Question:According to Newton's laws, an external force is needed to stop a car when the brakes are applied. Where is this force and what is its origin? Be careful to distinguish between internal forces and external forces.

Answer:You know that a car is slowed by applying pressure to the brake pedal causing a frictional force between the brake pads and the wheel. But these forces are internal forces and cannot stop the car. If the car is rolling smoothly in a straight line with no brake applied, the forces between the tires and the road are essentially only normal forces. The operation of the brakes retards the rotation of the wheels resulting in frictional forces between the tires and the road. These friction forces are parallel to the road surface. The tires push forward on the road and the road pushes backward on the tires. This behavior in not only true for skidding when the wheels are locked, but is also true even when the wheels are rolling. This backward or retarding force of the road on the tires is an external force acting on the car. Except for air resistance there are no other external forces along the line of motion, so the net force opposes the forward motion of the car causing it to slow to a stop.