Force
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Based on the Newton’s Second Law of motion,
LESSON 5 The effects of a force Definition of force Force is defined as anything that changes the state of rest or motion of an object moving in a straight line.
a
α F
a
α1
we obtain
Force is a vector quantity which has both direction and magnitude
m a
α F m
a =kF m
The S.I. unit for force is N or kg m s-2
F = kma
Force is measured by using a Spring Balance
In S.I. units , the definition of a force of 1 N is given as the amount of force which is applied on a mass of 1 kg and which causes it to experience an acceleration of 1 ms-2 Thus, 1 N = k ( 1 kg ) ( 1 ms-2 ) Thus k=1
The effects of a force A force can • move a stationary object • stop a moving object • accelerate a moving object • decelerate a moving object • change the direction of a moving object • alters the size of the object • alters the shape of the object
Therefore
F = ma Where F = net force m = mass a=
acceleration
NEWTON ‘S LAWS OF MOTION Newton’s First Law : An object will remain at rest or continue with a constant speed in a straight line unless acted on by an unbalanced force. Newton’s Second Law : The acceleration of a body ,a, is directly proportional to the net force acting on it, F , and inversely proportional to its mass, m.
Example 1
Newton’s Third Law : For every action there is an equal and opposite reaction. Two forces act horizontally to a block of wood of mass 4 kg as shown in figure above. What is the acceleration and the direction of motion of the block.
The relationship between the acceleration,a ,the net force ,F and the mass , m. 32
Solution
Example 2 An object of mass 2 kg is pulled on the floor by a force of 5 N and having a constant velocity. (a) What is the frictional force between the object and the floor. (b) Calculate the acceleration of the object if the object is pulled by a 17 N force?
Example 4 When traveling at 120 km h-1 a car has to overcome a drag force due to air resistance of 900 N. If the car has a mass of 1000 kg, determine the average force that the motor needs to apply if it is to accelerate at 5.0 ms2 .
Solution
Solution
Example 3 A bus of mass 2 000 kg travels with a uniform velocity 40 ms-1 for a distance 2500 m before it comes to rest. Calculate (a) the average deceleration of the bus (b) the average force applied by the brakes to bring the bus to a standstill Solution
33
C 3
4
5
Mass of M > Mass of N
Which of the following graph is true to show the relationship between the acceleration,a , the mass , m and the force ,F Which of the following objects acted on by a pair of forces will accelerate?
The figure shows forces , F1 and F2 , exerted on a wooden block placed on a table surface. The friction between the block and the table surface is 2 N.
TUTORIA L5 1
1 N is equivalent to A C
2
B D
1 kg ms-2 1 kg s m-1 Which pair of forces F1 and F2 causes the wooden block to move with an acceleration?
Which of following is not effects of a constant force?
A B C object D object 3
1 kg ms-1 1 ms kg-1
Change the direction of motion Change the shape of an object Change the velocity of a moving
A B C D E
Change the acceleration of a moving
The figure shows the graph of motion for two objects M and N is acted on by a same force.
6
A B
7
Mass of M = Mass of N Mass of M < Mass of N 34
F2/ N
5 3 6 3 4
7 5 4 2 1
An object of mass 0.2 kg is pulled on a smooth floor by a force of 8 N. What is the acceleration of the object ? A C E
Which comparision is correct about the mass of M and N?
F1 / N
20 ms-2 60 ms-2 160 ms-2
B D
40 ms-2 80 ms-2
Figure shows an object of mass 2 kg is pulled on a floor by a force of 15 N. The frictional force between the object and the floor is 3 N.
placed on a table surface. The friction force between the block and the table surface is 2 N.
Two forces P and Q exerted on the wooden block in three difference situations.The magnitude of P and Q are 5 N and 12 N respectively. The following table shows the situations and the resultant force occurred. . Situation
What is the acceleration of the object? A 2 ms-2 B 5 ms-2 -2 C 6 ms D 8 ms-2 -2 E 10 ms 9
A B C
A box with a mass 3 kg is pushed along a rough surface with a force 10 N. If the acceleration of the box is 3 ms-2 , what is the frictional force between the box and the rough surface? A
0.5 N
B
1.0N
C
1.5 N
D
2.0 N
E
Resultant force 17 N 13 N 7N
(a) What is meant by the resultant force? ………………………………………………… ……….. (b) Draw the forces P and Q in figure (a) ,(b) and (c) which produced the resultant force as shown in the table above.
2.5 N
10 A car of mass 1 200 kg travelliling at 20 ms-1 is brought to rest over a distance 40 m. Find the average braking force. A C
5N 2 400 N
E
1 2000 N
B
240 N D 6 000 N
11 A particle of mass 0.02 kg is moving with a velocity 10 ms-1 on a smooth surface . A force 5 N acts on the particle such that its velocity increases to 15 m s-1. For how long will the force act on the particle? A C E
0.01 s 0.1 s 1.0 s
B
(c) Later on the wooden block is pulled by a force of 20 N on a rough table
0.02 s D 0.2 s
surface as shown in figure (d) If the acceleration of the block is 6 ms-2,
12 Figure shows a wooden block of mass 2 kg resting on a frictionless surface. 35
(i) Calculate the magnitude of the friction force between the block and the rough table surface.
