A car of mass 1200 kg moves at 20 m/s and comes to a stop in 4 s. What is the average stopping force?

• A. 4000 N
• B. 6000 N
• C. 8000 N
• D. 1200 N

Answer: (B)

Forces come from changing motion. The deceleration needed to stop is found from F = ma, using the car’s mass and how quickly its velocity changes.

The car slows from 20 m/s to 0 m/s in 4 s, so the acceleration is a = Δv/Δt = (0 − 20) / 4 = −5 m/s^2. The negative sign shows the acceleration is opposite to the direction of motion.

The average stopping force is F = m a = 1200 kg × (−5 m/s^2) = −6000 N. The magnitude is 6000 N, acting opposite to the car’s initial motion.

Other options would require different decelerations or stopping times: for example, 4000 N would imply a ≈ −3.33 m/s^2 and a stop in about 6 s; 8000 N would imply a ≈ −6.67 m/s^2 and a stop in about 3 s; 1200 N would imply a ≈ −1 m/s^2 and a stop in about 20 s.