Physics Chater 3 Class 9 CBSE /ICSE
Chapter 3
Laws of Motion
Class 9 - Concise Physics Selina Solutions
Exercise 3(A)
Question 1
Explain giving two examples each of —
(a) Contact forces, and (b) Non-contact forces.
Answer
(a) Contact Forces — The forces which are applied on bodies by making a physical contact with them, are called contact forces.
Examples of contact force:
- Frictional force — When a body slides (or rolls) over a rough surface, a force starts acting on the body in a direction opposite to the motion of the body, along the surface in contact. This is called frictional force or force of friction.
- Normal reaction force — When a body is placed on a surface, the body exerts a force downwards, equal to it's weight, on the surface, but the body does not move (or fall) because the surface exerts an equal and opposite force on the body normal to the surface which is called normal reaction force.
(b) Non-Contact forces — The forces experienced by bodies even without being physically in touch, are called non-contact forces or forces at a distance.
Examples of non-contact force:
- Gravitational force — In the universe, each particle attracts the other particle due to it's mass. This force of attraction between them is called gravitational force.
- Electrostatic Force — Two like charges repel, while two unlike charges attract each other. The force between the charges is called electrostatic force.
Question 2
Classify the following amongst contact and non-contact forces —
(a) frictional force, (b) normal reaction force, (c) force of tension in a string, (d) gravitational force, (e) electrostatic force, (f) magnetic force.
Answer
(a) Frictional force — Contact force
(b) normal reaction force — Contact force
(c) force of tension in a string — Contact force
(d) gravitational force — Non-contact force
(e) electrostatic force — Non-contact force
(f) magnetic force — Non-contact force
Question 3
Give one example in each case where —
(a) the force is of contact, and
(b) force is at a distance.
Answer
(a) The force of contact — When a body slides over a rough surface, a force starts acting on the body in a direction opposite to the motion of the body, along the surface in contact. This is frictional force and is an example of contact force.
(b) The force at a distance or non contact force — In the universe, each particle attracts the other particle due to it's mass. This force of attraction between them is called gravitational force and it is an example of a non contact force.
Question 4(a)
A ball is hanging by a string from the ceiling of the roof. Draw a neat labelled diagram showing the forces acting on the ball and the string.
Answer
The diagram below shows the forces acting on the ball and the string.
Question 4(b)
A spring is compressed against a rigid wall. Draw a neat labelled diagram showing the forces acting on the spring.
Answer
The diagram below shows the forces acting on the spring when it is compressed against a rigid wall.
Question 4(c)
A wooden block is placed on a table top. Name the forces acting on the block and draw a neat and labelled diagram to show the point of application and direction of these forces.
Answer
The diagram below shows the point of application of force and the direction of these forces when a wooden block is placed on a table top.
The forces acting are —
(i) The block exerts a force ( = weight ) on the table top, downwards.
(ii) The table top exerts an equal reaction force upwards normal to the top of the table.
Question 5
State one factor on which the magnitude of a non-contact force depends. How does it depend on the factor stated by you?
Answer
The magnitude of non - contact forces between two bodies depends on the distance of separation between them.
It decreases with the increase in separation and increases as the separation decreases. It's magnitude varies inversely as the square of distance of separation i.e., on doubling the separation, the force becomes one fourth.
Question 6
The separation between two masses is reduced to half. How is the magnitude of gravitational force between them affected?
Answer
As we know that, force of attraction acting between two bodies is inversely proportional to the square of the distance between them.
Hence, magnitude of gravitational force will become four times.
Question 7
State the effects of a force applied on (i) a non-rigid, and (ii) a rigid body. How does the effect of the force differ in the two cases?
Answer
(i) Non-rigid force — A force when applied on a non-rigid object, changes the inter - spacing between it's constituent particles and therefore causes a change in it's dimensions and can also produce motion in it.
(ii) Rigid-force — A force when applied on a rigid object does not change the inter-spacing between it's constituent particles and therefore it does not change the dimensions of the object, but causes only motion in it.
Question 8
Give one example in each of the following cases where a force —
(a) stops a moving body.
(b) moves a stationary body.
(c) changes the size of a body.
(d) changes the shape of a body.
Answer
The examples are as follows —
(a) Stops a moving body — A fielder on the ground stops a moving ball by applying force with his hands.
(b) Moves a stationary body — A ball lying on the ground moves when it is kicked.
(c) Changes the size of a body — By loading a spring hanging from a rigid support, the length of the spring increases.
(d) Changes the shape of a body — On pressing a piece of rubber, it's shape changes.
Multiple Choice Type
Question 1
Which of the following is a contact force —
- electrostatic force
- gravitational force
- frictional force ✓
- magnetic force
Answer
The forces which are applied on bodies by making a physical contact with them, are called contact forces.
One of the examples of contact force is frictional force.
