chapter 7: Dynamics

1. PREPARED BY:
Mdm Heiana Nadia Hamzah

2. 1. A force is a push or a
pull acting on an
object.
2. When we push or pull
on an object, we are
exerting a force on it.

3. Identify whether the activities below are
push or pull actions:
a) Pressing the plasticine
b) Stretching the plasticine
c) Climbing a tree
d) Ride on a bicycle
e) Walk on the ground
f) Chopping onion with a knife
push
push
push
pull
pull
push

4. plasticine
PUSH
PULL

5. A B C
State the effect of force on each of the following activities:
A : Press & stretch the lump of plasticine
B : Shifting a wooden block to the side of the table
C : Blow gently & hard at the ping-pong ball in the opposite direction
Changes the shape of the plasticine
Changes the position of the wooden block
Changes the speed & direction of movement of ping-pong ball.

6. Let‘s do Activity 7.1
ACTIVITY OBSERVATION
A
B
C
D
E
F
G
Force change the shape & size of the spring.
Force change the shape & size of the span.
Force move a stationary trolley.
Force increase the speed of a moving trolley.
Force slow down a moving trolley.
Force stop a moving slotted weight.
Force change the direction of the magnet.

7. 3. Applying forces to an object can cause;
• A change of shape of an object

8. 3. Applying forces to an object can
cause;
• Move a stationary object.
• Change its position or direction.
• Increase or decrease the speed of a
moving object.
• Slow down & stop a moving object.

9. a)GRAVITATIONAL FORCE
• It is the force that pulls object to the Earth’s surface
A stone that is
thrown upward
will falls down
because of
gravitational
force.

10. • The force of gravity acts
equally on all objects.
• In a vacuum, all objects fall
with the same velocity.
Figure 1: objects fall with the same
acceleration in a vacuum (left) but
with different accelerations through
air (right)
Air resistance.

11. Different parts of the Earth experience
different gravitational forces.
Equator
North pole
South pole
Explanation
•This is because the Earth
is oval.
•The force of gravity is
stronger at the poles &
weaker at the equator.
•Gravity is stronger when
you are closer to the
centre of the Earth.

12. b)FRICTIONAL FORCE
•A force that opposes the movement of one surface
over another.
•Produced when two surfaces touch one another.

13. c)ELECTROSTATIC FORCE
• It is the force of attraction
between unlike charges or the
force of repulsion between like
charges.

14. c) ELECTROSTATIC FORCE
• It is produced by charged objects, which
are non-conductors of electricity. This
may be a plastic, glass & rubber.
• When a plastic ruler is rubbed with a dry,
woolen cloth, the ruler becomes
charged. It can attract small pieces of
paper that are not charged.

15. d)MAGNETIC FORCE
• It is the force of attraction or repulsion between magnets or
magnetic material.
• Like poles of magnets repel each other.
• Different poles of magnet attract each other.

16. e) ELECTROMAGNET
• An electromagnet is a type
of magnet in which the
magnetic field is produced
by the flow of electric
current.
• An electromagnet is a
temporary magnet.
• The magnetic field
disappears when the
current is turned off.
solenoid

17. 1. The unit of force is the newton (N)
2. A spring balance is used to measure the
magnitude.
3. If a spring is 5 cm long and a load
stretches it to 8 cm long, then the
extension of the spring is…
8 – 5 = 3 cm

18. • The extension of a spring is directly proportional to the
weight of a load.
• The extension of the spring increases when the load on
the spring increases.

19. Try this out...!
QUESTION
Figure shows the extension of a spring by 2 cm when a
load of 4 N was suspended on it.
Calculate the increase in length when a load of 6 N is
suspended on it.

20. SOLUTION
4 N extend 2 cm
6 N extend ? cm
6/ 4 x 2 = 3 cm
Extension of the spring = 3 cm

21. QUESTION 2
A spring without any load has the length of 10 cm. This
spring stretches to 35 cm if a load of 25 N is attached to
it. What is the extension of the same spring if a load of
10 N is attached to it.
ANSWER
10 cm

22. A cup that is
placed on a
table does not
move. WHY?
The cup does not move because
of the friction between the
table & the cup.

23. The magnitude of the frictional force that acts
between two surfaces depends on the…
a) Nature of the surfaces
• If the surfaces are rough, the
frictional force will be greater.
• If there is oil or grease between
the surfaces, the frictional force
will be smaller.

24. Rough surface will generate more friction compare to the
smooth surface.
A
B

25. b. Weight of the moving object
The greater the weight, the greater the frictional
force.
A
B

26. Frictional force does not depend on
the area of contact surface.

27. EXPERIMENT 7.1:
HOW DIFFERENT TYPES OF SURFACES AFFECT
FRICTIONAL FORCE
TYPES OF SURFACES MAGNITUDE OF
FRICTIONAL FORCE (N)
Table
Sand paper
Glass rods
Beans
CONCLUSION:
______________________________________________The rougher the surface, the greater the frictional force.

