A20.0 kg mass moving at a velocity of + 3.0 m/s is stopped by a constant force of 15.0 n. how many seconds must the force act on the mass to stop it? html editor keyboard shortcuts

The mass is 20 kg, initial speed is 30 m/s and the final speed is 0 m/s (rest), 
while the force is 15 N
Using second newton's law of motion which states that the rate of change in momentum is directly proportional to the resultant force. Then
 Ft = MV-MU
15 t = 20 (0-3)
15t = -60
  t = -4 sec ( negative because the force is acting on opposite direction to the motion)
Therefore, the time taken is 4 seconds

2. they have negligible mass.