually very very small cells there's a mesh covering that body there and then we created a computer uh a cad of uh his actual wheelchair and we stuck the .

Two together and then we put him in a virtual wind tunnel this virtual wind tunnel is called computational fluid dynamics and this is the way that most of the elite sports people equipment designers look at aerodynamics nowadays you can either put it in a wind tunnel or you .

Can put it in computational food amps inside a computer so let's have a look what you get right so there's dave in his worst position possible he's just about to push off on the push rims the push rims are these things here so you push downwards on the front and you have some special gloves to do .

That so you don't rip your fingers off and we've got some airflow going across dave so you can see what's happening so you've got if i follow the stream here okay it slows down and you get some turbulence around here you can see this turbulence wake turbulent wake behind now the size of .

The wake tells you how much drag bit much like a boat if you see a boat going on the water you can see very clearly the wake behind the boat and this is our way of visualizing this with dave so this is another way of looking at it so we can just see behind and you can see behind here these .

Vortices in a very large wake you want to minimize that wake as much as possible now interesting what the athletes do is the athletes put tape underneath their seats and they also pull their t-shirts down over their knees not through any knowledge of physics .

Particularly but they know that it works they have an intuition a feel that it works if you look at that again you can see the size this very large wake behind and you want to try and get rid of that if you can we can look at pressures pressures is the force per unit area .

Red means very high pressure and blue which is on the back of him is very low pressure if you think of a pressure on the front and the back of the body the difference between those two pressures over a particular area tells you the force the drag force and so we can look at those and you can .

See there right in the middle of the body there's a huge high pressure point which is really bad you want to try and get rid of that and that tells you why the t-shirt works because if you pull that t-shirt forward you deflect some of the air and you decrease that pressure there which makes it slightly better .

So we decided okay let's try some novel features let's see what we can do so uh let's put a helmet on him um we will put some eye holes in there so we can see where he's going at some point then you've got the t-shirt there it looks looks a bit like an apron but that was the best way we could do it at .

The time we've got some front deflectors on there and we've got some deflectors under the seat so we just tried all these different things just to see how much better could you get and what you find i haven't shown the numbers here but what you find if you try all these things .

Just by these small changes in design you can give dave such an advantage that instead of getting the bronze in the last olympics he would have got the gold by doing nothing more by playing around with design which is allowed within the rules of the sport .

Itself it's interesting one of the most important features that you've got to look for is lightness of weight and so you see some designs here these monocoque frames carbon fiber glass fiber frames you can see how the athlete is able to sit forward and put their face in a .

Cowling inside the cowling to reduce the cross-sectional area reduce those high pressures on on the on the chest but really the weight of this is is very very important particularly when they're accelerating so and this is probably the way that we'll go for the future for in wheelchair .

Athletics in this country right you might remember that goal some of you uh that was a goal that got us to the last what was it world cup wasn't it yes and that was the last goal in the la the last minute that that got us there so it was a beckham 11 against greece um and everyone knows that beckham is .

Renowned for for doing shots like that but we'll look at the physics of that and try and explain what is going on first of all he kicks the ball now what happens when you kick the ball okay the ball deforms and you can see the ball deforming there and the shoe deforms you can see the .

Shoe deformity and you can actually see the foot moving backwards kind of bending backwards these points on here by the way are so that you can just monitor the shape of the leg and take some measurements and stop it frame by frame so what's interesting is that shoe when .

You kick the football that shoe kind of bends back that way now what some of these shoes have now is they have a little hinge across the middle because of course you've got to be able to run so you want to be able to bend very easily that way so that when you run off it hinges forwards but it doesn't hinge .

Backwards you want as stiff as possible so that your feet don't bend back otherwise you might break your toes for a start so you make it as stiff as possible that's one of the designs that football boot manufacturers have got now so you've got an impact there with very very high forces .

Of the order of thousands of newtons there's another impact here um from a guy who got hit by a ball this was only a few weeks ago a newcastle player uh playing against money united um there's about 2000 newtons of force at that point hitting him in the face luckily i think he'd broken his nose 17 .

Times before so no one actually noticed afterwards that he'd been hit but there are very high forces and very large deformations and what you find is find the ball wraps around bits of your body wherever it hits so if it hits you on the foot it kind of wraps around your foot that .

