Automobiles have anti-lock braking systems to prevent the wheels from locking when the brake is suddenly applied. Rather than applying a constant force on the wheel, an anti-lock braking system exerts a series of forces of short duration on the wheel. Why would this be true?

Question

1. The kinetic friction force is larger than the static friction force
2. When the tire is locked, the frictional force is greater but the car cannot be steered
3. The static friction force is larger than the kinetic friction force
4. The coefficient of kinetic friction of the road in motion with respect the skidding tire is larger than the static coefficient of the road

Answer 1

The correct reason why an anti-lock braking system (ABS) exerts a series of forces of short duration on the wheel rather than a constant force is:

3. The static friction force is larger than the kinetic friction force.

When a tire is rolling normally (without slipping), the friction is static friction, which is generally greater than kinetic friction (which occurs when the tire skids). If the tires lock up during braking, the friction force decreases from static to kinetic, resulting in less effective braking and loss of control (the ability to steer). By modulating the braking force to maintain rolling (preventing lock-up), the ABS allows the tires to maintain static friction with the road, providing better braking effectiveness and control.