Ice Skating Spin

1. Sit in the chair or stand on the spinning exercise platform with your arms fully extended.
2. Have your friend spin you slowly (several revolutions) to get used to the speed of the spin.
3. Spin again with your arms extended and then quickly bring your arms into your chest. If the demonstration works, the velocity of your spin will increase when your arms are brought inward toward your chest.
4. To slow yourself down while you're spinning quickly, stick your arms out as before.
5. You'll find that your friend is helpful when it comes to spinning you and catching you as you spin out of control and go flying across the room. 

How does it work?

Watch a figure skater – when they do a spin they start slowly with their arms spread out and speed up when they bring their arms in toward their chest.  Why do they speed up when their arms are tucked?  To understand it, we have to understand the principle of the conservation of momentum. 

Linear momentum – or energy in a straight line – is calculated by multiplying the object's mass by its velocity.  This means a car’s momentum is the car’s weight times its speed.  When it comes to angular momentum – or momentum from spinning – physicists use two forces that only apply to spinning objects, angular velocity and moment of inertia.  Angular velocity is just like how fast something is moving in a straight line, but instead it measures how fast an object is spinning.  Moment of inertia is both the mass of an object and how far the mass is from the axis. 

Conservation of angular momentum means that unless an outside force acts on a spinning object, the object's momentum will stay the same.  You might be wondering what all this means when talking about a figure skater.  Well, if a figure skater starts their spin with their arms spread wide, they have a large moment of inertia.  When the skater tucks their arms to their chest, the moment of inertia decreases.  Conservation of angular momentum tells us this – if the moment of inertia gets smaller, the object's angular velocity is going to be faster.  This is how the angular momentum of the object stays the same.

This principle is the same for someone spinning on a rotating stool or chair.  If they start spinning with their arms outstretched and bring them in towards their chest, they will start spinning faster.  The exact opposite happens if they spin with their arms tucked in and extend them out.

An object spinning eventually slows down, but why does it happen?  Conservation of momentum says that momentum can be changed by an outside force.  This outside force can be gravity, someone stopping the spinning person, or friction.  Friction is what causes a person spinning on a chair to eventually stop.  But don’t be too angry with friction, it’s also what stops a car.