(c) If the wooden block is pulled by a 18 N force , calculate (i) the acceleration of the wooden block?
(ii) Suggest two methods to reduce the magnitude of the friction force.
(ii) the velocity of the wooden block after 4 s.
…………………………………………… ….
(d) Two identical wooden blocks B and C is placed on the wooden block A as shown in figure (b).
…………………………………………… …. …………………………………………… …. …………………………………………… …. 13 Figure (a) shows a spring balance which is pulling a wooden block A with a force of magnitude 10 N on a rough surface. The block is moving with a uniform velocity of 2 ms-1.
(i) What is the friction force between the wooden blocks and the rough surface?
(ii) What is the force is needed to move the wooden blocks with an acceleration is same as the acceleration in (c)(i).
(a)What is the friction force between the wooden block A and the rough surface.
(b) (i) What is observed on the reading of the spring balance if a smaller surface area of the wooden block A to touch the rough surface.
14 Figure (a) shows a car is pulling a boat and moving without the boat respectively. Figure (b) shows the car is moving without the boat. Observe the Figure(a) and (b). Compare the mass and the acceleration of the moving objects. Relate the mass and the acceleration of the moving objects to deduce a relevant physics concept.
…………………………………………… …................ (ii) Give a reason for your answer in (b)(i). ………………………………………………… ………. ………………………………………………… ……….
15 36
The motocycle and the lorry stop Figure (a)
The motocycle and the lorry accelerate Figure (b) Each figure below shows a motocycle and a lorry in two situations at a traffic light. Observe the situations of the motocycle and the lorry of each diagram. Based on the observations: State one suitable inference that can be made. (b) State one apporopriate hypothesis for an investigation. (c) With the use of apparatus such as trolley, ticker timer and other apparatus , describe an experimental framework to test your hypothesis. In your description , state clearly the following: (i) Aim of the experiment (ii) Variables in the experiment (iii) List of apparatus and materials (iv) Arrangement of the apparatus (v) The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable. (vi) Way you wouldtabulate the data (vii) Way you would analysis the data (a)
37
LESSON 5 The effects of a force Definition of force Force is defined as anything that changes the state of rest or motion of an object moving in a straight line.
a
α F
a
α1
we obtain
Force is a vector quantity which has both direction and magnitude
m a
α F m
a =kF m
The S.I. unit for force is N or kg m s-2
F = kma
Force is measured by using a Spring Balance
In S.I. units , the definition of a force of 1 N is given as the amount of force which is applied on a mass of 1 kg and which causes it to experience an acceleration of 1 ms-2 Thus, 1 N = k ( 1 kg ) ( 1 ms-2 ) Thus k=1
The effects of a force A force can • move a stationary object • stop a moving object • accelerate a moving object • decelerate a moving object • change the direction of a moving object • alters the size of the object • alters the shape of the object
Therefore
F = ma Where F = net force m = mass a=
acceleration
NEWTON ‘S LAWS OF MOTION Newton’s First Law : An object will remain at rest or continue with a constant speed in a straight line unless acted on by an unbalanced force. Newton’s Second Law : The acceleration of a body ,a, is directly proportional to the net force acting on it, F , and inversely proportional to its mass, m.
Example 1
Newton’s Third Law : For every action there is an equal and opposite reaction. Two forces act horizontally to a block of wood of mass 4 kg as shown in figure above. What is the acceleration and the direction of motion of the block.
The relationship between the acceleration,a ,the net force ,F and the mass , m. 32
Solution
Example 2 An object of mass 2 kg is pulled on the floor by a force of 5 N and having a constant velocity. (a) What is the frictional force between the object and the floor. (b) Calculate the acceleration of the object if the object is pulled by a 17 N force?
Example 4 When traveling at 120 km h-1 a car has to overcome a drag force due to air resistance of 900 N. If the car has a mass of 1000 kg, determine the average force that the motor needs to apply if it is to accelerate at 5.0 ms2 .
Solution
Solution
Example 3 A bus of mass 2 000 kg travels with a uniform velocity 40 ms-1 for a distance 2500 m before it comes to rest. Calculate (a) the average deceleration of the bus (b) the average force applied by the brakes to bring the bus to a standstill Solution
33
C 3
4
5
Mass of M > Mass of N
Which of the following graph is true to show the relationship between the acceleration,a , the mass , m and the force ,F Which of the following objects acted on by a pair of forces will accelerate?
