Download 5. Momentum - Rougemont School

1 of 28
© Boardworks Ltd 2007
2 of 28
© Boardworks Ltd 2007
Stopping moving objects
Why is it a good idea to avoid a large object moving quickly?
3 of 28
© Boardworks Ltd 2007
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.
4 of 28
© Boardworks Ltd 2007
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).
5 of 28
© Boardworks Ltd 2007
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.
6 of 28
© Boardworks Ltd 2007
Calculating momentum question
An aircraft carrier has a
mass of 1,000,000 kg
and a velocity of 15 m/s.
What is its momentum?
momentum = mass x velocity
= 1,000,000 x 15
= 15,000,000 kg m/s
7 of 28
© Boardworks Ltd 2007
Momentum calculations
8 of 28
© Boardworks Ltd 2007
9 of 28
© Boardworks Ltd 2007
Momentum and collisions
10 of 28
© Boardworks Ltd 2007
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.
11 of 28
© Boardworks Ltd 2007
Using conservation of momentum
12 of 28
© Boardworks Ltd 2007
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
13 of 28
© Boardworks Ltd 2007
Investigating momentum
14 of 28
© Boardworks Ltd 2007
Momentum in explosions
15 of 28
© Boardworks Ltd 2007
Momentum: true or false?
16 of 28
© Boardworks Ltd 2007
17 of 28
© Boardworks Ltd 2007
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 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).
18 of 28
© Boardworks Ltd 2007
Change in momentum question 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
19 of 28
© Boardworks Ltd 2007
Change in momentum question 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 is rolled?
change in momentum
force =
time
= (0.1 x 0.5) – ( 0.1 x 0)
0.1
= 0.05
0.05
= 1N
20 of 28
© Boardworks Ltd 2007
Change in momentum calculations
21 of 28
© Boardworks Ltd 2007
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?
22 of 28
© Boardworks Ltd 2007
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
23 of 28
© Boardworks Ltd 2007
How do car safety features work?
24 of 28
© Boardworks Ltd 2007
25 of 28
© Boardworks Ltd 2007
Glossary
 conservation of momentum – The principle stating
that when two objects interact with no external forces, their
total momentum will not change.
 momentum – A property of a moving object equal to its
mass times velocity.
 scalar – A quantity that has magnitude only. An example is
speed.
 vector – A quantity that has magnitude and direction.
Examples are velocity and momentum.
 velocity – The speed of an object in a given direction.
26 of 28
© Boardworks Ltd 2007
Anagrams
27 of 28
© Boardworks Ltd 2007
Multiple-choice quiz
28 of 28
© Boardworks Ltd 2007