Force and Laws of Motions For Class 9 Science Extra Question Answer

Q 1. Name the scientist who proved for the first time that objects move with constant speed when no force acts on them.

Ans. Galileo.

Q 2. If an object is not moving, does it mean that no force is acting on it?

Ans. No, it implies that all the forces acting on the body are balanced.

Q 3. A horse continues to apply a force in order to move a cart with a constant speed. Explain why?

Ans. A horse exerts a continuous force to move the cart with a constant velocity to overcome the force of friction between ground and cart.

Q 4. Why are the wheels of vehicles provided with mudguards?

Ans. The rotating wheels of a vehicle throw out mud sticking to it tangentially due to the inertia of direction. The mudguards stop this mud to fall on another vehicle just moving behind the vehicle.

Q 5. Name the physical quantity that measures inertia.

Ans. Mass of body measures its inertia.

Q 6. Why do bicycles begin to slow down when we stop pedalling?

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

Q 7. There are three solids made up of aluminum, steel, and wood, of the same shape and same volume. Which of them would have the highest inertia?

Ans. Steel ball would have the highest inertia as steel is denser than the other two.

Q 8. Action and reaction are equal and opposite but even they do not cancel each other.” Explain, why?

Ans. Two equal and opposite forces can cancel each other if they act on the same body. But action and reaction do not act on the same body. They act on two different bodies. Hence cannot cancel each other.

Q 9. Which law of motion gives the measure of force?

Ans. Newton’s second law of motion.

Q 10. Name the factors on which the momentum of a body depends.

Ans.

  • Mass and
  • The velocity of the body.

Q 11. Define one newton of force.

Ans. Force is said to be 1 newton if it produces an acceleration of 1 rn/s2 in a body of 1 kg.

Q 12. Mention any two effects of force.

Ans.

  • It changes the state of rest or motion of a body.
  • It changes the shape of the body.

Q 13. Write down SI unit of (i) force (ii) momentum.

Ans.

  • newton (N)
  • kg rn/s.

Q 14. Why does an athlete run a certain distance before taking a leap?

Ans. An athlete runs a certain distance before taking a leap so that the inertia of motion of his body at the time of leaping may help him in his muscular efforts.

Q 15. Write the C.G.S unit of force.

Ans. Dyne.

Q 16. Can every force produce motion in every object?

Ans. No.

Q 17. When a force is applied to a body, what are the two essential effects it can produce?

Ans.

  • It can bring about the change in the state of motion of a body or
  • It can deform a body, i.e., it can change its shape.

Q 18. Define 1 newton force.

Ans. 1 newton is the magnitude of force which produces an acceleration of 1 m/s2 in a body of mass 1 kg

Q 19. What do you mean by an impact force?

Ans. The force produced by the impact of a fast moving object on another is called impact force.

Q 20. Define force of friction.

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

Q 21. Define electrostatic force.

Ans. The force exerted by an electrically charged body is called electrostatic force.

Q 22. If the body is found to be accelerated, is the force acting on it balanced or unbalanced?

Ans. Unbalanced.

Q 23. What do balanced forces usually do to a body?

Ans. Balanced forces usually produce a change in the shape of the body.

Q 24. When a body moves on flat surface, will its speed change?

Ans. No.

Q 25. What did Galileo conclude on the basis of his experiments on the motion of objects?

Ans. A body continues to move with the same velocity if no unbalanced force acts on it.

Q 26. What do you mean by a resultant force?

Ans. When two or more forces act on a body simultaneously, then the single force which produces the same effect as produced by all the forces acting together is known as the resultant force.

Q 27. Do action and reaction act on the same body?

Ans . No, action and reaction act on different bodies.

Q 28. Write the SI unit of impulse.

Ans. Ns.

Q 29. What is the total momentum of a bullet and a gun before firing?

Ans. Zero.

Q 30. Name the principle on which a rocket works.

Ans. Newton’s third law of motion

Q 31. Body A is heavier than body Question Which has more inertia?

Ans. A has more inertia.

Q 32. A body is moving with uniform acceleration. Is its momentum constant?

Ans. No

Q 33. Name the physical quantity that corresponds to the rate of change of momentum.

Ans. Force

Q 34. Which principle is involved in the working of a jet plane?

Ans. Newton’s third law of motion.

Q 35. Two balls of the same size of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer. 

Ans. Yes, the balls will start rolling in the direction in which the train was moving. Due to the application of the brakes, the train comes to rest but due to inertia the balls try to remain in motion, therefore, they begin to roll. Since the masses of the balls are not the same, therefore, the inertial forces are not same on both the balls. Thus, the balls will move with different speeds.

