Accelerating car

A front-wheel drive car accelerates forward from rest such that all wheels roll without slipping. A simple way to figure out what friction is doing is to first see what happens without friction.

With no friction and the car stopped, pushing down on the accelerator makes the front wheels spin clockwise. They spin on the frictionless surface, the rear wheels do nothing, and the car goes nowhere.

Friction on the front wheels opposes the spinning, so friction must point in the forward direction. For the front wheels to roll without slipping, the friction must be static.

If there is friction on the front wheels only, the car accelerates forward, with the back wheels dragging along the road without spinning.

When there is friction on the rear wheels, friction opposes the sliding of the rear wheelsthis motion, so it must point opposite to the way the car is going. Again, it must be static friction.

The static friction force on the front wheels accelerates the car forward. This much smaller friction force on the rear wheels merely provide sufficient torque to give the rear wheels the correct angular acceleration.