Work, energy and momentum
Revise the following:
• Calculating work
• Calculating kinetic energy
• Momentum
Work and energy
When a force causes a body to move through a distance,
energy is transferred and work is done.
What is the link between work and energy?
work done = energy transferred
This means the units for work are the same as the
units for energy – joules.
For example, if a person does 500 J of work, then 500 J
of energy is transferred.
In the same way, if a person transfers 250 J of
energy, then 250 J of work is done.
How is work calculated?
The work done on an object can be calculated using this
equation:
work done = force
(Joules, J)
x
distance moved
(Newtons, N)
(metres, m)
 Force is measured in newtons (N).
 Distance moved is measured in metres (m).
 Work done is measured in joules (j).
done against frictional forces is mainly
transformed into heat
How is kinetic energy calculated?
(Higher tier)
The kinetic energy (KE) of an object depends on 2 things:
1. mass
and
2. speed
The kinetic energy (KE) of an object can be calculated
using this equation:
KE = ½ x mass x velocity2
= ½mv2
 Mass is measured in kilograms (kg).
 Velocity is measured in metres per second (m/s).
 KE is measured in joules (j).
What is momentum?
All moving objects have momentum.
This is a measure of how difficult it is to stop a moving object.
If these two cars have the same mass but one is quicker than
the other, which has the most momentum? The faster car.
If both cars travel at the same velocity, but one is full with
luggage and the other is empty, which will have the most
momentum? The heavier car.
The bigger an object is and the faster it moves, the more
momentum it will have and the more difficult it will be to stop.
How is momentum calculated?
The momentum of an object can be calculated using
this equation:
momentum = mass x velocity
 Mass is measured in kilograms (kg).
 Velocity is measured in metres per second (m/s).
 Momentum is measured in kilogram metres per
second (kg m/s).
When two objects collide, and no other forces
act, then conservation of momentum applies.
What does the term conservation of momentum
mean?
When the total momentum
after the collision equals
the total momentum
before the collision
Force and change in momentum
(higher tier)
When a force is applied to an object, the object’s velocity
changes. This means that its momentum will also change.
The change in momentum depends on the size of the force and
the time for which it is applied.
The relationship between this values is shown by this equation:
force = change in momentum
time
 Momentum is measured in kilogram meters per second
(kg m/s).
 Time is measured in seconds (s).
 Force is measured in newtons (N).
Gravitational potential energy depends on mass and height above
ground
Books on a shelf have gravitational
potential energy.
Book A has more than book C as it's
higher.
Book B has more than book A because it
has a greater mass
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Gravitational field strength on earth
The gravitational field at the Earth's surface produces a force of approximately
10N (Newtons) on every mass of 1kg.
Gravitational field strength is symbolised by the letter 'g'. On larger planets, like
Jupiter where the gravitational field strength is greater, the gravitational
potential energy would also be greater
10 N/Kg
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Gravitational potential energy is the energy that an
object has by virtue of its position in a gravitational field.
Ep = m x g x h
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Gravitational potential energy is the energy that an
object has by virtue of its position in a gravitational field.
Ep = m x g x h
Worked example
Abid the IT technician lifts a 15kg monitor from the floor onto a table 1.2m
high.
Calculate its gain in GPE.
GPE = mgh
GPE = 15 × 10 × 1.2
GPE = 180 J
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A force acting on an object may cause a change in shape of the
object.
Forces don’t always make an object move, sometimes they make an object
change shape
Work is done whenever a force is applied to an object. This means that if a force
changes the shape of an object, work must be done
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A force applied to an elastic object such as a spring will result in
the object stretching and storing elastic potential energy
Elastic objects such as elastic bands and squash balls can change their
shape.
They can be stretched or squashed, but energy is needed to change
their shape.
This energy is stored in the stretched or squashed object as elastic
potential energy.
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For an object that is able to recover its original shape, elastic potential energy is
stored in the object when work is done on the object to change its shape.
1. A falling ball transfers GPE into KE.
2. When the ball hits the ground,
its shape changes and the KE is transferred into EPE.
3. As its shape is restored, the EPE changes back into KE.
4. As the ball gets higher in the air, KE is transferred back into GPE.
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The extension of an elastic object is directly proportional to the force applied,
provided that the limit of proportionality is not exceeded: F= k x e
F is the force in newtons, N
k is the spring constant in newtons per
metre, N/m
e is the extension in metres, m
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