Welcome to the new Simple Science Series!
In this series I will explain some really easy science demonstrations and experiments that you can do with your kids. Some of the other experiments I have shared in the past ( Elephant Tootpaste, Magnetic Induction, … ) have required buying a few special supplies, but the Simple Science Series will explore science with just the supplies that you typically have in your house. Actually this first one does not need any supplies at all.
In today’s experiment we will see the effects of angular momentum and centripetal force.
Have your children (or yourself) stand with their arms out and feet about shoulder with apart. The goal is to jump and rotate as far as possible. It helps if you wind up by twisting before you jump. Keep your arms straight like this:
Now try it again, but this time pull your arms in to your chest as you jump.
Did you rotate farther this time?
In the next 2 videos, you can see how Gideon only makes it half way around before his feet hit the ground when his arms are out, but with his arms in, he turns more than 3/4 of the way.
When you pull your arms in, it increases the speed of your rotation.
Why does it work?
In this experiment, your hands start far from your body. When you start your spin, your hands are moving at a certain speed, and they want to continue moving at that speed (this is called angular momentum). When you pull your arms in, it shortens the distance that they have to travel in order to get all the way around your body. Since your arms are trying to continue at the same speed and now traveling a shorter distance, they complete the rotation faster.
When the kids were trying to spin with their arms in, Elijah noticed that it was hard to pull his arms into his body. This is because of centripetal force, which is really just the result of inertia. An object in motion tends to continue in a straight line at constant speed unless an outside force acts upon it. Your arms are moving and they don’t want to move in a circle; then want to move in a straight line. That straight line is not pointed straight out from your body, but since the path is straight, it is pointed out at an angle. Therefore your arms get pulled out (unless you apply that outside force with your muscles).
This concept is used in ice skating, snowboarding, and parkour.
I hope you enjoyed this Simple Science experiment and did not get too dizzy.