28. What are the
advantages of
frictional force?

29. ADVANTAGES OF FRICTIONAL FORCE
1. Enable us to carry out daily activities
such as walking, running, climbing,
dancing etc.

30. ADVANTAGES OF FRICTIONAL FORCE
2. Braking system making moving
vehicles slow down or stop.

31. ADVANTAGES OF FRICTIONAL FORCE
3. Enable us to hold things.
4. Enables us to move without slipping.

32. ADVANTAGES OF FRICTIONAL FORCE
5. Enable things to lean on.

33. What are the
disadvantages of
frictional force?

34. DISADVANTAGES OF FRICTIONAL FORCE
1. More energy is needed to move a
vehicle or object
2. Causes the surface in contact to wear
out.
3. Produces unnecessary heat –
Car engines becomes hotter because of friction.

35. The wooden block is just about to move when the
spring balance shows 4 N.
Which statements below is correct regarding the
magnitude and direction of friction.
A. The magnitude of friction is twice that shows on the
spring balance and acts in the same direction as the
pulling force.
B. The magnitude of friction is 4N and it acts in the
opposite direction to movement.
C. The magnitude of friction 2N and it acts in the
opposite direction to movement
pulled

36. Methods of reducing friction
1. Rollers or ball bearings reduce
friction.
2. Lubricants such as oils & grease are
applied to moving parts to reduce
friction.

37. Methods of reducing friction
3. A space shuttle is streamlined to reduce
friction & provide very little resistance to
a flow of air.
4. Hydrofoil boat The layer of air
between the hull of a hydrofoil &
the water allows the hydrofoil to
move at high speed.

38. Methods of reducing friction
5. HOVERCRAFT 
Vehicle that moves above
water or land by gliding on a
cushion of air it creates by
blowing downward.

39. Methods of reducing friction
6. MAGLEV TRAIN
This train use the basic principle of magnet. As the train
floats on the track, there is no contact with the ground,
so less friction is produced.

40. 1. Work is the product of the force acting on an object & the
distance moved by the object in the direction of the force.
2. S.I. unit for work is newton metre (Nm) or joule (J).
3. Work is also a measure of the amount of energy used.
WORK
Work done (J) = Force (N) x Distance moved in the
direction of the force (m)
FORMULA

41. WORK
wall
Is work done in the above situation?
nO work is done in above situation.
Work is done when a force moves an object
over a distance.

42. Try this out...!
QUESTION 1
Jamie uses 200 J of energy to lift a 10 kg box to a certain
height. What is the height?
Solution
[Assume 1 kg = 10 N]
200 J = 100 N x d
d = 200/ 100
d = 2 m

43. 1. Power is the rate of work done
2. The S.I unit for power is joule per second ( Js⁻ ˡ ) or
watt (W).
3. Power is a measure of how fast work is done.
POWER
Power (W) = Work done (J)
Time taken (s)
FORMULA

44. Try this out...!
QUESTION 2
A boy weighs 55 kg carries
a box of 2kg when
climbing a stairs of vertical
height 5 metres. Calculate
the work done by him.
ANSWER
2850 Nm/ 2850 J

45. Try this out...!
QUESTION 3
A girl weighs 45 kg carries
books of 3kg. She climbs a
stairs in 20 seconds.
Calculate the power
generated by her.
ANSWER
72 Watt

46. Try this out...!
QUESTION 4
A student climbs up a 6m high staircase with a 3.5 kg
parcel in 40 seconds. If the power used by the
student is 87 W, what is the mass of the student?
ANSWER 54.5 kg

47. Try this out...!
QUESTION 5
A soldier weighing 750 N carries a knapsack & a rifle
weighing 100 N and 50 N respectively. He is ordered to go on
a 10 km march. How much power does the soldier have if he
completes the march in 5 hours?

48. Solution
[Assume 1 km = 1000 m]
Work done (J) = (750 + 100 + 50) N x (10 km x 1000) m
= 9 000 000 Nm
Power (W) = 9 000 000 Nm / (5 hours x 60 x 60) s
= 500 W

49. 1. A man carrying a sack of rice walks for 40 m in 20
seconds. He weighs 50 kg and the sack of rice weighs
10 kg. How much power does he produce?
2. A pupil weighing 60 kg climbs 15 steps up a staircase
in 40 seconds. Each step of the stairs is 20 cm high.
How much power is produced as he climbs the stairs?
3. A car weighing 500 kg is being towed through a
distance of 0.4 km. How much power in kW is needed
to tow a car in 10 seconds?

50. QUESTION 1
A spring of 7 cm is stretched to 10 cm when a 10 N load is
placed on it. A load X is placed on it & the spring stretched
to 13 cm. Find the weight of load X?
ANSWER
Weight of load X = 20 N