Laws of Motion Chapter Science Class 9 CSE Board

Laws of Motion | Chapter 3 | Science | Class 9 | ICSE Board

Laws of Motion

Q1. What is force?

Ans: A force is that physical cause that changes (or tends to change) either the size or the shape or the state of rest or of motion of a body.

Q2. What are the effects of force on an object?


a) Force can make a body that is at rest to move.

b) It can stop a moving body or slow it down.

c) It can accelerate the speed of a moving body.

d) It can also change the direction of a moving body along with its shape and size.

Q3. What are contact forces? Give example.

Ans: The force which acts on the body when they are in physical contact is called contact forces.

Eg: friction forces, normal reaction forces, tension forces applied through strings, the force exerted by a spring, the force experienced by a collision. etc

see also: MCQ on Linear Equations in two variables

Q3. What are non contact forces? Give example.

Ans: A non-contact force is any force applied to an object by another body without any contact. For example, magnetic force, gravitational force and electrostatic force.

Q4. What is gravitational force?

Ans: It is the force of attraction between any two bodies.

Q5. What is electrostatic force?

Ans: The electrostatic force is an attractive and repulsive force between particles caused due to their electric charges. It is a noncontact force.

Q6. What is Magnetic Force?

Ans: Magnetic Force is a force that attracts certain metal objects (like iron and iron filings) towards a magnet.

Q7. Classify the following forces amongst contact and noncontact forces :

a) gravitational force

Ans: Noncontact Force

b) magnetic force

Ans: Noncontact Force

c) frictional force :

Ans: contact Force

d) normal reaction force :

Ans: contact Force

e) force of tension in a rope :

Ans: contact Force

f) electrostatic force :

Ans: Non contact Force

Q8. Define the Normal Reaction force.

Ans: A force that is applied by a surface to counterbalance the weight force when the object is kept on a surface. 

Q9. Give one example in each case

a) the force is of contact

Ans: frictional force, normal force, tension force.

b) force is at a distance

Ans: Electrostatic force, gravitational force

Q10. State one factor on which the magnitude of a non-contact force depends. How does it depend on the factor stated by you?

Ans: The magnitude of the non-contact force depends on the distance. Distance and magnitude of force are inversely related. The magnitude of force decreases as the distance increases.

Q11. A force is applied to a non-rigid body and a rigid body. How does the effect of the force differ in the above two cases?

Ans: A force, when applied to a rigid object, does not change the inter-spacing between its constituent particles and therefore it does not change the dimensions of the object but causes motion in it. On the other hand, a force, when applied to a non-rigid object, changes the inter-spacing between its constituent particles and therefore causes a change in its dimensions.

Q12. Give one example in each of the following cases where a force —

(a) Stops a moving body

Ans: A fielder on the ground stops a moving ball by applying force with his hands.

(b) Moves a stationary body

Ans: A ball lying on the ground moves when it is kicked.

c) Changes the size of a body

Ans: By loading a spring hanging from rigid support, the length of the spring increases.

(d) Changes the shape of a body

Ans: On pressing a piece of rubber, its shape changes.

Q13. State Newton’s first law of motion.

Ans: Newton’s First Law of Motion states that a body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it.

Q14. State Newton’s second law of motion.

Ans: Newton’s Second Law of Motion states that the force acting on a body is directly proportional to the rate of change of linear momentum of the body and the change in momentum takes place in the direction of the force.

Q15. State Newton’s third law of motion.

Ans: Newton’s Third Law of Motion states that for every action, there is an equal and opposite reaction. They always act on two different bodies.

Q16. Name the scientist who proved for the first time that objects move with constant speed when no force acts on them.

Ans: Galileo

Q17. Why do bicycles begin to slow down when we stop pedaling?

Ans: This is because of the frictional forces acting opposite to the direction of motion.

Q18. Which law of motion gives the measure of force?

Ans: Newton’s second law of motion.

Q19. Define the force of friction.

Ans: The force acting between any two surfaces in contact and tending to oppose motion is called force of friction.

Q20. A ball moving on a tabletop eventually stops. Explain the reason.

Ans: Force of friction between the ball and the table top opposes the motion, as a result of which the ball stops.

Q21. what is Galileo’s Law of Inertia?

Ans: Galileo’s law of inertia is as stated below: “An object, if once set in motion, moves with uniform velocity if no force acts on it.”

Q22. Give two examples to show that the greater the mass, the greater is the inertia of the body.

Ans: a) It is easy to push or pull the cycle than the truck, because the mass of the truck is much greater than the mass of the cycle, hence inertia is more for the truck than the cycle.

(b) It is easy to carry cotton than iron because the inertia of cotton is less than the inertia of iron.

Q23. Two equal and opposite forces act on a moving object. How is its motion affected? Give reason.

