Question

2) A 20 kg mass moving at a speed of 3 m/s is stopped by a constant force of 15 N. How many seconds must the force act on the mass to stop it? 0.20 s O 1.3s O 5.0 s O 4.0 s​

Answer 1

Take the object's starting direction of motion to be the positive direction, so that a stopping force acts in the opposite direction. By Newton's second law, the object undergoes an acceleration a such that

-15 N = (20 kg) a

Solve for a :

a = - (15 N) / (20 kg) = -0.75 m/s²

The object's velocity v at time t is then given by

v = 3 m/s + (-0.75 m/s²) t

so the time it takes for the object to slow to a rest is

0 = 3 m/s + (-0.75 m/s²) t

t = (3 m/s) / (0.75 m/s²) = 4.0 s