One's wrapped its way around his face okay once the ball is flying in the air once you've imparted some momentum to that ball okay so you've got some you've got a mass of your foot moving at a velocity and you hit the ball .

And you transfer some of that momentum to the ball so you've got to matter the ball traveling with some velocity fly through there then you look at the aerodynamics and what we can do is we can stick this is actually a wind tunnel experiment that we did so we've got a football there and we've got the airflow going from left to right .

And this is just some smoke that's in the airflow so you can see what the air is doing and you can see the air leaves about there and it leaves about there and you get some large vortices behind quite a large wake again like you had behind dave holding that tells you it's quite a large drag .

Force on that particular football at that particular speed now what's interesting is you can look at the seams and the seams do actually have an effect and so the seams on a football the fluff on a tennis ball and certainly the dimples on a golf ball all have an effect on how the air flows .

Over the object now there you've got a very smooth sphere this bar behind it that's just a way of holding it in the airflow so again you've got the airflow going from left to right you've got some smoke coming down over it and you've got a very large wake behind which telling you that there's .

Quite a large drag force on that you can kind of see in here there's it looks like a little bit of a band and what you've got there is you've got the airflow thickening around the ball at that point and that's very critical and what you can do is you can put .

Yourself a wire a trip wire across there just where that thickening will start starting to happen what happens is you get the air to stick to the ball surface and look at the size of this now look at the size of that wake in comparison to that one .

So just by putting a very small trip wire on there you can get that drag to go down now you can do this experiment yourself go along to the garage and get yourself one of those cheap footballs that'll all come out this this summer it'll say copa mondial on them but the .

The actual panels they're not real panels they're just painted on so it's one of those smooth footballs with painted names on it and so on get yourself an elastic band and just wrap yourself an elastic band around the football and suddenly if you try and kick those .

Smooth footballs they do all sorts of strange things and you can't get them to go very far but put an elastic band around it in fact put two one that way and then that way and try kicking that and you'll be able to bend that ball like beckham probably better than you can even bend one of these because they're very very light .

And you're basically doing that you're decreasing this drag on the ball we can look at football design again it's very difficult to do wind tunnel experiments so we tend to go to the virtual wind tunnel route this computational fluid dynamics and we have our football there and you can see .

These vortices uh and the way the air comes off the ball and you can even look a little bit closer at that and see the effect of seams so you can look at different seam designs we can work for mighty work for a deep this is a pick an adidas shape might have a slightly different .

Shape and see what the effects of those are and particularly this new football that's come out which is a very very smooth very very round football which is now glued together rather than this one which is actually stitched together okay now david's .

Just running up he's gonna kick the ball in fact i'll i'll just run it to the next one right let's stop him there so okay what happens he comes up to the ball and you can see the way he plants his left foot and look at the angle of his leg it's about 45 degrees so his foot is .

Like this and his ankle is very very much turned over now you get the boots you've got to have an appropriate style of studs a predator kind of boot is what he uses he's got fairly large studs in there make sure he doesn't slip over as he actually did when it was a bit too uh .

Muddy not so long ago that angle allows him to wrap his foot around the outside of that ball so you can see him wrap his foot around the outside there flies through the air he's got some spin on there and he's got some spin on this ball and the spin on it .

If i kick my kick around the outside of the ball the ball rotates like that and the axis of the spin isn't necessarily like that because he's over an angle the axis is like that and that's very important the spin axis okay so we do a computational model now it's very .

Difficult to a get david beckham and b get david beckham to do what you want him to do he costs about a million pounds a day so we can't afford that in this particular experiment so instead we've got a computational one which is much easier to control and what you can do .

Is you can use that foot and look at the ball move the foot around move uh make the foot hit the ball at different speeds and see what happens to the ball as it takes off now what you can see there is you can see the ball deforming as we saw before you can see the foot deforming backwards as we saw before .

That ball is actually taking off with a small amount of backspin it's going off at about 10 15 degrees with a small amount of backspin the center of mass again is the center of that ball and if you hit it slightly off center that's how you get spin because you're applying a torque to the side of .

A center of mass applying force to the side of central mass which is a talk right what happens okay from our computational model of david beckham what you find up the side here is either speed or or spin and this is the position along the axis of the ball so if you hit it in the middle then what .

You get in the middle you get quite a large velocity quite a large speed but you get almost zero spin hit it right in the middle if you hit it right at the edge so that your foot is almost just glancing it again you get zero velocity and zero spin because your .

