lesson 5

111EquationChapter (Next) Section 1

Understanding the effect of force

Learning outcomes1. describe the effects of balances forces acting on an object2. describe the effects of unbalanced forces acting on an object

3. determine the relationship between force, mass and acceleration i.e.

4. solve problems using

Force and its effects

1. The action of a force can cause(a) a stationary object to move(b) a moving object to change its speed(c) a moving object to change its direction of motion(d) an object to change in size and shape

2. Force is a vector quantity. It has magnitude and direction.

Effects of balanced forces acting on objects

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.

4. Figure below shows a flower pot hanging at rest

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Lesson 5

A

B

(a) The weight of the pot, W is balanced by the upward force of the tension, T of the rope.(b) The net force acting on the flower pot is zero.

5. Figure below shows a bird perching (rest) on a branch of a tree

(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

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(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.

Effects of unbalanced forces acting on objects

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.

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C

(b) The resultant force, F causes the bicycle to slow down or decelerate.

Balanced force Unbalanced forceForces in opposite direction cancel out each other

Forces in opposite directions do not cancel out each other.

Net force // Resultant force = zero Net force // Resultant force ≠ zeroObject are

- at rest- moving with constant velocity

Objects are- accelerating- decelerating

Acceleration = zero Acceleration ≠ zero

Relationship between Force, Mass and Acceleration

1. Newton’s 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 Newton’s second law,

Combining both

Or

where k is a constant3. By defining 1 Newton (1 N) as the unit force that causes an object with mass of 1 kg to accelerate

at .

Therefore,

Hence,

where m is constant where is constant

Newton’s Third Law of Motion

1. Newton’s third law of motion states that every action has a reaction that is same in magnitude but opposite in direction.

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D

E

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.

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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,

are acting on the car.

(a) Name the four forces, .(b) Write down the relationship between the magnitudes of

(i)

(ii)

3. The figure below shows two objects on two surfaces.

Find the acceleration, a for both objects.

4. The figure below shows the forces acting on an object.

If the frictional force, is 10 N, find the value of force, F.

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5. The figure below shows forces acting on an object of mass, m.

Calculate the value of m.

6. A stationary car of mass 800 kg accelerates from rest to a velocity of 15 m s-1 in 30 s.

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

. If the mass of the particle is 200 g, what is the magnitude of the force?

10. A car is moving with constant velocity of . When the brake is applied, the car stops in 4 seconds. If the mass of the car is 960 kg, what is the magnitude of the force applied by the brake of the car?

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?

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12. Determine the net force that is required to produce an acceleration of on a particle of mass 500 g.

13. An object accelerates with on a smooth surface when it is being pushed by a force of 10 N. What is the mass of the object?

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

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 C

B D

20. Which of the following is a situation where the forces are balanced?

A A car moving at constant accelerationB A tennis player hitting a stationary tennis ballC A rock falling from a cliffD 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 C ShapeB Size 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.

Minimum resultant force Maximum resultant forceA 1 N 11 NB 2 N 13 N

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C 3 N 14 ND 3 N 15 N

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Past year question

1. SBP 2008 Paper 1 Question 8Which of the following is not the effect of a force that is applied to moving toy car?A The toy car speeds upB The toy car change directionC The toy car stoppedD The mass of the toy car decreases

2. SBP 2009 Paper 1 Question 8Diagram 2 shows an aeroplane flying horizontally with increasing velocity.

Which of the following statements is correct?A Weight > LiftB Lift > WeightC Drag = ThrustD Thrust > Drag

3. SBP 2009 Paper 1 Question 9Three horizontal forces act on a car that is moving along a straight level road.

Which combination of forces would result in the car moving at constant velocity?

Air resistance Friction Forward thrustA 200 N 1000 N 800 NB 800 N 1000 N 200 NC 800 N 200 N 1000 ND 1000 N 200 N 800 N

4. SBP 2012 Paper 1 Question 7Which situation produced zero net force?

A C

B D

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5. SBP 2012 Paper 1 Question 8

Which of the following produces the largest acceleration when forces are applied on a block?

A C

B D

6. MRSM 2013 Paper 1 Question 8

Diagram 6 shows an aeroplane flying at constant altitude and velocity.

Which physics concept explains the above situation?

A Resultant force B Resolution of forces C Forces in equilibrium

7. SBP 2014 Paper 1 Question 7Diagram 7 shows Jamal drives a bike at a constant speed of 25 m s-1. The two forces resisting motion are air resistance of 30 N and frictional force of 80 N.

Determine the thrust force of the engine.

A 50 N B 55 N C 85 N D 110 N

8. SPM 2007 Paper 1 Question 7Diagram 8 shows a car with mass of 1000 kg moving with an acceleration of 2 m s-2. There is a frictional force of 900 N acting on the car.

What is the force exerted by the engine of the car?

A 900 N B 1100 N C 2000 N D 2900 N

9. SPM 2008 Paper 1 Question 7

are forces with the same magnitude acting on a wooden block.Which diagram shows the maximum resultant force acting on the wooden block?

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A C

B D

10. SPM 2009 Paper 1 Question 9Which diagram shows the forces in equilibrium?

A C

B D


Diagram 11 shows a toy car of mass 4 kg. The car is pushed forward by a force of 10 N. The frictional force between the toy car and the floor is 2 N.

What is the acceleration of the toy car?

A 20 m s-2 B 12 m s-2 C 5 m s-2 D 2 m s-2


Diagram 12 shows a stationary object when forces are applied.

Which statement is correct?

A B C D


Which diagram shows the unbalanced forces acting on an object?

A A car stops at a junction C An aeroplane take off the runaway

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B A train moves with constant velocity D A submarine stay stationary at a certain depth

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