Why do you spin faster when you tuck your arms and legs in?

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So far all the explanations I’ve seen have basically just said, “cause physics”.

In: Physics

7 Answers

Anonymous 0 Comments

You’re going to get a lot of answers that say *conservation of angular momentum* : that, for a given mass (like your arms), the angular momentum of that mass about an axis remains constant, i.e. decreasing the distance between that rotating mass and the axis of rotation increases the speed of rotation. This is true, but doesn’t help you understand *why* things speed up when pulled closer to the axis of rotation. Even when you experience it first hand, it seems counterintuitive.

Inertia, on the other hand, seems intuitive. Ignoring resistance, if a mass is moving it’s going to keep moving in the same direction *at the same speed.*

Focus on the “at the same speed” part of inertia, and then think about the relationship between the circumference of a circle and its radius. An object moving in a straight line with velocity V, tangential to (that is, along the edge of) a circle is moving past it with an *angular velocity* (rotations per of unit time) of V divided by its circumference. If you reduce the size of the circle you decrease its circumference, increasing angular velocity. If you increase the size of the circle you increase its circumference, decreasing angular velocity. The increase (or decrease) in angular velocity is proportional to the decrease (or increase) in the radius of the circle.

Anonymous 0 Comments

I’m gonna go and use the example of ice skating. Now if you’re going in a straight line theres very little force acting on you and without any effort on your part you’ll go on a long way as long as your skates are sharp (this is called momentum) but if they’re blunt then theres going to be resistance in the form of friction.

Let’s assume they are sharp. Then the force that’s acting on you is air resistance. Well the larger your profile the more air resistance the less force acting your momentum. Tucking in achieves this but that’s not the whole story.

If you hold a string with a ball attached to it and spin it around by spinning around then as you lengthen the string the more effort required to keep the ball spinning at the same speed. This is because centrifugal force. Instead of spinning the ball is pushed further away from you like a hammer thrower. Bring it in and theres less centrifugal force meaning that for the same effort you can go faster. Again this is what Tucking in achieves.

Anonymous 0 Comments

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Anonymous 0 Comments

Arms and legs out = big circle

Arms and legs in = small circle

If you have the same oomph, but less distance to move, you move faster.

Anonymous 0 Comments

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Anonymous 0 Comments

Energy and momentum are always conserved, so if momentum changes in something it has to come from or go somewhere else. This statement is where I’ll draw the “cause physics” line as discussing why that is true gets into above ELI5 territory, and is at the limit of my understanding.

Now we can discuss how conservation of momentum affects your question about spinning. Anything that spins has a particular amount of angular momentum. The amount of momentum is a combination of how fast it spins, how much mass it has, and how far away from the center (axis of rotation) of the spin that mass is (further out = more momentum). For a figure skater their total mass doesn’t change. If they have their arms or legs out then that mass is farther out from their axis of rotation. When they bring their limbs in their mass is concentrated near the axis of rotation. For momentum to be conserved they have to be spinning slower with their limbs out and faster with their limbs in.

Anonymous 0 Comments

There is something called “Conservation of angular momentum”. Essentially for spinning things, the momentum is how fast it rotates times something called the “moment of inertia”. Well, the inertia of an item is a measure of how far stuff is from the center of mass. So something spread out like a pancake, would have a bigger moment of inertia than something that got smashed down into a small ball.

So, by tucking your arms and legs in, you are reducing your “moment of inertia”. Since angular momentum is your moment of inertia times how fast you rotate, your body has to rotate faster to have the same angular momentum for your smaller moment of inertia.