Work, Energy, and Power
| Key Words Work, distance moved in direction of force, Energy, Power, Conservation, Specific Heat |
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.
Units
are newton metres (Nm) or joules
(J).
Work
is actually a scalar quantity
despite being the product of a vector quantity.
Normally
we consider the line of action of the force and the line of displacement to
be at zero degrees to each other. The
cosine of zero is 1. However if
we have the line of action of the force and the displacement at an angle, we have to use the cosine
function to take this into account.
When
work is done, there must be movement. This
can result in acceleration, a rise in temperature, or deformation in shape.
| Why is Work a Scalar? | Click HERE |
It is wrong to say that Work = Force
× displacement. If we push the box in the animation 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. | ANSWER |
| 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?
|
ANSWER |
Energy
Energy and work are very closely related.
Energy is the ability to do work. When work is done, energy is transferred.
Energy comes in many forms.
Some kinds of energy can be stored, while others cannot.
Energy is always conserved.
| 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
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).
1 watt = 1 joule
per second.
Also kilowatt (kW). 1kW = 1000 W.
megawatt (MW). 1 MW = 1 × 106 W.
|
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:
Work done = force x distance moved. W = Fs
Power = energy ÷ time. P = W/t
Speed = distance ÷ time: v = s/t
So we can write:
P = W/t = Fs/t
Therefore:
P = Fv
Power (W) = force (N) × speed (m/s)
|
(a) What force must the locomotive produce? Explain your answer. (b) What power does it develop? |
| Now try Topic Test | Home | Unit 2 | Physics AS |
