Lesson 5
Lesson 5
Lesson 5
Chapter (Next)
Section 1
Lesson 5
Learning outcomes
1. describe the effects of balances forces acting on an object
2. describe the effects of unbalanced forces acting on an object
3. determine the relationship between force, mass and acceleration i.e.
F ma
4. solve problems using
A
F ma
1. When the forces acting on an object are balanced, they cancel each other out.
2. The net force acting on the object is equal to zero.
3. The object is either at rest or moving at a constant velocity.
(a) The weight of the pot, W is balanced by the upward force of the tension, T of the rope.
1
(a) The force of gravity, W (its weight) pulls it downward and the reaction of the branch, R pushes
the bird upward to balance the weight.
(b) The net or total force acting on the bird is zero.
6. Figure below shows a golf buggy moving at constant velocity in a golf course.
(a) The engine force, T which drives the buggy forward, has the same magnitude but acts in the
opposite direction with the air resistance and frictional forces, F. The net horizontal force
acting on the golf buggy is zero.
(b) The weight of the buggy, W is balanced by the reaction, R of the ground on the buggy. The net
vertical force on the golf buggy is zero.
7. Figure below shows an aircraft flying at constant velocity
(a) The lift force, L from its wings is equal in magnitude but acts in the opposite direction to its
weight, W. The net vertical force on the aircraft is zero.
(b) The forward thrust of the engine, T is equal but acts in the opposite direction to the drag or air
resistance, F acting on the aircraft. The net horizontal force acting on the aircraft is zero.
1. When the forces acting on an object do not cancel out each other, a net force acts on the object.
2. The forces are known as unbalanced forces and the net forces is known as resultant force.
3. Figure below shows a boy uses force to lift a box from the floor.
(a) The lifting force has a greater magnitude than the weight of the box.
(b) The resultant force, F causes the stationary box to accelerate upward.
4. Figure below shows a lorry accelerating forward.
(a) The forward thrust of the engine of a lorry is greater in magnitude than the frictional forces acting
on the lorry.
(b) The resultant force, F causes the lorry to accelerate forward.
5. Figure below shows a cyclist slowing down (deceleration) when sees a cat in front of him.
(a) The cyclist presses the brakes hard causing the magnitude of the total frictional forces to be bigger
than forward force.
(b) The resultant force, F causes the bicycle to slow down or decelerate.
Balanced force
Forces in opposite direction cancel out each other
Net force // Resultant force = zero
Object are
- at rest
- moving with constant velocity
Acceleration = zero
D
Unbalanced force
Forces in opposite directions do not cancel out
each other.
Net force // Resultant force zero
Objects are
- accelerating
- decelerating
Acceleration zero
1. Newtons second law of motion states that when a net external force is acting on a body, the
acceleration of the body is directly proportional to the magnitude of the force and is inversely
proportional to its mass.
2. From Newtons second law,
a Fnet
a
1
m
Combining both
F
a net
m
Or
Fnet kma
where k is a constant
3. By defining 1 Newton (1 N) as the unit force that causes an object with mass of 1 kg to accelerate at
1 m s 2
.
1 k (1)(1)
k 1
Therefore,
F ma
Hence,
a Fnet
where m is constant
E
where
Fnet
1
m
is constant
1. Newtons third law of motion states that every action has a reaction that is same in magnitude but
opposite in direction.
2. For example, a swimmer pushes the water backward with his hands and less with a force, F, while the
reaction force of the water, -F pushes the swimmer forward.
3. For example, a fighter jet taking off from an airbase. The action of the exhaust gas rushing out from
the nozzle with a force, F causes a forward reaction force, -F that moves the jet forward.
Try it yourself
1. For each of the situations in diagrams below, analyse and state whether the forces acting on the body
are balanced or unbalanced.
(a)
(c)
(b
)
(d)
2. Diagram below shows a car travelling on level road with a uniform acceleration. Four forces,
F1 , F2 , F3 and F4
are acting on the car.
