#### Accelerating car

A front-wheel drive car accelerates from rest such that all the wheels roll without slipping. In what direction is the frictional force on the front wheels? Is it static friction or kinetic friction?

We should also answer this question - in what direction is the frictional force on the rear wheels, and is that static or kinetic? The force on the rear wheels is:

1. kinetic friction acting in the direction you are traveling.
2. kinetic friction acting opposite to the direction you are traveling.
3. static friction acting in the direction you are traveling.
4. static friction acting opposite to the direction you are traveling.

Remember that this analysis assumes the car is accelerating forward. The simplest way to figure out what friction is doing is to see what happens when you turn friction off.

With no friction at all 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 it must point in the direction the car wants to go. For the front wheels to roll without slipping, the friction must be static.

If we turn on friction to the front wheels only, the car accelerates forward with the back wheels dragging along the road without spinning. Friction opposes this motion, so it must point opposite to the way the car is going. Again, it must be static friction.

Note that the static friction force acting on the front wheels is the force that accelerates the car forward. It is considerably larger than the friction force on the rear wheels - all that force has to do is to provide sufficient torque to give the rear wheels the correct angular acceleration.