9th Standard Chapter 8 NCERT Solution with Answers

Force and Laws of Motion - Class 9 Exercises

1.

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 object.

Answer: Yes, if the object is moving with constant velocity in a straight line without any unbalanced force acting on it, it will continue moving with that velocity as per Newton's First Law.

2.

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

Answer: The carpet moves when beaten, but the dust tends to remain at rest due to inertia and separates out from the carpet.

3.

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

Answer: To prevent the luggage from moving forward and falling off due to inertia if the bus stops suddenly.

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).

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).

Answer: Using s = ut + 0.5at^2:
400 = 0.5 * a * 400
a = 2 m/s².
Mass = 7000 kg, Force = 7000 * 2 = 14000 N.

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 travelling a distance of 50 m. What is the force of friction between the stone and the ice?

Answer: Using v² = u² + 2as:
0 = 400 + 2 * a * 50 → a = -4 m/s².
Force = 1 * -4 = -4 N (opposite direction).

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.

Answer: Total mass = 8000 + 10000 = 18000 kg.
Net force = 40000 - 5000 = 35000 N.
Acceleration = 35000 / 18000 = 1.94 m/s².

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?

Answer: Force = 1500 * -1.7 = -2550 N (opposite to motion).

9.

What is the momentum of an object of mass m, moving with a velocity u?

• (a) (mu)²
• (b) mu²
• (c) ½ mu²
• (d) mu

Answer: (d) mu

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: Friction force = Applied force = 200 N (as velocity is constant).

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.

Answer: The student is wrong. Action-reaction forces act on different objects, so they don't cancel. The truck does not move because the force applied is not enough to overcome friction.

12.

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

Answer: Mass = 0.2 kg, initial velocity = 10 m/s, final velocity = -5 m/s.
Change in momentum = 0.2 * (-5 - 10) = -3 kg m/s.
Magnitude = 3 kg m/s.

13.

A bullet of mass 10 g travelling horizontally with a velocity of 150 m/s 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.

Answer: Mass = 0.01 kg, u = 150 m/s, v = 0, t = 0.03 s.
a = (0 - 150)/0.03 = -5000 m/s².
s = 150 * 0.03 + 0.5 * (-5000) * (0.03)² = 2.25 m.
Force = 0.01 * -5000 = -50 N, magnitude = 50 N.

14.

An object of mass 1 kg travelling in a straight line with a velocity of 10 m/s 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.

Answer: Total momentum before = 1 * 10 + 5 * 0 = 10 kg m/s.
After impact: 6 * v = 10, v = 1.67 m/s.
Total momentum after = 10 kg m/s.

15.

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

Answer: Initial momentum = 100 * 5 = 500 kg m/s.
Final momentum = 100 * 8 = 800 kg m/s.
Change = 300 kg m/s.
Force = 300 / 6 = 50 N.

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.

Answer: According to Newton's Third Law, both exert equal and opposite forces on each other, but due to the insect's small mass, it undergoes a large change in velocity (acceleration), leading to its death, while the car’s velocity changes very little.