Class 9 Physics: Force & Law of Motions of Solutions exercise and Intext Questions

In-Text Questions Solved

NCERT Textbook for Class 9 Science – Page 118

Question 1. Which of the following has more inertia:

(a) A rubber ball and a stone of the same size?

answer: A stone of the same size

(b) A bicycle and a train?

answer: A train

(c) A five-rupees coin and a one-rupee coin?

answer: A five-rupees coin

Explanation: As the mass of an object is a measure of its inertia, objects with more mass have more inertia.

Question 2. In the following example, try to identify the number of times the velocity of the ball changes.

answer:

“A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team.”

Answer: The velocity of the football changed four times.

Explanation: Each kick involves a change in velocity. The agents supplying the force are the football players and the goalkeeper.

Question 3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

answer: When the branch of a tree is shaken vigorously, the branch attains motion but the leaves tend to remain at rest due to inertia of rest. This inertia causes some leaves to detach and fall.

Question 4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

answer: When a bus brakes to a stop, our lower body comes to rest while our upper body continues to move forward due to inertia of motion, causing us to fall forward.

When a bus accelerates from rest, our lower body is pushed backward due to the bus's motion while our upper body tends to remain at rest due to inertia of rest, causing us to fall backwards.

Class 9 Science NCERT Textbook – Page 126-127

Question 1. If action is always equal to the reaction, explain how a horse can pull a cart?

answer: According to Newton's third law of motion, for every action, there is an equal and opposite reaction. When a horse pulls a cart, the horse exerts a forward force on the ground with its feet. In response, the ground exerts an equal and opposite force on the horse's feet. This force from the ground propels the horse forward, allowing it to pull the cart.

Question 2. Explain, why is it difficult for a fireman to hold a hose, which ejects a large amount of water at a high velocity.

answer: When water is ejected from a hose with a high velocity, it carries a large momentum. According to Newton's third law, the hose experiences an equal and opposite force backward. This recoil force makes it difficult for a fireman to hold the hose steady.

Question 3. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m/s. Calculate the initial recoil velocity of the rifle.

answer: To calculate the initial recoil velocity of the rifle, apply the principle of conservation of momentum.

Given: Mass of rifle, m1 = 4 kg

Mass of bullet, m2 = 50 g = 0.05 kg

Initial velocity of bullet, u2 = 35 m/s

Let the initial velocity of rifle, u1 = -v (negative sign indicates opposite direction)

Using conservation of momentum:

m1 * u1 + m2 * u2 = 0

4 * (-v) + 0.05 * 35 = 0

-4v + 1.75 = 0

-4v = -1.75

v = 0.4375 m/s

Therefore, the initial recoil velocity of the rifle is 0.4375 m/s in the opposite direction of the bullet.

Question 4. Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 m/s and 1 m/s respectively. They collide and after the collision the first object moves at a velocity of 1.67 m/s. Determine the velocity of the second object.

answer: To find the velocity of the second object after collision, we can use the principle of conservation of momentum.

Given:

Mass of first object, m1 = 100 g = 0.1 kg

Velocity of first object before collision, u1 = 2 m/s

Velocity of first object after collision, v1 = 1.67 m/s

Mass of second object, m2 = 200 g = 0.2 kg

Let velocity of second object before collision, u2 = v2 (since it's moving in the same direction)

Using conservation of momentum:

m1 * u1 + m2 * u2 = m1 * v1 + m2 * v2

0.1 * 2 + 0.2 * u2 = 0.1 * 1.67 + 0.2 * v2

0.2 + 0.2 * u2 = 0.167 + 0.2 * v2

0.2 * u2 - 0.2 * v2 = 0.167 - 0.2

0.2 * (u2 - v2) = -0.033

u2 - v2 = -0.165

v2 = u2 + 0.165

v2 = 2 + 0.165

v2 = 2.165 m/s

Therefore, the velocity of the second object after collision is 2.165 m/s.

Class 9 Physics Exercises: Force and law of Motion

1. An object experiences a net zero external unbalanced force. Is it possible for the object to be traveling with a non-zero velocity? If yes, state the conditions that must be placed on.

answer: Yes, it is possible for an object to travel with a non-zero velocity even if the net external unbalanced force is zero.

Therefore, for the object to maintain a non-zero velocity:

• The object could be moving with a constant velocity due to balanced forces.
• If initially moving with a non-zero velocity, no external force acts to change its state of motion.

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

answer: Beating a carpet with a stick causes vibrations that dislodge dust particles trapped in the carpet fibers. The force of beating overcomes static electricity or adhesive forces holding the dust, allowing it to come loose and be expelled.

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

answer: Luggage on the roof of a bus is exposed to external forces like wind and sudden movements. Tying it with a rope prevents the luggage from sliding or being blown off, which could cause accidents or damage.

4. 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.

answer: (c) There is a force on the ball opposing the motion. Friction between the ball and the ground acts opposite to the direction of motion, gradually reducing its kinetic energy until it stops.

5. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 tonnes (Hint: 1 tonne = 1000 kg.)

6. A stone of 1 kg is thrown with a velocity of 20 m/s across the frozen surface of a lake and comes to rest after traveling a distance of 50 m. What is the force of friction between the stone and the ice?

answer: Given data:

• Mass of the stone, m = 1 kg
• Initial velocity of the stone, u = 20 m/s
• Distance traveled before coming to rest, s = 50 m
• Final velocity (since the stone comes to rest), v = 0

First, calculate the acceleration (a) of the stone using the equation of motion:

2.Determine the coefficient of kinetic friction (𝜇) between the stone and the ice. This value is not provided directly, but typically, for ice and stone, the coefficient of kinetic friction can be estimated to be around μ=0.02.

Therefore, the force of friction between the stone and the ice is approximately 0.196 N. This frictional force acts in the direction opposite to the motion of the stone and is responsible for bringing it to rest over the distance traveled.

7. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate: (a) the net accelerating force and
(b) the acceleration of the train.

8. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 m/s²?

9. What is the momentum of an object of mass m, moving with a velocity v? - (a) (mv)2 (b) mv2 (c) ½ mv2 (d)mv

answer:(d) Momentum (p) = mass × velocity = m × v

10. Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?

answer:

11. According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.

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12. A hockey ball of mass 200 g travelling at 10 m s–1 is struck by a hockey stick so as to return it along its original path with a velocity at 5 m s–1. Calculate the magnitude of change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.

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13. A bullet of mass 10 g travelling horizontally with a velocity of 150 m s–1 strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.

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14. An object of mass 1 kg travelling in a straight line with a velocity of 10 m s–1 collides with, and sticks to, a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.

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15. An object of mass 100 kg is accelerated uniformly from a velocity of 5 m s–1 to 8 m s–1 in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.

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16. Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.

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17. How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be 10 m s–2.

answer: