Ask Dr Bec: The science in our daily lives

How do figure skaters spin so quickly? asks 7-year-old Madison.

Great question – as these athletes are relying on quite complex science to create these spectacular spins!

Let’s highlight some basic ideas from physics to keep things simple and instead of getting into technical details or using specific measurements, we’ll focus on a few key concepts.

With any object in motion we need to consider these three things:

1. Mass: This is simply how much the skater weighs. It stays the same no matter what the skater does, so it’s constant.

2. Moment of inertia: This is a measure of how hard it is for the skater to spin. It depends on how the skater’s mass is spread out from their center. When their arms are stretched out, the moment of inertia is higher. When their arms are pulled in, it’s lower.

3. Angular velocity: This is how fast the skater is spinning.

Both the moment of inertia and the angular velocity can change, so they are our variables.

To understand how spinning works, we need to consider both constants (like mass, which stays the same) and variables (like moment of inertia and angular velocity, which can change).

In science, controlling variables is key to understanding and predicting outcomes, and this principle applies to a skater’s spin.

To achieve an impressive spin, the skater needs to manage their moment of inertia and angular velocity effectively.

Here’s the key point: When the skater pulls their arms in close to their body, they take advantage of the things that can be changed (the variables) making it easier for themselves to spin faster.

The moment of inertia of a rotating mass is measured from centre or axis to the radius, which in our example will be the skaters out stretch arms and hands, let’s give this a value of 10.

The angular velocity is how fast the skater will spin; we will also simply give this a value of 10.

Now the basic law of momentum is that as you reduce the moment of inertia by moving it toward the axis or centre of rotation you will increase the angular velocity.

As the skater pulls their arms and hands in toward the centre of their body or axis from 10 to five, their rotational speed will double from 10 to 20.

So much cool science going on behind those incredibly mesmerising spins!