Q 36. Why mass is sometimes called coefficient of linear inertia?

Ans. It is easier to pull a lighter body than a heavier body. Therefore, more the mass more will be the inertia. That is why mass is sometimes termed as coefficient of linear inertia.

Q 37. When a force acting on a body has equal and opposite reaction, then why should the body move at all?

Ans. The action and reaction act on different bodies. Therefore, the body moves under the action of the acting force.

Q 38. Does Newton’s third law apply to a system where bodies do not actually touch each other?

Ans. Yes, whenever the bodies are in actual contact or even if there is an interaction between the bodies (e.g., attraction or repulsion between two magnets charges, etc.), Newton’s third law is applicable.

Q 39. What is the ratio of SI units to CGS units of momentum? How do you measure the effect of an impulsive force on the body?

Ans. A ratio of SI units to CGS units of momentum is (kg m/s)/(g cm/s) i.e., 10s. The effect of an impulse force on the body is measured only in terms of impulse.

Q 40. Suppose a ball of mass m is thrown vertically upwards with an initial speed v, its speed decreases continuously till it becomes zero. Therefore, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final momenta of the ball are same. Yet, it is not an example of conservation of momentum. Explain why.

Ans. Law of conservation of momentum is applicable to isolated system (no external force is applied). In this case, the change in velocity is due to the gravitational force of earth.

Q 41. On which factors does friction depend?

Ans. The force of friction is directly proportional to the weight of the body sliding over the surface. The force of friction also depends on the nature of the surfaces in contact.

Q 42. Why can a small mass such as a bullet kill a person when fired from a gun?

Ans. It is so because even if the mass of the bullet is small, it moves out of the gun with a very high velocity, due to which the momentum produced is high (p = mv). This high momentum of the bullet kills a person.

Q 43. Why does a boat tend to leave the shore, when passengers are alighting from it?

Ans. When the passengers alight from the boat, they push the boat in backward direction. As a result, the boat has a tendency to slip back into water. This difficulty is usually overcome by the boatman by tying the boat to some rigid support.

Q 44. Describe our walking in terms of Newton’s third law of motion.

Ans. When we walk on the ground or road, our foot pushes the ground backward (action) and the ground pushes our foot forward (reaction). Thus, the forward reaction exerted by the ground on our foot makes us walk forward.

Q 45. There are three solids made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?

Ans. Steel has the highest inertia. As the mass is a measure of inertia, the ball of same shape and size, having more mass than other balls will have highest inertia. Since steel has greatest density and greatest mass, therefore, it has highest inertia.

Q 46. Why does a cricket player moves his hand backward while catching the ball?

Ans. A fast moving cricket ball has a large momentum. In stopping or catching this ball, its momentum has reduced to be zero. Now, when a cricket player moves back his hands on catching the fast ball, then the time taken to reduce the momentum of ball to zero is increased. Due to more time taken to stop the ball, the rate of change of momentum of ball is decreased and hence a small force is exerted on the hands of player. So, the hands of player do not get hurt.

Q 47. A bullet fired against a glass window pane makes a hole in it, and the glass pane is not cracked. But on the other hand, when a stone strikes the same glass pane, it gets smashed. Why is it so?

Ans. When the bullet strikes the glass pane, the part of the glass pane which comes in contact with the bullet immediately shares the large velocity of bullet and makes a hole, while the remaining part of the glass remains at rest and is therefore not smashed due to inertia of rest.

But when a slow moving stone strikes the same glass pane, the various parts of the glass pane gets enough time to share the velocity of the stone, and the glass is smashed.

Q 48. Two balls of the same size but of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer

Ans. Yes, both the balls will start rolling in the direction opposite to the motion of the train. The speed of two balls will be different as the inertia of the two balls are different.

Q 49. Two identical bullets are fired on by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulders more and why?

Ans. The light rifles will hurt more as the recoil velocity of the light rifle will be greater.

Q 50. Two identical bullets are fired one by a light rifle and the other by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why? [

Ans. According to conservation of momentum, the rifle recoils with same momentum as that of bullet. As momentum = mass X velocity; so light rifle will recoil with larger velocity and hence, will hurt the shoulder more.

Q 51. Water sprinkler used for grass lawns begins to rotate as soon as as the water is supplied. Explain the principle on which it works

Ans. The working of the rotation of sprinkler is based on third law of motion. As the water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play. So the sprinkler starts rotating.

Q 52. When small boy is trying to push a heavy stone, mention various forces acting on the stone.