When a body slides (or rolls ) over a rough surface, a force starts acting on the body in a direction opposite to the motion of the body, along the surface in contact. This is called frictional force or force of friction.
Question 2
The non-contact force is —
- force of reaction
- force due to gravity ✓
- tension in string
- force of the friction
Answer
The forces experienced by bodies even without being physically in touch, are called non contact forces.
One of the examples of non contact force is the force due to gravity.
The earth, because of it's mass attracts all other masses around it. The force on a body due to earth's attraction is called the force of gravity or the weight of the body.
Exercise 3(B)
Question 1
Name the physical quantity which causes motion in a body.
Answer
Force is the physical quantity which causes motion in a body.
Question 2
Is force needed to keep a moving body in motion?
Answer
No, force is not needed to keep a moving body in motion.
If a body is set in motion, it will remain in motion even when the force applied to set the body in motion is withdrawn, provided that there is no other force such as friction etc., to oppose the motion.
Question 3
A ball moving on a table top eventually stops. Explain the reason.
Answer
As the ball moves on the table top, force of friction comes into play and it opposes the motion of the ball. Hence, the ball stops.
A ball moving on a table top stops, as the force of friction between the moving ball and the table top opposes the motion.
Question 4
A ball is moving on a perfectly smooth horizontal surface. If no force is applied on it, will it's speed decrease, increase or remain unchanged?
Answer
According to Newton's first law of motion —
If a body is in a state of rest, it will remain in the state of rest and if it is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it.
Hence, when no force is applied on a ball moving on a perfectly smooth horizontal surface, it's speed will remain unchanged.
Question 5
What is Galileo’s law of inertia?
Answer
According to Galileo’s law of inertia — "An object, if once set in motion, moves with uniform velocity if no force acts on it."
Thus, a body continues to be in a state of rest or in a state of uniform motion unless an external force is applied on it.
Question 6
State Newton's first law of motion.
Answer
According to Newton's first law of motion, if a body is in a state of rest, it will remain in the state of rest and if it is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it.
Question 7
State and explain the law of inertia (or Newton’s first law of motion).
Answer
The law of inertia or Newton’s first law of motion states that —
If a body is in a state of rest, it will remain in the state of rest and if it is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it.
It implies that,
(i) if a body is at rest, it remains at rest unless a force is applied on it.
(ii) If a body is moving, it will continue to move with the same speed in the same direction unless a force is applied on it.
Question 8
What is meant by the term inertia?
Answer
The property of an object by virtue of which it tends to retain its state of rest or of motion is called inertia.
If the object is in the state of rest, it will remain in the state of rest and if it is moving in some direction, it will continue to move with the same speed in the same direction unless an external force is applied on it.
Example — A book lying on a table top will remain placed at it's place unless it is displaced. Similarly, a ball rolling on a horizontal plane keeps on rolling unless the force of friction between the ball and the plane stops it.
Question 9
Give qualitative definition of force on the basis of Newton's first law of motion.
Answer
The qualitative definition of force on the basis of Newton's first law of motion is —
Force is that external cause which tends to change the state of rest or the state of motion of an object.
Example — A book lying on a table gets displaced from it's place when it is pushed.
Question 10
Name the factor on which inertia of a body depends and state how it depends on the factor stated by you.
Answer
The factor on which inertia of a body depends is mass.
More the mass, more is the inertia of the body. Thus, a lighter body has less inertia than a heavier body. In other words, more the mass of a body, more difficult it is to move the body from rest (or to stop the body if it initially in motion).
Hence, mass is a measure of inertia.
Question 11
Give two examples to show that greater the mass, greater is the inertia of the body.
Answer
Below examples illustrate that mass is a measure of inertia i.e., greater the mass, greater is the inertia of the body:
- A cricket ball is more massive then a tennis ball. The cricket ball acquires much smaller velocity than a tennis ball when the two balls are pushed with equal force for the same duration.
In case when they are moving with the same velocity, it is more difficult to stop the cricket ball (which has more mass) in comparison to the tennis ball (which has less mass). - It is difficult (i.e., larger force is required) to set a loaded trolley (which has more mass) in motion than an unloaded trolley (which has less mass). Similarly, it is difficult to stop a loaded trolley than an unloaded one, if both are moving initially with the same speed.
Question 12
'More the mass, more difficult it is to move the body from rest'. Explain this statement by giving an example.
Answer
As we know, more the mass, more is the inertia of the body. So mass is a measure of inertia.
If we take the example of a loaded trolley, then we observe that, it is difficult (i.e., larger force is required) to set a loaded trolley (which has more mass) in motion than an unloaded trolley (which has less mass).
Question 13
Name the two kinds of inertia.
Answer
The two kinds of inertia are —
- Inertia of rest
- Inertia of motion
Question 14
Give one example of each of the following —
(a) Inertia of rest, and (b) inertia of motion.
