If a car accelerates forward, how does the friction between tires and road relate to the car's acceleration?

If a car accelerates forward, how does the friction between tires and road relate to the car's acceleration?

• A. The friction force is optional and does not affect acceleration.
• B. The frictional force from the road on the tires provides the net forward force; it must be sufficient to produce m a, and its maximum value is μ_s N or μ_k N depending on slipping.
• C. The friction force only affects rotational motion, not translation.
• D. Friction acts backward at all times, reducing acceleration.

Answer: (B)

The key idea is that friction between the tires and the road provides the forward push that makes the car accelerate. When the engine turns the wheels, they push backward on the road; in response, the road pushes the tires forward. This frictional forward force is the net force that accelerates the car, so it must be large enough to produce the required m a. The amount of friction isn’t unlimited. On level ground, the maximum static friction that can act without slipping is μ_s times the normal force N (N ≈ m g). If the wheels start to slip, the friction is kinetic and has the value μ_k N, which is typically smaller than μ_s N. So the forward acceleration is constrained by these friction limits: the friction must be sufficient to provide m a, up to the maximum values set by μ_s N (no slip) or μ_k N (slipping). Friction isn’t optional, and it’s not just about rotational motion. It directly determines the car’s translational acceleration by providing the forward force; it doesn’t always act backward.

The key idea is that friction between the tires and the road provides the forward push that makes the car accelerate. When the engine turns the wheels, they push backward on the road; in response, the road pushes the tires forward. This frictional forward force is the net force that accelerates the car, so it must be large enough to produce the required m a.

The amount of friction isn’t unlimited. On level ground, the maximum static friction that can act without slipping is μ_s times the normal force N (N ≈ m g). If the wheels start to slip, the friction is kinetic and has the value μ_k N, which is typically smaller than μ_s N. So the forward acceleration is constrained by these friction limits: the friction must be sufficient to provide m a, up to the maximum values set by μ_s N (no slip) or μ_k N (slipping).

Friction isn’t optional, and it’s not just about rotational motion. It directly determines the car’s translational acceleration by providing the forward force; it doesn’t always act backward.