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Difficulty: HardFriction and Resistance

A driver must execute an emergency stop on a dry asphalt road. The vehicle is equipped with an anti-lock braking system (ABS) that uses threshold braking, allowing the tires to continue rotating just on the verge of locking up. If the ABS were disabled and the driver fully locked the brakes, causing the tires to skid continuously across the surface, how would the frictional resistance between the tires and the road change?

  1. The resistance would decrease, because the skidding tires rely on kinetic friction, which is generally weaker than the maximum static friction maintained during threshold braking.Answer
  2. B
    The resistance would increase, because the skidding tires deform and flatten, creating a larger contact surface area with the road.
  3. C
    The resistance would remain unchanged, because frictional force depends on the vehicle's mass and is completely unaffected by its gravitational weight or rotation.
  4. D
    The resistance would increase, because the forward inertia of the skidding vehicle creates a larger opposing reaction force from the asphalt.

Answer

The resistance would decrease, because the skidding tires rely on kinetic friction, which is generally weaker than the maximum static friction maintained during threshold braking.
As long as a tire is rolling without slipping (as is maintained during threshold braking), the exact point of the tire touching the road is momentarily stationary relative to the ground. This means it utilizes static friction. Once the brakes lock and the tire skids, the rubber slides across the surface, switching the force to kinetic friction. Because the coefficient of static friction is greater than the coefficient of kinetic friction, a sliding tire actually provides less overall stopping resistance than a tire rolling on the verge of locking.

Step-by-Step Solution

1
Identify the type of friction present during threshold braking when the tires are still rotating.
The bottom of a rolling tire does not slide against the road, meaning the system utilizes static friction at the contact patch.
Understanding the physical state of rolling without slipping is necessary to determine the active frictional forces.
2
Identify the type of friction present during a full brake lockup (skid).
When the tires lock, they slide across the asphalt, meaning the system transitions to utilizing kinetic (sliding) friction.
Skidding fundamentally changes the interaction between the surfaces from gripping to sliding.
3
Compare the magnitudes of the relevant friction coefficients.
For standard materials like rubber on asphalt, the coefficient of static friction is greater than the coefficient of kinetic friction.
This physical rule dictates which braking method provides the most stopping force.
4
Determine the net effect on frictional resistance.
Because kinetic friction is weaker than maximum static friction, locking the brakes and skidding results in decreased frictional resistance.
Combining the physical state with the friction coefficients provides the final answer.

Key Concept

Static versus Kinetic Friction in Rolling Mechanisms
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Topics

Friction and Resistance

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