Momentum is a measure of how difficult it is to stop a moving object. It is defined by the equation momentum (p) equals mass (m) times velocity (v). In a closed system where no external forces act, the total momentum before an event like a collision will equal the total momentum after the event, according to the principle of conservation of momentum.

1. Why is it a good idea to avoid a large
object moving quickly?
Stopping moving objects

2. 6.5.5 Momentum (HT only)
6.5.5.1 Momentum is a property of moving
objects
• Momentum is defined by the equation:
• momentum = mass × velocity
• p = m v
• momentum, p, in kilograms metre per
second, kg m/s
• mass, m, in kilograms, kg
• velocity, v, in metres per second, m/s

3. 6.5.5.2 Conservation of momentum
• In a closed system, the total momentum
before an event is equal to the total
momentum after the event.
• This is called conservation of momentum.
• Students should be able to use the
concept of momentum as a model to
describe and explain examples of
momentum in an event, such as a
collision.

4. MOMENTUM
• Understand the term ‘momentum’
• Recall and use the equation for momentum:
p=mv
• Describe how velocity and mass affect
momentum
• Know what a closed system is
• Explain the term ‘Conservation of momentum’
• Use the concept of momentum as a model to
describe and explain examples of momentum
in an event, such as a collision.

5. What is momentum?
• All moving objects have momentum.
This is a measure of how difficult it is to
stop a moving object. Sort of!!

6. All moving objects have
momentum. This is a measure of
how difficult it is to stop a moving
object. Sort of !!
What is momentum?

7. If these two cars have the same mass but
one is quicker than the other, which has
the most momentum?
The faster car.
What is momentum?

8. 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?

9. 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.
What is momentum?

10. All moving objects have
momentum. This is a measure
of how difficult it is to stop a
moving object.
What is momentum?

11. So why do we need to
know about momentum?
Simply, it is a tool.
It can help us understand
what happens when
objects collide or
EXPLODE!
What is momentum?

12. How is momentum calculated?
The momentum of an object can be
calculated using this equation:
momentum = mass × 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).

13. An aircraft carrier
has a mass of
1,000,000 kg and a
velocity of 15 m/s.
What is its
momentum?
Calculating momentum
= 1,000,000 × 15
momentum = mass × velocity
= 15,000,000kg m/s

14. Calculating momentum
• A ball of mass 20kg has a velocity of
5m/s, Show that its momentum is
100kg m/s?

15. Momentum in collisions
• We now calculate the total
momentum of two objects
6 kg
10 m/s
10kg
20 m/s

16. 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 different.
A ‘+’ and a ‘–’ are often used
to indicate the opposite directions of momentum.

17. Momentum
• Momentum is a vector quantity
• This means it has both a size and direction
• Look at this, both cars have a mass of 100 kg and both
have a speed of 10m/s. What are their momentum?
+

18. Momentum
• Calculate change in momentum
10 kg
10 m/s
10kg
10 m/s
Before After
Change
+

19. Which are the units of momentum?
A. kg s/m
B. N/m
C. kg m/s
D. kg m/s2


20. You can increase momentum by increasing
the object’s ?
A. weight
B. temperature
C. energy
D. velocity 

21. To calculate momentum you need to know
an object’s?
A. mass
B. position
C. direction
D. speed


22. What is the momentum of a 30kg mass with
a velocity of 5 m/s?
A. 35 J/s
B. 130 kg m/s
C. 150 J/s
D. 150 kg m/s 

23. A 5kg ball hits a wall at 4m/s and bounces off
at 2m/s. What is its change in momentum?
A. 6 kg m/s
B. 30 kg m/s
C. 10 kg m/s
D. 20 kg m/s


24. Car crashes and momentum
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?
Two cars travelling very quickly collide
A very large change of momentum in a short space of
time means that the car occupants will experience a
large force.
Using this principle, how could you improve the safety of cars?

25. Momentum and collisions

26. Using conservation of momentum

27. 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.
What is conservation of momentum?
Therefore, whenever two objects collide or
interact, momentum is conserved.

28. Momentum: Collisions and explosions
In a collision or explosion we can say
The momentum
before
(in a particular direction)
The momentum after
(in a particular direction)
=
As long as there are no other
external forces
Some examples

29. 1) Describe an object that has momentum.
2) Is momentum a scalar or vector quantity?
3) How does increasing velocity affect momentum?
4) How does increasing mass affect momentum?
5) What is the formula for momentum? Include the units.
6) What is meant by a closed system?
7) When a moving object collides with another object, what
happens to its momentum?
8) What does the term ‘Conservation of momentum’ mean?
9) If marble A has a momentum of 10kg m/s, and it hits marble B,
which moves away with a momentum of 4kg m/s, what is the
new momentum of marble A?
10) If one ball on a Newton’s Cradle is lifted to a height of 5cm and
then let go, what happens to the momentum of this ball?
11) Why does the ball at the other end move to almost the same
height as the ball at the start?
Momentum Questions

30. 1) A moving object has momentum.
2) Momentum is a vector quantity.
3) Increasing velocity increases momentum.
4) Increasing mass increases momentum.
5) P=mv (p=kg m/s, m=kg, v=m/s
6) A closed system has no external forces acting on it
7) When a moving object collides with another object some of its
momentum is transferred to the other object (i.e. it shares some
of its momentum)
8) ‘Conservation of momentum’ means that the total momentum
before an event is the same as the momentum after the event.
9) 10kg m/s - 4kg m/s = 6kg m/s
10) The momentum is transferred along the row of balls.
11) The ball moves to almost the same height as the ball at the
start because momentum is conserved; ball at the end has
same momentum as the first ball that hit the row of balls.
Momentum ANSWERS

31. Momentum: Collisions and explosions
before after
mm mm
vv
m 2m2mm
u
2u

32. Momentum: Collisions and explosions
before after
mm mm
vv
m 2m2mm
u
2u

33. Momentum: Collisions and explosions
before after
mm mm
vv
m 2m2mm
u
2u

34. Momentum: Collisions and explosions
before after
mm mm
vv
m 2m2mm
u
2u
Explain why each trolley has its new velocity

35. Momentum: Collisions and explosions
• A 2kg gun fires a bullet of mass 0.01kg
• The velocity of the bullet is 350m/s, what is the recoil
velocity of the gun
2 kg
350
m/s
Before After

36. Momentum: Collisions and explosions
• Two cars are travelling in the same direction
• They collide, then stick together
• What is their new velocity
12 kg
10 m/s
10kg
20 m/s
Before After

37. Momentum: true or false?

38. Multiple-choice quiz

39. A past exam question…
Two lorries are travelling in the same direction along a motorway.
1) Calculate the momentum of Lorry A as it travels along the motorway.
2) Calculate the momentum of Lorry B as it travels along the motorway.
(3 marks)
3) Lorry B collides with Lorry A and they stick together. Calculate the common
speed of the lorries immediately after the collision.
(3 marks)
June 2000
Lorry A
Mass = 20,000kg
Speed = 14m/s
Lorry B
Mass = 30,000kg
Speed = 20m/s

40. Assume ‘positive’ means ‘to
the right’ and
‘negative’ will be ‘to the left’

41. Momentum in explosions

42. • How we can use our understanding to improve road safety
(B)
• To know what momentum is and how to calculate it
(C)
• To know momentum is conserved in collisions and
explosions allowing us to understand them (B/A)
What is momentum?

43. Momentum: true or false?

44. Glossary