An accelerating car

You are driving your front-wheel drive car on Comm. Ave. You are stopped at a red light, and when the light turns green you accelerate smoothly so that there is no slipping between your car tires and the road. During the acceleration period, in what direction is the force of friction from the road acting on your front tires? Is it static friction or kinetic friction?

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

During the acceleration period, in what direction is the force of friction from the road acting on your rear tires? Is it static friction or kinetic friction?

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

Accelerating car

A front-wheel drive car accelerates from rest such that all the wheels roll without slipping. 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.