Ans: Its motion will not be affected because these forces are equal and opposite and cancel each other.

Q24. What is Inertia of Rest?

Ans: The ability of a body to maintain its state of rest is known as the inertia of rest.

Example: When we shake the branch of a tree, the leave or the fruits fall down because the branches come in motion whereas the leaves or the fruits tend to remain at rest and hence get detached.

Q25. What is Inertia of Motion?

Ans: The ability of a body to maintain its state of uniform motion is known as the inertia of motion.

Example: An athlete runs a certain distance before taking a long jump so that he can cover a long distance.

Q26. Give a qualitative definition of force on the basis of newton’s first law of motion.

Ans: According to Newton’s first law, force is an external factor that can change the state of rest or state of uniform motion of an object”.

Q27. Why does a ball thrown vertically upwards in a moving train, come back to the thrower’s hand?

Ans: The ball gains the same horizontal velocity as that of the train. Hence, it moves the same distance as the train in the horizontal direction and it falls back to the thrower’s hand.

Laws of Motion | Chapter 3 | Science | Class 9 | ICSE Board

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Q28. Why does a person fall when he jumps out from a moving train? 

Ans: If a person jumps out of a moving train and tries to stop immediately, he falls due to inertia of motion. This is because his body tends to move forward with the velocity of the train  while his feet are stationary.

Q29. 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.

Ans: If the net force is not zero the plane would be moving upward or downward. Since its height is constant the net force is zero.

Q30. What is the force on a body of mass m when it is moving with a constant velocity ?

Ans: zero

Q31. Explain the following : 
When a train suddenly moves forward, the passenger standing in the compartment tends to fall backwards.

Ans: It is because the lower part of the passenger’s body is in close contact with train. When the train starts moving, the lower part started to move at once, but the upper part try to maintain its state of rest due to inertia. So the passenger inside the compartment fall backward.

Q32. Explain the following : 
Dust particles are removed from a carpet by beating it.

Ans: The section of the carpet which comes in contact with the carpet in motion at once, but the dust particles present on it, due to rest, stay in the state of rest.
Consequently, the section of the carpet moves along the stick leaving dust particles behind which tend to fall down to the pull of gravity.

Q33. 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.

Ans: It is because if they were to halt at once, their feet would suddenly come to rest but their upper body would still continue to stay in its state of motion and hence they tend to fall forward.

Q34. Explain the following : 
When a corridor train suddenly starts, the sliding  doors of same compartments may open.

Ans: It is because the frame of sliding door is in contact with the floor of the train also comes in motion when the train starts but due to inertia, the sliding door remains in its position.

Q35. Explain the following : 
People often shake branches of a tree for getting down its fruits.

Ans: It is because when the branches are shaken they are motion but the fruits remain in the state of rest due to inertia. Hence the fruits that are weakly attached to the branches, detach and fail due to the pull of gravity.

Q36. Explain the following : 
It is advantageous to run before taking a long jump.

Ans: It is advantageous to run before taking a long jump as the athlete sets his body in the state of motion hence it becomes to take the jump.

Q37. Why does a coin placed on a card, drop into the tumbler when the card is flicked with the finger?

Ans: A momentary force acts on the card when it is flicked with fingers which causes it to move away. But the coin placed on it is bot does not share the motion at once and continues to stay at rest which is attributed to inertia of rest. Eventually, the coin falls down into the tumbler due to the gravitational pull.

Q38. Your hands are wet, and the restroom towel dispenser is empty. What do you do to get drops of water off your hands? How does the motion of the drops exemplify one of Newtons laws? Which one?

Ans: When the hands are shaken, there is a large acceleration of the surfaces of the hands. If the water drops were to stay on the hands, they must accelerate along with the hands. The only force that can provide this acceleration is the friction force between the water and the hands. (There are adhesive forces also, but let’s not worry about those.) The static friction force is not large enough to keep the water stationary with respect to the skin at this large acceleration. Therefore, the water breaks free and slides along the skin surface. Eventually, the water reaches the end of a finger and then slides off into the air. This is an example of Newton’s first law in action in that the drops continue in motion while the hand is stopped.

Q39. Under what condition does a force take the form F=ma?

Ans: When the mass of the body is constant.

Q40. Define linear momentum and state its S.I. unit.

Answer: Linear momentum of a body can be defined as the product of its mass and velocity. The S.I. unit of linear momentum is kg m/s. It is a vector quantity.

Q41. What information do you get from Newton’s second law of motion?

Ans: Newton’s second law of motion provides the quantitative value of force. We could relate the physical quantity force to the other measurable quantities such as mass and acceleration.