Ans. The various forces acting on the stone are:’’

  • The gravitational force exerted by the earth which pulls the stone downwards.
  • The force of reaction exerted by the ground on the stone vertically upwards.
  • The force of pushing exerted by the boy.
  • The force of friction exerted by the stone.

Q 53. Newton’s first, second, and third law of motion.

Ans.

Newton’s first law of motion: An object remains in a state of rest or of uniform motion along a straight line unless compelled to change that state by an applied force.

Newton’s second law of motion: The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

Q 54. Describe in brief an activity to illustrate the property of inertia of resource.

Ans.

  • Make a pile of similar carom coins on a table, as shown in figure.
  • Attempt a sharp horizontal hit at the bottom of the pile using another carom coin or the striker. If the hit is strong enough, the bottom coin moves out quickly. Once the lowest coin is removed, the inertia of the other coins makes them ‘fall’ vertically on the table.
  • The inertia of coins tries to maintain its state of rest even when one of the coin moves out.

Q 55. Give few examples of Newton’s third law of motion.

Ans.

  • Jet aeroplanes and rockets work on the principle of third law of motion.
    In this case, the hot gases come out of a nozzle with great force, i.e., action and the rocket moves ‘ with high speed upwards as a reaction.
  • If we fill a balloon with air and hold it with its mouth downwards, then when release the balloon, the air rushes out vertically downwards (action). The balloon moves vertically upwards (reaction).

Q 56. Two friends on roller-skates are standing 5 m apart facing each other. One of them throws a ball of 2 kg towards the other, who catches it. How will this activity affect the position of the two? Explain your answer.

Ans. Separation between them will increase. Initially the momentum of both of them are zero as they are at rest. In order to conserve the momentum the one who throws the ball would move backward. The second will experience a net force after catching the ball and therefore will move backwards that is in the direction of the force.

Q 57. Why does an athlete puts some sands or cushion on the ground while high jumping?

Ans. When a high jumper falls on a soft landing site(such as cushion or a heap of sand), then the jumper takes a longer time to come to stop. The rate of change of momentum of athlete is less due to which a smaller stopping force acts on the athlete. And the athlete does not get hurt. Thus, the cushion or sand, being soft, reduces the athlete’s momentum more gently. If however, a high jumping athlete falls from a height on to hard ground, then his momentum will be reduced to zero in a very short time. The rate of change of momentum will be large due to which a large opposing force will act on the athlete. This can cause serious injuries to the athlete.

Q 58. Describe balanced forces with the help of two examples.

Ans.

If the resultant of various forces acting on a body is zero, the forces are said to be ‘balanced forces’. These forces do not change the speed but usually change the shape of an object.

Examples:

(a) Consider a wooden block lying on a table, the strings tied to its two opposite faces, as shown in the figure.

If we pull at point P, it begins to move towards left. If we pull at point Q, it begins to move . towards right. But if we pull from both the sides with equal force, the block does not move. The two forces have now balanced each other.

(b) In a tug-of-war, the two teams pull the rope with equal effort; the rope is not moved in any direction. This is clearly because the forces exerted by the two teams are equal and opposite and thus get balanced.

Q 59. Why do the driver and the person seated in front seat need a seat belt?

Ans. In a car accident, a fast running car stops suddenly. Due to this the car’s large momentum is reduced to zero in a very short time. The stretchable seat belts tightened by the passengers of the car increases time taken by the passengers to fall forward. Due to longer time, the rate change of momentum of passengers is reduced and hence less stopping force acts on them. So, the passengers may either not get injured at all or may get less injuries. It is obvious that seat belts reduce the passengers’ momentum more gently and hence prevent injuries.

Q 60. Describe Newton’s first law of motion in detail, giving examples.

Ans. According to Newton’s first law of motion, a body at rest or in uniform motion will remain at rest or in uniform motion unless an unbalanced force acts upon it. This law consists of three parts:

  • The first part says that a body at rest continues in its state of rest. For instance, a boy standing in a train falls backward when the train suddenly starts moving forward. This is because when the bus moves, the lower part of his body begins to move along with the train while the upper part of his body continues to remain at rest due to inertia.
  • The second part says that a body in uniform motion continues to move in straight line path with a uniform speed, e.g., when a moving train stops suddenly a person sitting in it falls forward. This is because as the train stops, the lower part of the person’s body comes to rest along with the bus while upper part of his body continues to remain in ^notion due to inertia of motion and thus he falls forward.
  • Third part says that a body moving with a uniform speed in a straight line cannot change its direction of motion by itself. For example, when a bus takes a sharp turn, a person sitting in the bus gets force acting away from the centre of the curved path due to his tendency to move in the original direction.