F1 , F2 , F3 and F4
(a) Name the four forces,
.
(b) Write down the relationship between the magnitudes of
F1 and F2
(i)
F3 and F4
(ii)
FR
If the frictional force between the car and the surface of the road is 100 N, calculate
(a) the acceleration of the car
(b) the resultant force acting on the car
(c) the engine thrust (forward force) of the car.
7. The figure below shows an object with a mass of 6 kg moves at constant velocity when it is pulled by
a horizontal force of 4 N on a level surface.
What is the acceleration of the object if the object is pulled with a force of 22 N?
8. The diagram below shows a car of mass 1500 kg towing a caravan of mass 1000 kg. The frictional
force between the car and the road is 500 N and the frictional force between the caravan and the road
is 300 N. Both the car and the caravan have an acceleration of 2 m s-2.
Calculate
(a) the pulling force, F, of the car,
(b) the tension, T , in the coupling between the car and the caravan.
9. When a force acts on a stationary particle for 5 s, the speed of the particle increases from rest to
8 m s 1
. If the mass of the particle is 200 g, what is the magnitude of the force?
24 m s 1
11. A resultant force of 3 N is acting on a particle of mass 2 kg. What is the acceleration of the particle
due to this force?
5 m s 2
12. Determine the net force that is required to produce an acceleration of
on a particle of mass
500 g.
4 m s 2
14. When a horizontal force of 6 N is applied to a wooden block of mass 2 kg on a horizontal surface, the
wooden block moves with constant velocity. The horizontal force is then increased to 20 N. What
would be the resultant acceleration?
15. Two masses, P and Q are placed on a smooth horizontal surface. P moves with an acceleration of
4 m s 2
12 m s 2
when a force of 7 N is acting on it, while Q moves with an acceleration of
when the
same force is acting on it. If P and Q are stick together, what would be the acceleration when a force
of 7 N is acting on it?
16. A rocket is moving upwards with an acceleration of 6g m s-2 (g = acceleration due to gravity). If the
mass of astronaut is 60 kg, what is the force acting on the seat by the astronaut?
17. A rocket of mass 104 kg is moving vertically upwards with an acceleration of 10 m s -2. What is the
upwards force acting on the rocket? Given that the acceleration due to gravity is 10 m s-2.
18. Diagram shows a fully loaded trolley which has a mass of 24 kg. To move the trolley, a person pushes
the trolley with a force of 50 N. The trolley accelerates with an acceleration of 1.5 m s -2. What is the
frictional force acting on trolley?
19. Which pair of forces acting on the identical objects of the same mass causes the highest acceleration?
A
20. Which of the following is a situation where the forces are balanced?
A A car moving at constant acceleration
B A tennis player hitting a stationary tennis ball
C A rock falling from a cliff
D A lift travelling upwards with constant velocity.
21. Which physical property of an object will not change when a force is applied on it?
A Acceleration
B Size
C Shape
D Mass
22. Three forces of magnitude 4 N, 5 N and 6 N respectively are acting on an object in the horizontal
direction. Calculate the minimum and maximum possible magnitudes of the resultant of these three
forces.
A
B
C
D
3N
3N
14 N
15 N
Which combination of forces would result in the car moving at constant velocity?
A
B
C
D
Air resistance
200 N
800 N
800 N
1000 N
Friction
1000 N
1000 N
200 N
200 N
Forward thrust
800 N
200 N
1000 N
800 N
Which of the following produces the largest acceleration when forces are applied on a block?
A
Resolution of forces
C Forces in equilibrium
B 55 N
C 85 N
110 N
B 1100 N
C 2000 N
2900 N
10
B 12 m s-2
C 5 m s-2
D 2 m s-2
F1 , F2 , F3 and F4
are applied.
F1 F2
F1 F2
F3 F4
F4 F3
11
12