The figure shows forces , F1 and F2 , exerted on a wooden block placed on a table surface. The friction between the block and the table surface is 2 N.
TUTORIA L5 1
1 N is equivalent to A C
2
B D
1 kg ms-2 1 kg s m-1 Which pair of forces F1 and F2 causes the wooden block to move with an acceleration?
Which of following is not effects of a constant force?
A B C object D object 3
1 kg ms-1 1 ms kg-1
Change the direction of motion Change the shape of an object Change the velocity of a moving
A B C D E
Change the acceleration of a moving
The figure shows the graph of motion for two objects M and N is acted on by a same force.
6
A B
7
Mass of M = Mass of N Mass of M < Mass of N 34
F2/ N
5 3 6 3 4
7 5 4 2 1
An object of mass 0.2 kg is pulled on a smooth floor by a force of 8 N. What is the acceleration of the object ? A C E
Which comparision is correct about the mass of M and N?
F1 / N
20 ms-2 60 ms-2 160 ms-2
B D
40 ms-2 80 ms-2
Figure shows an object of mass 2 kg is pulled on a floor by a force of 15 N. The frictional force between the object and the floor is 3 N.
placed on a table surface. The friction force between the block and the table surface is 2 N.
Two forces P and Q exerted on the wooden block in three difference situations.The magnitude of P and Q are 5 N and 12 N respectively. The following table shows the situations and the resultant force occurred. . Situation
What is the acceleration of the object? A 2 ms-2 B 5 ms-2 -2 C 6 ms D 8 ms-2 -2 E 10 ms 9
A B C
A box with a mass 3 kg is pushed along a rough surface with a force 10 N. If the acceleration of the box is 3 ms-2 , what is the frictional force between the box and the rough surface? A
0.5 N
B
1.0N
C
1.5 N
D
2.0 N
E
Resultant force 17 N 13 N 7N
(a) What is meant by the resultant force? ………………………………………………… ……….. (b) Draw the forces P and Q in figure (a) ,(b) and (c) which produced the resultant force as shown in the table above.
2.5 N
10 A car of mass 1 200 kg travelliling at 20 ms-1 is brought to rest over a distance 40 m. Find the average braking force. A C
5N 2 400 N
E
1 2000 N
B
240 N D 6 000 N
11 A particle of mass 0.02 kg is moving with a velocity 10 ms-1 on a smooth surface . A force 5 N acts on the particle such that its velocity increases to 15 m s-1. For how long will the force act on the particle? A C E
0.01 s 0.1 s 1.0 s
B
(c) Later on the wooden block is pulled by a force of 20 N on a rough table
0.02 s D 0.2 s
surface as shown in figure (d) If the acceleration of the block is 6 ms-2,
12 Figure shows a wooden block of mass 2 kg resting on a frictionless surface. 35
(i) Calculate the magnitude of the friction force between the block and the rough table surface.
(c) If the wooden block is pulled by a 18 N force , calculate (i) the acceleration of the wooden block?
(ii) Suggest two methods to reduce the magnitude of the friction force.
(ii) the velocity of the wooden block after 4 s.
…………………………………………… ….
(d) Two identical wooden blocks B and C is placed on the wooden block A as shown in figure (b).
…………………………………………… …. …………………………………………… …. …………………………………………… …. 13 Figure (a) shows a spring balance which is pulling a wooden block A with a force of magnitude 10 N on a rough surface. The block is moving with a uniform velocity of 2 ms-1.
(i) What is the friction force between the wooden blocks and the rough surface?
(ii) What is the force is needed to move the wooden blocks with an acceleration is same as the acceleration in (c)(i).
(a)What is the friction force between the wooden block A and the rough surface.
(b) (i) What is observed on the reading of the spring balance if a smaller surface area of the wooden block A to touch the rough surface.
14 Figure (a) shows a car is pulling a boat and moving without the boat respectively. Figure (b) shows the car is moving without the boat. Observe the Figure(a) and (b). Compare the mass and the acceleration of the moving objects. Relate the mass and the acceleration of the moving objects to deduce a relevant physics concept.
…………………………………………… …................ (ii) Give a reason for your answer in (b)(i). ………………………………………………… ………. ………………………………………………… ……….
15 36
The motocycle and the lorry stop Figure (a)
The motocycle and the lorry accelerate Figure (b) Each figure below shows a motocycle and a lorry in two situations at a traffic light. Observe the situations of the motocycle and the lorry of each diagram. Based on the observations: State one suitable inference that can be made. (b) State one apporopriate hypothesis for an investigation. (c) With the use of apparatus such as trolley, ticker timer and other apparatus , describe an experimental framework to test your hypothesis. In your description , state clearly the following: (i) Aim of the experiment (ii) Variables in the experiment (iii) List of apparatus and materials (iv) Arrangement of the apparatus (v) The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable. (vi) Way you wouldtabulate the data (vii) Way you would analysis the data (a)
37
