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

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

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.