Work, energy and momentum

Work, energy and momentum

Revise the following:

• Calculating work• Calculating kinetic energy• Momentum

In the same way, if a person transfers 250 J of energy, then 250 J of work is done.

This means the units for work are the same as the units for energy – joules.

work done = energy transferred

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?

For example, if a person does 500 J of work, then 500 J of energy is transferred.

Work and energy

How is work calculated?The work done on an object can be calculated using

this equation:

work done = force x distance moved (Joules, J) (Newtons, N) (metres, m)

Distance moved is measured in metres (m). Work done is measured in joules (j).

Force is measured in newtons (N).

How is kinetic energy calculated? (Higher tier)

The kinetic energy (KE) of an object can be calculated using this equation:

Velocity is measured in metres per second (m/s).

Mass is measured in kilograms (kg).

KE is measured in joules (j).

KE = ½ x mass x velocity2= ½mv2

The kinetic energy (KE) of an object depends on 2 things:

1. mass and 2. speed

All moving objects have momentum.

This is a measure of how difficult it is to stop a moving object.

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.

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.

What is momentum?

How is momentum calculated?

The momentum of an object can be calculated using this equation:

momentum = mass x velocity

Velocity is measured in metres per second (m/s).

Momentum is measured in kilogram metres per second (kg m/s).

Mass is measured in kilograms (kg).

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

When a force is applied to an object, the object’s velocity changes. This means that its momentum will also change.

Force and change in momentum (higher tier)

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:

Time is measured in seconds (s).

Momentum is measured in kilogram meters per second (kg m/s).

Force is measured in newtons (N).

force = change in momentumtime

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Gravitational potential energy depends on mass and height above ground

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


Gravitational field strength on earth 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


Gravitational potential energy is the energy that anobject has by virtue of its position in a gravitational field.

Ep = m x g x h


Ep = m x g x h



Ep = m x g x h


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


A force acting on an object may cause a change in shape of the object.

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


A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energyA 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.


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.

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.


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

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, Nk is the spring constant in newtons per

metre, N/me is the extension in metres, m

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Work, energy and momentum