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P3 PHYSICS
BISHOPSTON COMPREHENSIVE
SCHOOL
PHYSICS DEPARTMENT
P3 PHYSICS REVISION
BOOK 1 – MOMENTUM
Name: _______________ Form: ___ Teacher: ______________
BOOK 1 – MOMENTUM
P3 PHYSICS
MOMENTUM
Momentum is a quantity possessed by all MOVING bodies. It is found by multiplying the
mass of a body by its velocity.
So,
Momentum (kgm/s) = Mass (kg) x Velocity (s)
Momentum has a direction as well as a size, what other quantities have we met that have a
direction as well as a size
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So you could say that big, fast objects have lots of momentum but small, slow objects have
little momentum.
Calculating Momentum - Examples
1.
A bus has a mass of 4000kg and is moving at a velocity of 2m/s, calculate the
momentum of the bus.
Momentum
2.
3.
=
Mass x Velocity
=
4000 x 2
=
8000 kgm/s
A child runs at a velocity of 4m/s and in doing so builds up 80kgm/s of momentum.
Calculate the mass of the child.
Momentum
=
Mass x Velocity
80
=
Mass x 4
Mass
=
80 / 4
=
20 kg
A 1000kg car has a momentum of 5000kgm/s, calculate the velocity of the car.
Momentum
=
Mass x Velocity
5000
=
1000 x Velocity
Velocity
=
5000 / 1000
=
5 kg
BOOK 1 – MOMENTUM – PAGE - 1
P3 PHYSICS
Calculating Momentum – Practise Questions
1.
A car has a mass of 1000kg and is moving at a velocity of 6m/s, calculate the
momentum of the car.
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2.
A motorbike has a mass of 400kg and is moving at a velocity of 10m/s, calculate the
momentum of the motorbike.
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3.
A box has a mass of 20kg and is moving at a velocity of 0m/s, calculate the momentum
of the box.
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4.
A child runs at a velocity of 8m/s and in doing so builds up 160kgm/s of momentum.
Calculate the mass of the child.
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5.
A car moves at a velocity of 12m/s and in doing so builds up 1200kgm/s of momentum.
Calculate the mass of the car.
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BOOK 1 – MOMENTUM – PAGE - 2
P3 PHYSICS
6.
A lorry moves at a velocity of 4m/s and in doing so builds up 16000kgm/s of
momentum. Calculate the mass of the lorry.
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7.
A 800kg car has a momentum of 4000kgm/s, calculate the velocity of the car.
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8.
A 50kg girl has a momentum of 350kgm/s, calculate the velocity of the girl.
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9.
A 60kg boy has a momentum of 0kgm/s, calculate the velocity of the boy.
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BOOK 1 – MOMENTUM – PAGE - 3
P3 PHYSICS
CONSERVATION OF MOMENTUM
The law of conservation of momentum states that:
The total momentum of objects before a collision equals their total momentum after a
collision providing no external forces act.
EXAMPLE
A body of mass 4kg moving with a velocity of 2m/s collides head on with a body of mass 3kg
moving in the opposite direction with a velocity of 5m/s. If the bodies stick together on
impact, what velocity will they have after the collision?
STEP 1 - Draw a `before` and `after` diagram showing the bodies with their masses and
velocities.
2m/s
5m/s
BEFORE
4kg
3kg
v
AFTER
7kg
STEP 2 - Choose a sign convention.
STEP 3 - Calculate the total momentum before the collision.
+ (4 x 2) - (5 x 3)
= - 7 kgm/s
STEP 4 - Calculate the total momentum after the collision.
= + 7v kgm/s
STEP 5 - Equate the total momentum before and the total momentum after.
- 7 = 7v
therefore
v = -1 m/s
The -ve sign indicates that the 7kg mass moves in the opposite direction to the one
indicated on the `after diagram`. i.e. it moves to the left not the right.
BOOK 1 – MOMENTUM – PAGE - 4
P3 PHYSICS
MOMENTUM - PRACTICE QUESTIONS
1. A body of mass 3kg moving with a velocity of 4m/s collides head on with a body of mass
2kg moving in the opposite direction with a velocity of 5m/s. If the bodies stick together on
impact, what velocity will they have after the collision?
BEFORE DIAGRAM
AFTER DIAGRAM
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2. Body A of mass 7kg moves with a velocity of 2m/s and collides head on with body B of
mass 1kg moving in the opposite direction with a velocity of 2m/s. If body A keeps the same
direction with a new velocity of 1m/s calculate the new velocity of body B.
BEFORE DIAGRAM
AFTER DIAGRAM
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BOOK 1 – MOMENTUM – PAGE - 5
P3 PHYSICS
3. A body of mass 5kg moving with a velocity of 4m/s collides head on with a body of mass
2kg moving in the opposite direction with a velocity of 4m/s. If the bodies stick together on
impact, what velocity will they have after the collision?
BEFORE DIAGRAM
AFTER DIAGRAM
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4. Body A of mass 10kg moves with a velocity of 8m/s and collides head on with body B of
mass 4kg moving in same direction with a velocity of 3m/s. If body A keeps the same
direction with a new velocity of 6m/s calculate the new velocity of body B.
BEFORE DIAGRAM
AFTER DIAGRAM
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BOOK 1 – MOMENTUM – PAGE - 6
P3 PHYSICS
HOW DO WE CHANGE THE MOMENTUM OF AN OBJECT?
The momentum of any object can be changed by applying an external force to the object. In
fact you can say that:
When a force acts on a body that is moving or able to move then a change in
momentum occurs
Just imagine that your car breaks down a few metres from home, if you want to increase its
momentum to get it that extra few metres you will have to apply an external force – i.e. push
the car.
How is the car’s momentum lost as it enters the garage? Where does the force come from to
do this?
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The equation below is used to calculate the force necessary to change the momentum of an
object:
Force (N) = Change In Momentum (kgm/s)
/ Time Taken (s)
Example
1.
A driver puts his foot down to make his car accelerate. In doing so the momentum of
the car increases from 20kgm/s to 50kgm/s. If this took 5 seconds to do then calculate
the driving force provided by the cars engine.
Change in momentum = 50 - 20 = 30kgm/s
Force = Change In Momentum / Time taken
Force =
30 / 5
Force =
6N
BOOK 1 – MOMENTUM – PAGE - 7
P3 PHYSICS
Practise Questions
1.
A driver puts his foot down to make his car accelerate. In doing so the momentum of
the car increases from 200kgm/s to 800kgm/s. If this took 4 seconds to do then
calculate the driving force provided by the cars engine.
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2.
A cart driver whips his horse to make the cart accelerate. In doing so the momentum
of the cart increases from 100kgm/s to 900kgm/s. If this took 8 seconds to do then
calculate the increased force provided by the horse on the cart.
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3.
A carpenter hammers in a nail. Upon impact the momentum of the nail increases from
0kgm/s to 50kgm/s. If the hammer was in contact with the nail for 0.5 seconds then
calculate the force of the hammer on the nail.
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4.
A driver slams on the brakes in his car to avoid an accident. When he does this he
decelerates. If his momentum changes from 800kgm/s to 200kgm/s in 2 seconds then
calculate the force of the brakes on the car.
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BOOK 1 – MOMENTUM – PAGE - 8