We have already posted twice, talking about Shotput and Javelin and about the Hammer-throw, analyzing how they differ from one another and why athletes use certain methods to achieve incredible distances. So what is so different with the Discus? And why is the Discus more like an airplane wing than you realize?
Firstly, lets watch a video of the discus world record:
So, what is a Discus? A Discus, as the name suggests, is a disc, usually made of plastic or wood with a metal core and a metal rim. These are useful for a few things we will come back to later. The Discus thrower spins a couple of times in a way similar to in the Hammer-throw, and throws the discus as far as they can. For men, the discus weighs 2kg and for women it weighs 1kg. Ok, so that is most of the boring stuff done so lets get to the juicy physics. There is a lot more to the Discus than it looks. In fact, the physics of the discus is more complicated than any of the other throwing events and now we can see why. There are two interested things about the discus’ time in the air, the flight itself and the spin imparted on it by the athlete. So lets start with the flight:
So if you watched the video, though it is quite difficult to tell, you may have noticed something interesting about the flight of the discus. Instead of a normal parabola (fancy mathematical term for a hill-shape) flight, it is a little different.
So why does it have this weird flight? Well it relates back to what I said before about airplanes. The Discus acts essentially like an airplane wing. It generates lift and this is what gives it its strange flight path. So lets take this useful cue and tackle a brilliant problem. How do airplanes fly? Well I’m sure you have heard one. The wing’s shape means that air over the top has to move faster to join up with air going down the bottom. An area of faster moving air exerts less pressure on the wing, meaning that it can move upwards. However there are problems with this, for example, shouldn’t stunt pilots flying upside down just fall out of the sky then? There must be a different reason (though the other does happen, it just isn’t the main force). And this is where the discus comes in. If you have ever visited a high place, or at least a very windy place, you will know that you can lean into the wind without falling over:
So here is the interesting part. We take it for granted, but in this situation the wind is not just pushing us backwards, but pushing us up at the same time. You could even say that it is giving us lift! And there is the way a plane flies. The plane moves very fast, increasing the lift, and uses its engines to continue moving forward. A plane wing is like a person leaning forward running very fast, in a way. And the Discus works in a similar way!
It is for this reason that the best condition to throw a Discus is with a light headwind. However, there is a problem. The discus doesn’t have engines and can’t overcome the air resistance for very long. So whilst it may fly high, the main reason it can travel the distance it does. It doesn’t go too far overall. You can see this overall by understanding, it is about 3.5 times lighter than a shot-put but only thrown about 2.5 times the distance.
If you are interested in finding out more about airplane flight, check out this fantastic video from Veritasium:
Anyway, now lets look at the other interesting thing about the Discus. The spin. When a discus is thrown, the thrower imparts a very large amount of spin into it. In fact, if you have played with a frisbee (a flying disc) you will know that spin is essential. But have you ever considered why? It is quite difficult to throw it with no spin, but that isn’t what is so important to Discus athletes. Instead it comes down to something called angular momentum, and gyroscopic stability. Now, before the horrible words scare you away, let me explain. We have already talked about angular momentum (and calculated what would happen if everyone in the world spun around at the same time!), but lets give a more basic explanation. It is just a combination of how fast an object is spinning, and now much it would like to spin. An object that is spinning fast and is very difficult to stop has a high angular momentum.
There is another thing about angular momentum. It is conserved. In other words, an object can’t lose angular momentum by itself. In the case of the discus, it can only lose angular momentum by giving it to air molecules. Not only is the spin conserved, so is the direction of the angular momentum. Ok this sounds weird and is misleading so let me give you an example. When you throw a frisbee without/with a very small amount of spin, the first thing you will notice is that it wobbles a lot. Its flight is far from stable. However, throw it with a lot of spin and it seems to glide, almost hovering in the air. The top of the frisbee always points where you chose (usually up) and this is what we call gyroscopic stability. Have you ever seen a discus wobble in the air when it is initially ascending? Exactly. When the athletes throw the discus, they spin it so it does not wobble in the air and the top will continue to point in the right direction (until it looses its spin).
Discus is actually a lot more physical, in that it involves more physics, than the other throwing sports. The athletes need to be both strong and understand the mechanism behind it and I hope you respect them as much as I do!
We hope you have enjoyed this post. If you have, then please check out our last two posts:
The Physics of Diving: Tom Daley and Angular Velocity – How does Tom Daley manage to complete his famous Front 4 1⁄2 dive?
A few weeks ago, Theo created a new scoring system for the Heptathlon, as the current one is biased towards Shotput and Javelin. He then followed up with how our system would have affected the 2012 Heptathlon.
We have now nearly reached the climax of our Physics of Sport series with the arrival of the Olympics, so have a look at the posts in the series here.
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