Answer
(a) Example of inertia of rest — When a train suddenly starts moving forward, the passenger standing in the compartment tends to fall backwards. The reason is that the lower part of the passenger's body is in close contact with the train. As the train starts moving, the person's lower part shares the motion at once. However, the upper part, due to the inertia of rest cannot share the motion simultaneously and so it tends to remain at the same place.
Hence, the lower part of the body moves ahead and the upper part is left behind, so the passenger tends to fall backwards.
(b) Example of inertia of motion — A cyclist riding along a level road does not come to rest immediately after he stops pedalling. The reason is that the bicycle continues to move due to inertia of motion even after the cyclist stops applying the force on the pedal.
Question 15
Two equal and opposite forces act on a stationary body. Will the body move? Give reason to your answer.
Answer
No, the body will not move when two equal and opposite forces act on a stationary body. As the net force on the body is zero, so the body will remain stationary due to inertia of rest.
Question 16
Two equal and opposite forces act on a moving object. How is it's motion affected? Give reason.
Answer
When two equal and opposite forces are acting on a moving object, the motion remains unaffected because the net force on the object is zero.
Question 17
An aeroplane is moving uniformly at a constant height under the action of two forces (i) upward force (lift) and (ii) downward force (weight). What is the net force on the aeroplane.
Answer
When the aeroplane is acted upon by two opposing forces then the net force acting on the aeroplane is zero as the two forces are in opposite direction and they cancel each other.
Question 18
Why does a person fall when he jumps out from a moving train?
Answer
A person falls when he jumps out from a moving train, because inside the train, his whole body was in a state of motion with the train. On jumping out of the moving train, as soon as his feet touch the ground, the lower part of the body comes to rest, while the upper part still remains in motion.
As a result, he falls in the direction of motion of the train and gets hurt.
To avoid falling, as soon as the passenger's feet touch the ground he should start running on the ground in the direction of motion of the train for some distance.
Question 19
Why does a coin placed on a card, drop into the tumbler when the card is rapidly flicked with the finger?
Answer
A coin placed on a card, drop into the tumbler when the card is rapidly flicked with the finger, because when the card is flicked the momentary forces acts on the card, so it moves away. But the coin kept on it does not share the motion at once and it remain at it's place due to inertia of rest. The coin then falls down into the tumbler due to the pull of gravity.
Question 20
Why does a ball thrown vertically upwards in a moving train, come back to the thrower’s hand?
Answer
A ball thrown vertically upwards in a moving train, comes back to the thrower’s hand because when ball was thrown, it was in motion along with the person and the train. It remains in the same state of forward motion even during the time the ball remains in air.
The person, the inside air and the ball all move ahead by the same distance due to inertia and so the ball falls back into his palm on it's return.
Question 21(a)
Explain the following:
When a train suddenly moves forward, the passenger standing in the compartment tends to fall backwards.
Answer
The reason is that the lower part of the passenger's body is in close contact with the train. As the train starts moving, his lower part shares the motion at once, but the upper part due to inertia of rest cannot share the motion simultaneously and so it tends to remain at the same place.
Consequently, the lower part of the body moves ahead and the upper part is left behind, so the passenger tends to fall backwards.
Question 21(b)
Explain the following:
When a corridor train suddenly starts, the sliding doors of some compartments may open.
Answer
The reason is that the frame of sliding door being in contact with the floor of the train also comes in motion on start of train, but the sliding door remains in it's position due to inertia.
Thus, the frame moves ahead with the train while door slides opposite to the direction of motion of the train. Thus, the door may open.
Question 21(c)
Explain the following:
People often shake branches of a tree for getting down its fruits.
Answer
The reason is that when the stem (or branches) of the tree are shaken, they come in motion, while the fruits due to inertia, remain in the state of rest. Thus, the massive and weakly attached fruits get detached from the branches and the fall down due to the pull of gravity.
Question 21(d)
Explain the following:
After alighting from a moving bus, one has to run for some distance in the direction of bus in order to avoid falling.
Answer
One needs to run for some distance in the direction of the bus after alighting from it because inside the bus, his whole body was in a state of motion with the bus. On jumping out of the moving bus, as soon as his feet touch the ground, the lower part still remains in motion due to inertia of motion. As a result he falls in the direction of motion of the bus.
Hence in order to avoid falling, as soon as the passenger's feet touch the ground he should start running on the ground in the direction of motion of the bus for some distance.
Question 21(e)
Explain the following:
Dust particles are removed from a carpet by beating it.
Answer
The reason is that the part of the carpet where the stick strikes, comes in motion at once, while the dust particles settled on it's fur, remain in position due to inertia of rest. Thus, the part of the carpet moves ahead with the stick, leaving behind the dust particles which fall down due to the earth's pull.
Question 21(f)
Explain the following:
It is advantageous to run before taking a long jump.
Answer
It is advantageous to run before taking a long jump because by running a person brings his entire body in the state of motion. When the body is in motion, it becomes easier to take a long jump.
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