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Sunday, 27 May 2012

The Physics of Field Athletics: Why do the Shot, the Javelin, the Discus and the Hammer fly differently?

Shot-put, Javelin, Discus and Hammer-Throw all have the same basic concept. Get the projectile as far away from you as possible before it bounces. However, the distances and methods of throwing are very different for all of them. So, what is the physics behind them?

There are a few differences between the sports. The distance in shot-put is noticeably smaller than in the others. In all but javelin, the athlete spins before throwing. I am going to go through and look at the differences between the events. This week, we will talk about shot-put and javelin, and in a few weeks time we will look at discus and hammer-throw

Firstly, lets look at shot-put. The men's involves "putting" a 7.25 kg (16lb) "shot" from a small 2.135m (7.00 ft) diameter circle. The put is thrown with a sort of push, instead of conventional throwing, due to its weight. It is also the weight that leads to the substantially smaller distances that a shot-put athlete achieves. Unlike javelin, shot-put requires not only explosive arm and abdominal strength, but also an even more powerful leg push. Anyway, lets take a look at the current world record, done by Randy Barnes, back in 1990:

What is interesting about shot-put is just how much force Randy has to put in to the shot in order to achieve his impressive 23.12 m (75'10"). For this, we have to use the laws of trajectory. At the moment, we know the distance that the shot travels and that is about it. What we want to find out is the initial velocity when the shot leaves Randy's hand. Here is the equation we will use:

V0 is the initial velocity, R is distance the shot travels, g is gravitational acceleration towards the ground (around 9.81 ms2)  and θ is the angle at which the shot is putted.
We don't know the value of the angle, however what we can do is accept the angle to be 45˚. This is the angle that would mean he would have to put in the least effort. So lets work it out:


Now, force is shown by the expression:

Where F is the force, m is the mass of the object and a is its acceleration

We can also change this into:
v is the final velocity, u is the initial velocity and t is the time
The initial velocity when he is throwing is 0 ms and the final velocity is, as we worked out, 15.06 ms.  All we  have to do now is know the time it takes for this velocity change. From the video, we can see it takes about 1 second. Therefore:

To put this in perspective. The average person is around 70kg so if we plug in the numbers, and I won't show it because it is tedious, we find that he could throw a person a quarter of a meter. This doesn't sound like a large distance, until you consider that they are throwing a dead weight fully grown adult from a dead stop. It is pretty impressive!

So, now to javelin. There are a few things that set javelin apart from the other throwing sports. It is the only one to have one method that the athletes have to use. Another difference arises from this, the allowed throw has to be an overarm throw as you would throw a ball, and the athlete doesn't spin at all - completely unlike all the others. This means that almost all the power put into the throw was done only by the arm of the athlete, with a little bit of momentum gained from a 30m run up.
Back in 1984, the javelin had to be redesigned. This is mainly due to this absolutely incredible throw two years earlier:


So now lets take a look at the more recent world record:

So what are the main differences? The first thing you notice is that the first throw seems to glide, going further with what appears to be a shallower curve. Secondly, in the first video, the javelin seems flat almost the whole time of flight and lands nearly parallel to the ground, where as the second javelin tilts down more and embeds itself in the ground nearly perpendicularly:
Blue is the first video, red is the second.
It depends on the style of the athletes as to whether these lines overlap.

So what were these changes and why were they put in place? Well in the first javelin video, the javelin was thrown 104.8 meters. It was decided that the throws were getting so long that they would soon become dangerous as they outgrew the length of stadiums they were held in. Another problem was that the landings started to become shallower and shallower, and seeing as the javelin must land point first, there were disagreements over some throws. Therefore the decision was made to move the centre of gravity of the javelin 4 centimeters forward. Though this doesn't sound like a large change, it caused the javelin to dip considerably more, shortening the distance by around 10%.


We hope you enjoyed this post, it was certainly very interesting to research and write. If you want to get in touch you can follow and mention us on twitter, @theaftermatter, email us at contactus@theaftermatter.com or search "The Aftermatter"on Facebook.


Ned Summers

Check out our last two posts:

The Physics of Gymnastics - The forces exerted on a gymnasts body during some routines are extreme, so how does it work?

The Physics Of Cycling: Why Does A Velodrome's Sides Need To Be Banked? - The sides of a velodrome are incredibly steep, so why is this?

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