# Does static friction cause a car to move forward?

## Main Question or Discussion Point

I get that when a car moves forward, its tires are rotating backwards, but the static friction from the ground, as a reaction force to the force from the tires, prevents the tires from moving backwards (not sure on this). So does the static friction only prevent the tires from moving backwards, or does it actually make the tires (hence, the car) move forward?

## Answers and Replies

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tiny-tim
Science Advisor
Homework Helper
I get that when a car moves forward, its tires are rotating backwards, but the static friction from the ground, as a reaction force to the force from the tires, prevents the tires from moving backwards (not sure on this). So does the static friction only prevent the tires from moving backwards, or does it actually make the tires (hence, the car) move forward?
Hi FisherDude!

A car moves forward because the engine forces the back axle to turn.

If the car was on ice, the back wheels would spin, but the rest of the car would be still.

The torque from the engine causes a force at the bit of the tyre in contact with the road. So long as that force does not exceed the maximum static friction, that force will equal the actual friction force, and the bit of the tyre in contact with the road will not move.

(Newton's first law on that bit of the tyre: zero total force means zero change in movement.)

The car will move, because the only external horizontal force on it is the actual friction force.

(Newton's second law on the car as a whole: net horizontal force means horizontal acceleration.)

So yes, the actual friction force prevents the tires from spinning, and also actually makes the car move forward.

Only an external force acting on the car can cause the car to move. (External meaning between the car and "something", where that something could be the ground, a wall, rocket exhaust, etc.).

So, the force acting on the car that causes it to move forward is the reaction force from the car pushing on the ground. This reaction force, described by Newton's third law, is due to the friction between the tire and the ground. Tire pushes on ground, ground pushes on car.

Ok, I think i understand it a little better...

how about when someone is pulling an object, such as a box, along a surface....is the static friction from the ground an actual reaction force to the force exerted by the person on the box? as in, is it part of the action-reaction pair, with the action being the force exerted by the person, or is the static friction just a result of the box sliding across the surface?

thanks btw!

Doc Al
Mentor
If the "action" is the person pulling on the box, then the "reaction" is the box pulling back on the person. Similarly, the box exerts a friction force on the ground, thus the ground will exert a friction force on the box.

Friction is an interaction between the ground and the box, so it's not part of any "action-reaction" pair with the person. (And if the box is sliding, it will be kinetic friction, not static.)

rcgldr
Homework Helper
In the case of the car, the tires exert a backwards force on the pavement, and the pavement exerts and equal and opposite forwards force on the tires, regardless of whether the tires are slipping (dynamic firction) or not (static friction), as long as the fricion isn't zero. The backwards force from the tires rotates the earth backwards a bit, and the forwards force from the pavement accelerates the car around the surface of the earth. Angular momentum of earth and car would be conserved if there was no aerodynamic drag.

In the case of the person pushing a box, the person exerts a backwards force to the floor which exerts an equal forwards force on the person. In a constant velocity situation, the person exerts a forwards force to the box, which in turn exerts a forwards force to the floor, and the floor exerts a backwards force on the box, and since there's no acceleration in this constant velocity case, all forces cancel. The work done by the person is converted into heat via dyamic friction between box and floor.

tiny-tim
Science Advisor
Homework Helper
… friction can be your friend … !

how about when someone is pulling an object, such as a box, along a surface....is the static friction from the ground an actual reaction force to the force exerted by the person on the box? as in, is it part of the action-reaction pair, with the action being the force exerted by the person, or is the static friction just a result of the box sliding across the surface
Hi FisherDude!

This is very different from the friction on a wheel.

Friction on the wheel is the only force which makes the car move. Paradoxically, friction is the car-driver's friend!

But it is you pulling which makes the box move … friction on the box opposes your force. Friction is the box-puller's enemy!