Mechanics Tutorial 10 - Work, Energy, and Power


Work is defined as:

The product of force and the distance moved in the direction of the force.


Work = Force distance moved in the direction of the force.



  It is wrong to say that Work = Force displacement.  If we push the box in the animation (see link above) back to where it started, the displacement is 0, but the distance in the direction of the force is 10 metres.


Question 1 

A car owner is trying to bump-start his car, but he cannot get it to move.  Sweat is pouring off him.  Explain why he has done no work.


Question 2

A horse is pulling a barge along a canal as shown in the diagram.  It pulls the barge with a force of 1000 N a distance of 75 m.  The angle the rope is at 15o to the direction of travel.


The situation is shown in the diagram:


(a) Can you explain why the answer is NOT 75000 J?

(b) What is the work done by the horse? 






Energy and work are very closely related. 

Question 3

A box is pushed 5 m across a room with a force of 30 N.  What is the work done and how much energy is used? 




Power is the rate at which energy is used.

Power = energy transferred (J) = work done (J)                     

time taken (s)              time taken (s)




Units of power are watts (W).  

Question 4

It takes 20 seconds to push the box in Question 3 across the room.  What is the power?



We can also relate power, force and speed:

P = W/t = Fs/t

P = Fv


Power (W) = force (N) speed (m/s)




Question 5

An electric locomotive is pulling a train at a constant speed of 30 m/s.  The train has a rolling resistance of 100 kN. 


(a) What force must the locomotive produce?  Explain your answer.

(b) What power does it use?