# 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.

250 views

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

Thanks.

In: Physics

The tire creates enough friction that the wheel can not spin freely. This causes it to move forward instead, not pushing the ground but pushing the car.

When a wheel rolls it’s center moves around. Your car is connected to the center of the wheel by axels, so the center of the wheel’s movement moves along with the center of the wheel. That leaves only two options, the wheel slips in place or the axel and car move. Wheels are pretty hard to slip, because cars are heavy, and tires have a lot of friction (high coefficient of friction).

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

That’s it basically.

Indeed, if there was no friction between the wheel and the road, the wheel would just spin.

The friction creates a force from the wheel to the ground, and in reaction the opposite form from the ground to the wheel, and this force, which applies to the car, results in the car going forward. Energy dissipation caused by the friction would result in the car stopping quickly, but as the engine keeps providing energy and causing the wheels to rotate, the movement keeps going.

ELI5: when you put heavy things on a tire (like a car) it squishes it a little bit. So it is not a perfect circle anymore. For the sake of visualization imagine that it is REALLY squished and looks more like tracks on a tank. As the tire/track spins the flatter part that is touching the ground is really only moving backwards for that short time. There are 3 things that could happen in this scenario. The ground could move, the car could move, or the tire could just spin without either of them moving.
Other than in sand or other loose soil the ground won’t normally move much, and other than times with slippery things like ice or snow normally the tire won’t just spin so that leaves the last option. Normally it makes the car move.

Because of friction mostly.
Not sure where you’re confusion is but you might just be over thinking it.

People keep mentioning friction, but they are missing one important part of the equation, a difference in force. The physics behind anything “turning” is a differential in lateral force being applied.

&#x200B;

Think of it this way, turning your wheel to the right puts more force on the right hand side of the wheels. Now think of the rate of travel for the left side vs the right side. The right side has more force to contend with so it travels slightly slower than the left, this results in the left traveling faster than the right and a net rightwards change in direction.