Why does the rotation of a wheel on a car force the car to move? I was told it was due to the wheel ‘pushing the ground and propelling the car,’ but this seems a little basic.

Question

I know it seems stupid but I can’t grasp the Physics behind it.

Thanks.

In: Physics

Answer 1

It helps not to think of it as rotation, just look at the patch where the tire touches the pavement.

The engine is twisting the wheel, trying to push the tire contact point backwards. But it can’t slide (because the tire is grippy) and the ground can’t go backwards (because it’s ground) so the only way for the bottom of the tire to move backwards is for the axle to move forwards. The friction between the tire and the ground is transmitted through the tire and wheel and shows up as a force pushing forward on the axle through the wheel bearings.

As the car (and axle) moves forward, that little patch of tire on the ground lifts away…but it’s a wheel, so as it rotates a new patch is coming into contact. Rotating the wheel makes sure that you’ve always got a piece of tire touching the ground to push off with.

If the tire *isn’t* grippy enough, the tire will slide over the ground, you spin your wheels, and don’t go anywhere. If the ground isn’t solid, the ground will go backwards and spray dirt/gravel behind you, and you won’t go anywhere.

Edit:typo