Transcript Momentum - Sackville School

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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.
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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).
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Scalar or vector?
Velocity is a vector quantity – this means it has a magnitude
(size) and direction.
Scalar quantities, such as speed, only have a magnitude.
As velocity is needed to calculate momentum, momentum
must also be a vector quantity and it therefore has a direction.
If two objects of the same mass are moving in opposite
directions but at the same speed (i.e. their velocities are
different), the momentum of each object will be of the
same magnitude but a different direction.
A ‘+’ and a ‘-’ are often used to indicate the direction of
momentum of moving objects.
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Momentum calculations
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What is conservation of momentum?
If two objects collide or interact, the
forces acting on each one will be the
same size but in opposite directions.
The same is true for the change in
momentum of each object.
This means that the momentum
lost by one of the objects will
be gained by the other object.
Therefore, whenever two
objects collide or interact,
momentum is conserved.
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Using conservation of momentum
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Conservation of momentum question
Two trolleys collide and stick
together. From the data below,
calculate the velocity of the
trolleys after the collision.
trolley A
mass = 3 kg
velocity = 8 m/s
momentum = 24 kg m/s (3 x 8)
trolley B
mass = 5 kg
velocity = -4 m/s
momentum = -20 kg m/s (5 x -4)
total momentum before collision = 4 kg m/s (24 + -20)
mass after collision = 8 kg (3 + 5)
momentum after collision = 4 kg m/s
velocity after collision = momentum / mass = 0.5 m/s
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Investigating momentum
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Momentum in explosions
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Momentum: true or false?
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Force and change in momentum
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 these 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).
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Change in momentum example 1
A rugby ball of mass 0.5 kg is
kicked from stationary to a velocity
of 8 m/s. The kicker’s foot is in
contact with ball for 0.1 seconds.
What force does the kicker use?
change in momentum
force =
time
= (0.5 x 8) – ( 0.5 x 0)
0.1
= 4
0.1
= 40 N
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Change in momentum example 2
A tennis ball is rolled at a toy car of mass 0.1 kg. The car
is moved with a velocity of 0.5 m/s. If the ball and car are
in contact for 0.05 seconds, with what force is the tennis
ball rolled?
change in momentum
force =
time
= (0.1 x 0.5) – ( 0.1 x 0)
0.05
= 0.05
0.05
= 1N
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Change in momentum calculations
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Car crashes and momentum
What happens if two cars travelling very quickly collide?
Both cars come to a stop
in a short space of time.
This means that the cars
and their occupants
experience a large change
of momentum very quickly.
Why could this cause a
very serious injury?
A very large change of momentum in a short space of time
means the car occupants will experience a very large force.
Using this principle, how could you improve the safety of
cars?
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Reducing force in car crashes
Many modern car safety features work by increasing the
amount of time taken for the person to decelerate in a
collision. How does this reduce the risk of serious injury?
A longer deceleration means that change in momentum
occurs over a longer time. There is therefore a smaller
force acting on the person.
What features of cars
use this principle?
 seatbelts
 airbags
 crumple zones
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How do car safety features work?
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Multiple-choice quiz
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