A 2500-kg vehicle traveling at 25 m/s can be stopped by gently applying the breaks for 20 seconds. What is the average force supplied by the breaks to stop the vehicle?

Momentum = (mass) x (speed)

Change in momentum = (force) x (time)

The initial momentum is (mass) x (speed) = 2500x 25 = 62,500 kg-m/s.

Since you want to stop the vehicle, that number is also the required change

in momentum ... you want the vehicle to wind up with zero momentum.

62,500 = (force) x (time) = 20 x force

Divide each side by 20:

force = 62,500 / 20 = 3,125 newtons