1. PREPARED BY:
PHYSICS DEPARTMENT
SBPI TEMERLOH

3. 2.5 Understanding the
effects of force

4. Learning Outcomes
A student should be able to :
• State the effects of balanced force acting
on an object.
• State the effects of unbalanced force
acting on an object.
•Determine the relationship between
force,mass and acceleration. F=ma

5.  The action of pushing or pulling is a force.
 It is a vector quantity
 Its SI unit is Newton (N) or kg m sˉ²
 A force is not visible but the effect of a force
can be observed

6. Situation 1
A bird perching on a branch of a tree
R (Reaction of the branch)
W (Weight of the bird)

7. A bird perching on a branch of a tree
R (Reaction of the branch)
 The forces of gravity, W pulls it downward
and the reaction of the branch, R pushed
the bird upward to balance the weight.
 Total force acting on the bird is
zero and the bird is at rest.
W (Weight of the bird)

8. Situation 2
A cyclist use a bigger force to
pedal his bicycle and
increased its speed.

9. A cyclist use a bigger force
to pedal his bicycle and
increased its speed
F
 The forces to the pedal is greater
than the frictional forces acting on
the bicycle.
 The resultant force, F causes the
bicycle to accelerate forward.

10. Situation 3
 Push the stationary toy car  A toy car collides with a wall
with the hand

11. Situation 3
Toy car starts to move Toy car stop the moving
 Push the stationary toy car  A toy car collides with a wall
with the hand
 The toy car is stop by the wall
 The toy car start to move
 The forces can stops the motion
 The forces can moves a of an object.
stationary object.

12. A golf buggy moving in a golf course
R (Reaction of the ground)
T (Engine Force) F (Frictional Force)
W (Weight of the golf buggy)

13. A golf buggy moving in a golf course
R (Reaction of the ground)
T (Engine Force) F (Frictional Force)
W (Weight of the golf buggy)
 T – drives the buggy forward
 T has the same magnitude with F (included air resistance), but
acts in the opposite direction.
 The net horizontal force acting on the buggy is 0
 W is balanced by the R on the buggy
 The net vertical force on the golf buggy is 0
 So the buggy moves with a constant velocity

14. Situation 5
Lorry moves from smooth surface to
a rough surface
smooth surface rough surface
Move to

15. Lorry moves from smooth surface to
a rough surface
smooth surface rough surface
Move to
 When a lorry moves from a smooth surface to a rough surface,
the forces of friction retards the motion of the lorry and slow down
a lorry moving (decelerate)

16. Situation 6
A tennis ball hit by the racket

17. A tennis ball hit by the racket
When a tennis ball is hit
by a racket, the reactive
force causes the tennis
all to change its direction.

18. Situation 7
Bending a straight ruler

19. Bending a straight ruler
Straight ruler
A straight ruler forms
an arc when bending
forces are applied to
Change to both ends. The forces
can change the shape
of an object.
Bend ruler

20. BALANCE FORCES
 An object may have several forces acting on it
 But if the forces are in balance, they cancel each other
out (no net forces)
 Then, the object behaves as if no force is applied to it.
 With balance forces on it, an object is either at rest or
moving at a constant velocity. (Newton’s first law of
motion).

21. Example of balance forces
A book resting on a hard surface. The weight, W book is acting
vertically downwards. At the same time, an equal and opposite
force (normal reaction, R) from the surface acted on the books.
The net force acting on the book is zero. The book remain at rest

22. Example of balance forces:
 A car moving at constant velocity along a straight road. The
engine provides a forward driving force, T. The wind and
frictional force, F. The net force acting on the car is zero.
The car travels with its original constant velocity

23. Example of balance forces:
An airplane is flying horizontally at a constant height with a
constant velocity. The engine provides a forward thrust,T. The
wind and air resistance provides a drag, F against the forwards
motion. The wings of the plane provide a lift, L vertically to
balance its downward weight, W. When these four forces are
balanced, the net force acting on the plane is zero. This means
that the lift force is equal to its weight, and the forward thrust is
equal to the drag.

24. UNBALANCED FORCE
• When two or more forces acting on a body are not balance, there
must be a net force acting on it.
• This net force is known as the unbalanced force or the resultant
force.
• An object will accelerate if the forces acting on it are not balance.
• Unbalanced forces acting on an object can cause the object:
(i) to start or stop moving
when a golfer hits a stationary golf ball, a force acts on
the ball and causes it to fly off from rest. The net force
or the unbalance force causes the golf ball to
accelerate.

25. (ii) to accelerate or decelerate
-when the engine of a moving car is shut down, the car will
slow down and finally stop. The net force due to friction is
acting against the direction of the motion of the car. It
causes the car to decelerate
(iii) to change its direction
-when a footballer kicks a fast moving ball towards him, the
ball bounces off and moves in other direction. The
unbalanced force causes the ball to change its direction.

26. • The motion of a body due to an unbalanced force is closely
related to Newton’s second law of motion
• the acceleration of moving body is related to the net force
applied on it and the mass of the body

27. Discussion group.
Two cars with different direction move with
zero acceleration. Suddenly, at the arc of
highway, this two car hit together. In your
group, discuss what effects of balanced and
unbalanced forces occur before, during and
after the collision?

28. Initially, two cars with different direction move with zero
acceleration.
Suddenly, at the arc of highway, this two car hit together.
In your group, discuss what effects of balanced and
unbalanced
forces occur before, during and after the collision?
Balanced force Unbalanced force
1. The cars move 1. Change of state.
with constant ( from move to stop)
velocity. 2. Change of shape.
( in front of the car
damaged)
3. Change of velocity
(decelerate)

29. RELATIONSHIP BETWEEN FORCE, MASS AND ACCELERATION
F 2F F
m m 2m
m
(a) (b) (c)
(a) When a net force, F acts on a mass, m it causes an accelerate, a
(b)When the force, F on the same mass is doubled, its acceleration
also doubled, 2a
(c) The same force applied to twice of the mass results is only half of
the acceleration, ½ a

30. • Newton’s second law of motion states
the acceleration produced by a net force on an object is directly
proportional to the magnitude of the net force applied and is
inversely proportional to the mass of the object. The direction
of the acceleration is the same as that of the net force
• Combining the relation ship, a ∞ F and a ∞ 1 , we get

31. (i) The graph of a against F and
(ii) The graph of a against m

32. Example:
• A wooden block of mass 3 kg is pulled along a table with a
constant velocity by a force of 6 N as in Figure 1(a). If the pulling
force is increased to 15 N as in Figure (b), what is
(a) the resultant force
6N
(b) the acceleration
(a)
Solution:
15N
(a) F = (15 - 6)N = 9N (b)
(b) Acceleration, a = F ÷ m
= 9 N ÷ 3 kg
= 3 N kgˉ¹ @ m sˉ²

33. Its time to do exercise!
Exercise 2.5 the effect of force