Foot's just about missing it so as you go from there to there the speed goes down as you go from there to there there comes a point where the spin is a maximum and then it drops down again and the point is around here now if you can get that point every time which david beckham appears to be able .

To you get the highest amount of spin possible and you still get quite a high velocity so you've got about half the velocity but you've got a really really high spin and that's what david beckham manages to do every time that's what makes him so .

Consistent and i've seen pictures of him actually hitting a football through the hole in a tire hanging from the side of a goal and he's that consistent you can get the ball through there so he gets the ball to spin so what well okay this is a baseball now in some fluid and in that .

Fluid you've got a little bit of dye going past it allows us to do it very slowly and very clearly so as the ball spins you can see the the die being deflected downwards and newton's third law for every action is an equal opposite .

Reaction that tells you there's going to be a force acting upwards the smoke flow's going to go downwards and the ball is going to go upwards so as the thing spins in that direction let's just run that again so as the ball spins in that direction you get a force acting upwards some people call this the .

Magnus force it's a spin force that's all it is really so or in this case it could be lift if the ball spins that way then it's negative lifted it's going to go downwards so how does bex use that well okay if we get our model okay you got david beckham he just .

Kicked the ball the spin axis is something like that and because it's spinning you've got a force acting downwards in that direction and as the ball flies over there like that this force is always acting down in that direction kind of down towards the side of that goal there .

And it's that that allows him to beat the goalkeeper so moving on from david beckham if we go on to tennis you find that you see a lot of the same principles starting to appear now um right so that was venus williams at the sydney 2000 olympics people don't really .

Think of tennis as particularly being an olympic sport but it was one of the very first olympic sports i think introduced in 1900 so at the second olympics it was introduced then and in fact there's an amusing anecdote one an irishman turned up as a spectator .

Because they didn't have enough players he was asked to play and he actually ended up coming back with a gold medal even though he just turned up to watch so those early days you could get bizarre scenarios like that so you've got venus there okay what does she do what has the technology moved on .

Um to allow her to do shots like that with such force if we look at what happens when the ball hits the rackets now again it's very expensive to get venus in um so instead of getting venus in we have a racket here and a ball hitting a racket that's more or less the same kind of impact you can .

Kind of run that in reverse and have the racket come back and hit the ball and it's more or less the same thing now what you see is you see the ball coming in hitting the the racket uh it deforms the strings deform the ball rebounds with some energy and .

The racket rebounds back with some energy now what happens these strings are very very efficient and they return a large amount of energy back to that ball very much like a trampoline does if you try and trampoline uh jump on a trampoline or jump on a .

Concrete surface the trampoline is much more efficient than the concrete surface and that's what the strings do there now also what happens is the racket vibrates rather a lot now there hopefully you can see we've got a racket all the way down there and you can see .

This racket vibrating so the ball hits there and if you watch the top there the racket vibrates backwards and forwards now this is a modern racket it's not a particularly old racket it's a very modern stiff racket and what manufacturers have done is they've done their best to make these .

Rackets as stiff as possible if you look here these are two rackets which are probably about 30 years apart so you've got an old wooden one and you've got this modern graphite racket how you see them change is first of all you can see the size of the head .

Has changed now people tried to make larger headed wooden rackets in the past but because wood isn't strong enough the whole thing as soon as you put the strings in they just buckle to a crumple down to a mess of cat guts and wood so you couldn't do it you couldn't get much bigger than that because the .

The material wasn't strong enough as soon as you ended up with a material like this it allowed you to get much bigger now one thing that you do when you play tennis is you always hit it around the same place and you hit it um around well you hit it in the center .

Of the face that's what we tend to do with this particular racket you get the maximum velocity off around here and so what the racket manufacturers did is they brought the bottom of the racket down so you can see there you hit it in the same place but because they brought the .

Bottom of the racket downwards you're more likely hit it because you try and hit it in the middle of the face every time so also you can see the thing is wider and this thing is wider and if you look at this one if i tried to spin that one it would be quite easy to spin .

If i had tried to spin that one because the mass is so much further out that's much harder to spin now that's good if you hit it off axis okay you're not a very good player like i'm not okay you hit it off axis it's not going to spin in your hand and it's not going to go off into the trees so with this one when i started playing .

