Hot Wheels on a treadmill? Why didn’t anyone think of that earlier? As you can see in the video, this guy put a whole bunch of tiny cars on an inclined treadmill. After that, he just slowly increased the track speed. And there you have it: an instant demolition derby. It’s kind of fun to watch as some cars collide and then get thrown off the back of the treadmill.

But of course there are questions here—physics questions. I’m going to answer them for you.

Why Are Some Cars Faster Than Others?

Let’s imagine that these cars are just blocks on a low-friction inclined plane. (It’s simpler that way.) With that, I can show the forces acting on each car (block).

Illustration: Rhett Allain

We have three forces. First there is the downward-pulling gravitational force. This depends on both the mass of the object (m) and the gravitational field (g = 9.8 Newtons/kilogram). That’s the easy force.

The next one is FN. This is the normal force. It’s an interaction between the car and the surface. The whole point of this force is to prevent the car from moving through the surface. It’s a force of constraint—that means it has just the right value to keep the block on the inclined plane.

Finally, there is the kinetic frictional force (FK). This force depends on two things: the magnitude of the normal force and a coefficient of friction that is set for the two materials that are interacting. For the actual Hot Wheels car, the kinetic friction is not between the wheels and the track, but rather between the wheels and the axles.

As an equation, the kinetic friction force can be modeled as:

Illustration: Rhett Allain