Why Do Figure Skaters Go Faster When They Pull Their Arms In?

Figure skating is a transfixing sport that involves skaters doing various tricks and jumps while gliding gracefully on the ice. A particular trick highlights one of the more curious aspects of figure skating. Skaters seem to speed up their spins when they pull their arms in. Do they simply spin faster to put on a show? Is it some sort of optical illusion? Continue reading to uncover the mystery.

The Law of Conservation of Momentum

Figure skaters seem to go faster when they tuck their arms in, but it is no trick or optical illusion. It is only physics. In fact, the increase in a skater’s speed is due to the law of conservation of momentum, which states that one’s momentum (momentum is an object’s speed times its velocity) must be conserved before and after any interaction.

For example, if a small car drives fast and crashes into a semi-truck, it will not make much of a dent. This phenomenon occurs because the momentum of the two cars was dominated by the massive semi-truck, causing the small car to mostly take on the bigger truck’s momentum.

The Physics of Speeding up While Tucking Your Arms in

Objects (and people) can also have angular or rotational momentum. Angular momentum is defined as an object’s rotational inertia times its rotational velocity. Inertia refers to an object’s resistance to motion. Therefore, a skater increases their inertia by making themselves bigger, and one main way to do this is to spread their arms out.

If a skater starts to spin with their arms outstretched, they have a lot of rotational inertia. Therefore, once they tuck their arms in, their rotational inertia shrinks, making them much smaller. Due to the law of conservation of momentum, the skater must have the same momentum after they tucked in their arms as before.

Since their rotational inertia decreased after folding in their arms, the only way to balance out their angular momentum (make it the same as before) is for them to increase their rotational velocity. Thus, by tucking their arms in, a skater will decrease their rotational inertia, which necessitates an increase in their rotational speed.

Conclusion

Figure skaters increase their speed when they tuck their arms into their bodies because this decreases their rotational inertia (which is their mass times their radius squared). The law of conservation of momentum requires that their momentum before tucking their arms in is the same as after tucking them in.

Since a skater’s rotational inertia is their mass times their radius squared, tucking their arms in greatly impacts their inertia. Therefore, the skater’s speed greatly increases to compensate for all this loss in rotational inertia, keeping their momentum conserved. When skaters tuck their arms in, it forces their bodies to spin faster and puts on a dazzling show for their audience.