Transcript work done = force distance

Work done

1.
2.
3.
Lesson Objectives:
Calculate work done
Explain that energy is needed for work to be done
Review calculations used so far
Starter
What is work? What does the word work mean to
you?

Work is when a force moves

If an object that has a mass is moved then
work is needed.

In pairs discuss: “ the work done by a force
depends on....”
Work done = force x distance moved in direction of force
(J)
(N)
(m)
This man has jumped 1m into the
air. He is moving against gravity.
Therefore he has done work in
moving his weight (force) off the
ground
Energy (J) is needed for work to
be done

The mass. If a larger object is moved more
work is done because a greater force is
needed to move it

The distance. If an object is moved a greater
distance more work is done.
Doing work
Doing work:
when you push or pull something
and make it move,
you do work.
How much work?
Imagine pushing a car along a road.
• You must apply a force . . .
• . . . and you push the car a certain distance.
work done (in joules)
= force (in newtons)  distance (in metres)
Energy transfer
As you push the car, you transfer energy from your
body to:
• the car – increasing its kinetic energy
• the surroundings (and bits of the car) – which heat
up because of friction
the amount of work you do
= the amount of energy transferred
An example
I push my car 30 m. I push with a force of 400 N.
work done = force  distance
= 400 N  30 m
= 12 000 J
Doing work against gravity
• If you lift something up, you must
exert enough force to balance the
force of gravity.
• The upward force is equal to the
weight of the object.
• You transfer some energy from
your body – and this increases the
gravitational energy of the thing
you lift.
Work done = energy transferred
So if you lift something through a certain distance
work done = force  distance
or
change in gravitational potential energy
= weight  vertical height gain
Lifting
A woman lifts up her baby.
The baby weighs 100 N.
The woman lifts the baby up by 2 m.
change in gravitational potential energy
= work done
= weight  vertical height difference
= 100 N  2 m
= 200 J
Pushing a car uphill
• If you push your car uphill, you do work:
work done = force  distance
• You increase the gravitational potential energy of your car:
change in gravitational potential energy
= weight  vertical height gain
• Are these the same? If not, why not?
The calculation
work done in pushing
= force  distance
= 450 N  30 m
= 13 500 J
gain in gravitational potential energy
= weight  vertical height gain
= 900 N  1 m
= 9000 J
So why the difference?
• Some of the energy is transferred to the
surroundings – heating them up.
• Only part of it goes to increasing the gravitational
potential energy of the car.



1) worksheet 4.26
2) Workbook page 66-67
3) extension: worksheet 4.27

Design your own question on calculating
work. Make up a scenario. Estimate the
forces, work done and distances used.

Write the correct answer in the back of your
book

Swap books and let someone else try your
question