The physics behind ballet and Irish dance might not have been too hard to understand. But what about gymnasts? They really do seem to defy gravity! But a quick hint before we start … they can’t be defying gravity because they always do come back down no matter how high they got off the floor in the first place!
When a gymnast is swinging round and round the bars (called a giant), he or she is using the principle of torque, a rotational force around an axis. The axis is the bar and the gymnast will actually be both helped and hindered by gravity. For simplicity, let’s assume that the gymnast started from a handstand on top of the bar. She has a lot of potential energy and begins to swing down. Gravity helps her to accelerate and convert her potential energy into kinetic energy. At the bottom of her swing, she has no more potential energy, but lots of kinetic energy and torque to overcome gravity and help her carry on to swing back up to the top of the bar.
Beam & Pommel
While women do beam and men do pommel, both of these are all about the gymnast’s centre of mass. To avoid falling off while completing skills, the gymnast’s centre of mass must always be over their base of support (normally hands or feet) and the apparatus. This is particularly key in beam skills because it is only 10 cm wide which means the gymnast has very little “wobble room”. For an apparatus like beam, it is helpful to think of the gymnast’s centre of mass as being around her belly button when she’s standing in a neutral position. If she lands slightly to one side, her centre of mass may shift over the beam and she may put out an arm or leg to bring her centre of mass back over the beam (but she will receive a deduction for wobbling). If she is too far over, she will fall off entirely with the help of gravity!
In pommel, the men must keep their centre of mass centred over their hands and the pommel itself. However, as they spin around it, the position of their centre of mass keeps changing. The key is for them to be spinning the top of their body over the other side as soon as they begin to fall and then immediately spin again as they begin to fall that way. In the end, they end up going round in a circle, hopefully without falling off.
This encompasses trampolining, floor and vault and all their flips and turns. In a simple front sault, the gymnast brings their arms down into their knees to generate angular momentum which allows them to rotate until they open up their body to overcome their momentum and land.
The same principle applies if they are doing a layout (like a front sault, but with a straight body) except that they have to generate a lot more angular momentum to actually rotate around fully.
Then it gets even more complicated when gymnasts adding twisting. In this scenario, conservation of angular momentum normally holds even when the gymnast is changing from a straightforward, ‘all-in-a-line’, front sault shown above to adding a twist to land facing the other way. The gymnast will lift one arm and then bring both in (as shown in the diagram) to change some of their rotation into twisting.
I hope you enjoyed this little overview of some of the physics that apply to gymnasts performing incredible stunts. If you haven’t already, check out these other awesome posts about the physics in ballet and Irish dance!