Chapter 3 - Dynamics


A stationary crate of mass 4.0 kg is free to move forward along a frictionless surface. For the first 20 s, a steady horizontal force of 15 N is applied to the crate.


a) Calculate the acceleration of the block and hence describe how the speed of the crate in the first 20 s changes. 

b) Calculate the speed of the crate after 20 s.

c) For the next 10 s, the forward force is removed. Describe and explain what happens to the crate. 




a) F = ma

    15N = 4 kg * a

    a  = 3.75 m/s2


    At 0s, the speed on the crate was 0m/s. After a stead force if 15N was applied to the crate, the speed of the crate increased at a constant

    rate of 3.75m/s2


b) a = (v2 - v1)/ t

    3.75 = (v2 - 0 ) / 20

    v2 = 75 m/s


c) Since the surface is frictionless, there is no loss in energy. As such, the crate will move at constant velocity after the forward force is removed. 



In the real world, due to the presence of friction, net force on the object will be lower as some force is constantly needed to overcome the frictional force. Therefore, when the forward force is removed, you will expect the object to slow down and stop.