Force and Laws of Motion Class 9 Worksheet Science Chapter 8

Q1: Fill in the blanks

(i) When a running car stops suddenly, the passengers are jerked forward.
(ii) In collisions and explosions, the total Momentum remains constant, provided that no external force acts.
(iii) If there were no unbalanced force of Friction and no air resistance, a moving bicycle would go on moving for ever.
(iv) When a stationary car starts suddenly, the passengers are jerked Backward.
(v) To every action, there is an Equal and opposite reaction.

Q2: The inability of the body to change its state of rest or motion is:
(a) Momentum
(b) Force
(c) Inertia
(d) Acceleration

Ans: Correct Answer is Option (c)

The inability of a body to change its state of rest or motion is called inertia. Inertia is the property of an object that resists changes to its motion, whether it's at rest or moving with a constant velocity.

Q3: The S.I. unit of weight is:
(a) Newton
(b) Newton m
(c) Newton/sec
(d) Newton m/s

Ans: Correct Answer is Option (a)

The S.I. unit of weight is the Newton (N), which is a measure of force. Weight is the force exerted by gravity on an object.

Q4: A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because
(a) the batsman did not hit the ball hard enough.
(b) velocity is proportional to the force exerted on the ball.
(c) there is a force on the ball opposing the motion.
(d) there is no unbalanced force on the ball, so the ball would want to come to rest.

Ans: Correct Answer is Option (c)

The cricket ball comes to rest because there is a force acting on it that opposes its motion, such as friction between the ball and the ground. This opposing force slows the ball down and eventually stops it.

Q5: If the initial velocity is zero then the force acting is :-
(a) Retarding
(b) Acceleration
(c) Both
(d) None.

Ans: Correct Answer is Option (d)

The force acting on an object depends on its acceleration, not just its velocity. If the initial velocity is zero, it doesn't directly tell us whether the force is retarding or accelerating. The nature of the force will depend on whether the object is being pushed forward (accelerating) or slowed down (retarding) after being in motion.

Q6: An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.

Ans: No, it is not possible for the object to be travelling with a non-zero velocity if an object experiences a net zero external unbalanced force since unbalanced forces cannot be equal to zero.

Q7: Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Ans: When a moving bus brakes to a stop, we fall in the forward direction because we are also moving with the speed of the bus due to the inertia of motion and when suddenly it puts brakes, i.e. comes to rest, the lower half of our body also comes to rest but the upper half of our body not being in close contact with the bus is still in the phase of motion so we fall in the forward direction.
When the bus accelerates from rest, we are also at rest, being on the resting seat. As the engine applies force in the forward direction, we fall backwards due to the inertia now.

Q8: Why is the weight of the object more at the poles than at the equator?

Ans: The weight of the object is more at the poles than at the equator because w=weight = mg. Because g at the equator is less because its radius is more, and hence, weight is less and vice–versa for poles.

Q9: Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Ans: Some of the leaves may get detached from a tree if we vigorously shake its branch because some of the leaves, due to the property of inertia, remain at rest while we vigorously shake the branch of the tree. As a result, those leaves detach and fall off.

Q10: Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Ans: It is advised to tie any luggage kept on the roof of a bus with a rope because when bus moves the luggage also gets moving with the velocity same as that of the bus and in the same direction but when bus changes direction or deaccelerates, due to inertia of motion luggage moves in the same direction and may get thrown away from roof of buses.

Q11: Why are the passengers sitting in a moving bus pushed in the forward direction when the bus stops suddenly?

Ans: The passengers sitting in the moving bus are pushed in the procured direction when the bus stops suddenly because of inertia due to rest. The bus comes to rest, so the passenger's feet come to rest, but due to inertia, the upper part of his body is still in motion, and so he falls forward.

Q12: State Newton’s first law of Motion?

Ans: According to Newton’s first law of motion, a body at rest will continue to be at rest and a body in motion will continue to be in motion until and unless it is acted upon by an external force.

Q13: Define force and what are the various types of forces?

Ans: Force is the push or pull which can produce the change in state or shape of the body. The various types of force are:

• Gravitational force
• Electrostatic force
• Electromagnetic force
• Nuclear force

Q14: State Newton’s second law of motion?

Ans: According to Newton’s second law of motion, the rate of change of momentum of a body is equal to the force acting on it, and the change in momentum takes place in the same direction as the force applied.

Q15: When a carpet is beaten with a stick, dust comes out of it. Explain.

Ans: When a carpet is beaten with a stick, dust comes out of it because carpet fibres vibrate in the forward and backward directions as the carpet is beaten, but the loosely bound dust particles, due to inertia, remain at rest, and so they come out.