With this one it used to take me about two weeks before i got a shot across the net with this one now you can do it within the day you can really learn quite quickly with one of these things now now the other thing the manufacturers do i don't if you can see on that picture if you look at the handle down .

Here right if i if i try and keep the dot stationary that handle doesn't really move very much and there's a point on a racket called the center of percussion where if you hit it the handle doesn't really move so you don't get a big jarring sensation at the handle .

And that's called the sweet spot and if you hit that sweet spot and what the manufacturer's trying to do is to try and get that sweet spot again in the middle of the racket so if the ball hits that sweet spot it feels really good and you don't get this jarring sensation now what will happen is if you play .

Tennis you'll get that occasionally happening as you hit that really good place and you'll go that felt good you don't know why but it did it's because it does and you hit it on the center of percussion there and that's the sweet spot so you've hit the ball .

It's flying through the air do some aerodynamics again you've seen these this is actually wind tunneler work rather than a computational fluid dynamics this one is a ball that's come straight out of the can and it's kind of quite fluffy this one is a really worn tennis ball where the fluff has been .

Worn away and you can see that that vortices those vortices behind really reduced um so you can see it happening there and where the air leaves the air leaves almost horizontal and you get this smaller wake so as the ball as the fluff gets worn away they go a .

Lot faster because of the reduced drag on them that's counterbalanced by the fact that these things start to lose pressure and they start to get really old and the rubber goes all soft so it kind of balances out depending on how old the ball is so what happens the ball hits the .

Surface and what's interesting about this is how far the ball actually slides as you think the ball hits there and see how far it slid to over there so let's look at that again hits there and slides over there this tells you why it's so difficult for line court judges .

To actually make that call because if you were to look at the ball coming in you'd say well okay it landed there if you look at the ball going out you would say okay it rebounded from there and somewhere between that you've got a line which is that thick .

And you've got to tell that that that line called judge in real time has got to decide whether it hit that line or not and the fact is the ball slides an awful long way and it's very difficult to tell that also makes these cyclops automatic line call .

Systems very very difficult to manufacture and difficult to get right see when you've got sunshine rain and all sorts of other things going on again we can go down to some we can go down really really technical routes and we can look at uh computational models there we've got a model where there's actually a whole .

Ball there but with this computational model you can just like hide the front half so you can see what's going on inside and what's interesting what you can see you can see the way the ball buckles and deforms and you get this funny kind of buckling here now if you've ever played squash i was .

Always told in squash that what you do is you get your squash racket and you stand at the front of the tee and that's the place to be now i was never particularly good at placing the ball and i was placing the wrong place at the back and this person would then whack that ball hit the sweet spot and it'd always get .

Me right there and that's because i'm in the wrong place that's their point what was interesting kind of was that the bruise that you get is kind of this big purple bruise and it's a ring bruise with a kind of white bit in the middle and the reason you get a bruise like .

That is because of the way the ball deforms as it hits your body and you can see this buckling inwards so you kind of have a ring of very very large forces and the middle bit is kind of buckled up inwards so if nothing else told me why i got these great bruises but didn't tell me how to play any better unfortunately .

So we're almost there now we're at the end the big question is it cheating to use physics engineering and technology in sport i was asked this question many many years ago when i started downers field and it did actually take me quite a while to formulate an answer that i felt satisfied with .

If we look at the hundred meter sprint the pole vault and javelin we saw that with a 100 meter sprint there wasn't really anything that we could put in that really made a big difference in the pole vault the change of materials over time produced a big difference .

Around the 1960s when the heights really started to take off and the rules allowed that change the javelin on the other hand they didn't they wanted to change it but they wanted to use technology to downgrade the distances thrown so that was changed as well so what you find is you find that the .

Laws of physics tell you what's possible so you've got this big big kind of sphere which tells you these are all the things that are possible but inside that you've got the laws of the game telling you what's allowed so the physics tells you what's possible and laws of the game tell you what's allowed .

And these rules of the game kind of just move around almost arbitrarily they shift very very slowly over time if you look back to the rules of tennis 100 years ago to what they are now they're very very different but they do move over time so it's not cheating if it's allowed by the rules that's all .

You can say because tomorrow the rules might be completely different so if it's allowed by the rules it's perfectly legitimate to do it to use technology if you can to enhance that sport thank you very much


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