Why a (Very Bad and Unsafe!) Playground Stunt Sends Folks Flying

WIRED | 4/15/2019 | Rhett Allain
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Why do people do such dumb things? In today's dumb thing, these kids decide to use the wheel of a gasoline-powered scooter to spin up a small merry-go-round at a park. Oh, don't worry, they have helmets on so everything should be fine. NO. Everything is NOT fine. This is why we can't have nice things.

Of course, these people aren't the first (or sadly the last) to try this stunt. It rarely ends well.

Trick - Center - Course - Answer - Physics

But what makes this such a difficult trick? Why do you want to keep as close to the center as possible? Of course, the answer is all about physics.

It starts with acceleration. If you have an accelerating object, you need a net force on that object. This is the basic model for motion. It looks like this.

M - Mass - Object - Acceleration - Arrows

Yes, the "m" is the mass of the object and the "a" is the acceleration. But what about the arrows over the F and the a? The arrow means that those quantities are vectors. Both acceleration and force have directions as well as magnitude. Pushing with a force of 1 Newton (the unit of force) to the left is different from pushing with 1 N to the right. But if the object is accelerating (also a vector), there must be a force.

Then what is the definition of acceleration? Over a short time interval, I can define acceleration as the following:

Acceleration - Change - Velocity - Object - Object

There will be an acceleration when there is a change in velocity. That means that an object that is speeding up or slowing down would be accelerating. But wait! What about an object that is moving in a circle? That is also acceleration. Since velocity is also a vector, just changing the direction of velocity would also be an acceleration.

The acceleration for an object moving in a circle is pointed towards the...