Q42. Name the physical quantity which is considered to be a measure of the quantity of motion of a body.
Ans: Momentum is the measure of the quantity of motion of a body

Q43. State whether momentum is scalar or vector.
Momentum is a vector quantity and is directed along the direction of velocity.

Q44. What is the total momentum of the bullet and the gun before firing?
The total momentum of the bullet and the gun before firing would be zero because the velocities of both of them will be zero.

Q45. What is the usual name of the forces which cannot produce motion in a body but only change its shape?
Balanced forces cannot produce motion in a body but can its shape

Q46. Explain why, it is easier to stop a tennis ball than a cricket ball moving with the same speed.
Since the speed of tennis ball and cricket ball is same, the momentum of cricket ball will be higher due to its mass being greater than mass of tennis ball. So, less force is required to stop a tennis ball than to stop a cricket ball.

Q47. Explain how, a karate player can break a pile of tiles with a single blow of his hand.
A karate player can break a pile of tiles with a single blow because he strikes the pile with his hand very fast. In doing so, the large momentum of his hand is reduced to zero in a very short time. This exerts a large force on the pile of tiles which is sufficient to break them apart.

Q48. Give one example each where :
(a) a force moves a stationary body.
(b) a force stops a moving body.
(c) a force changes the speed of a moving body.
(d) a force changes the direction of a moving body.
(e) a force changes the shape (and size) of a body.
a) Kicking a stationary football.
b) Applying brakes to a moving bicycle.
c) Pressing an accelerator to speed up a moving car.
d) A moving cricket ball hit by a bat.
e) Flattening of dough by a rolling pin to make chapatis.

Q49. A car is moving on a level road. If the driver turns off the engine of the car, the car’s speed decreases gradually and ultimately it comes to a stop. A student says that two forces act on the car which bring it to a stop. What could these forces be ? Which of these two forces contributes more to slow down and stop the car?
The two forces acting on the car are force of friction and air resistance. Force of friction contributes more to slow down and stop the car.

Q50. Why an athlete land on sand after taking a long jump?

Answer: Athlete is made to land on the sand after long jump so as to increase the time of impact. This reduces the momentum and force by which he is landing on the ground. Thus, athlete is saved from getting injured. 

Q51. State the usefulness of Newton’s third law of motion.

Ans: Newton’s third law of motion states that for every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. Forces always exist in pairs.

This can be used in propulsion of rockets in free space by ejecting gases out of it in backward direction.

Q52. State and explain the law of action and reaction, by giving two examples.

Ans: When a man fires a bullet from a gun, a force F is exerted on the bullet (action), and the gun experiences an equal and opposite recoil (reaction) and hence gets recoiled.

When a man exerts a force (action) on the boat by stepping into it, its force of reaction makes him step out of the boat, and the boat tends to leave the shore due to the force exerted by the man (i.e. action).

Q53. To move a boat ahead in water, the boatman has to push th ewater backwards by his oar. Explain.

Ans: Water should be pushed backward with oars because every action has an equal and opposite reaction. The more force you will apply to the water backward, the boat will move forward with an equal force.

Q54. Explain the motion of a rocket with the help of Newton’s third law

Ans:Newton’s third law of motion states that to every action there is an equal and opposite reaction. Similarly, when a rocket moves, it exerts the action force on the gases to expel them backwards which in turn exerts an equal and opposite reaction force to move the rocket forward.

Q55. Why does a gun recoil when a bullet is fired?

Ans: According to Newton’s Third Law of motion, every action has an equal and opposite reaction

When a gun fires a bullet, the gun puts a force on the bullet that propels it forward.

The bullet likewise exerts an equal and opposing force on the gun in the backward direction.

Q56. When you step ashore from a stationary boat, it tends to leave the shore. Explain.

Ans: When we step ashore from a rowing boat, it tends to leave the ashore because a force is exerted by us to step ashore and there is equal and opposite reaction on the boat. Thus, it tends to leave the shore.

Q57. When two spring balances joined at their free ends, are pulled apart, both show the same reading Explain.

Ans: Couple two spring balances A and B as shown in the figure. When we pull the balance B, both the balances show the same reading indicating that both the action and reaction forces are equal and opposite. In this case, the pull of either of the two spring balances can be regarded as action and that of the other balance as the reaction.


Q58. A person pushing a wall hard is liable to fall back. Explain.

Ans: If a person applies a force F on the wall, wall applies the same force on the person in opposite direction. Since wall is very massive as compared to the person, it doesn’t move, but the person is likely to fall back because of the reaction force.

Q59. ‘The action and reaction both act simultaneously.’ Is this statement true and what is the net force ?

Ans: Yes, the statement is true.

Action and reaction, both act simultaneously on different objects and in opposite directions. As soon as action force is removed, reaction force also ends.

Q60. The action and reaction are equal in magnitude’ is this statement true?

Ans: yes


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