NCERT Class 9 Science Solutions: Chapter 9 – Force and Laws of Motion Part 2

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Q 4 Image Showing Relative Velocities.

Q 4 Image Showing Relative Velocities

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Question 4

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

Answer:

Mass of one of the objects,

Mass of the other object,

Velocity of before collision,

Velocity of before collision,

Velocity of after collision,

Velocity of after collision

According to the law of conservation of momentum:

Total momentum before collision Total momentum after collision

Hence, the velocity of the second object becomes 1.165 m/s after the collision.

Question 1

Q. 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 condition that must be placed on the magnitude and direction of the velocity. If no, provide a reason.

Answer:

Yes. Even when an object experiences a net zero external unbalanced force, it is possible that the object is travelling with a non-zero velocity. This is possible only when the object has been moving with a constant velocity in a particular direction. Then, there is no net unbalanced force applied on the body. The object will keep moving with a non-zero velocity. To change the state of motion, a net non-zero external unbalanced force must be applied on the object.

Question 2

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

Answer:

Inertia of an object tends to resist any change in its state of rest or state of motion. When a carpet is beaten with a stick, then the carpet comes to motion. But, the dust particles try to resist in a state of rest, According to Newton’s first law of motion, the dust particles stay in a state of rest, while the carpet moves. Hence, the dust particles come out of the carpet.

Question 3

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

Answer:

When the bus accelerates and moves forward, it acquires a state of motion. However, the luggage kept on the roof, owing to its inertia, tends to remain in its state of rest. Hence, with the forward movement of the bus, the luggage tends to remain at its original position and ultimately falls from the roof of the bus. To avoid this, it is advised to tie any luggage kept on the roof of a bus with a rope.

Question 4

Q. 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.

Answer:

A batsman hits a cricket ball, which then rolls on a level ground. After covering a short distance, the ball comes to rest because there is frictional force on the ball opposing its motion.

Frictional force always acts in the direction opposite to the direction of motion. Hence, this force is responsible for stopping the cricket ball.

Question 5

Q. 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 metric tonnes (Hint: 1 metric tonne=1000 kg).

Answer:

Initial velocity, (since the truck is initially at rest)

Distance travelled,

Time taken,

According to the second equation of motion:

Where,

Acceleration

1 metric tonne kg (Given)

Mass of truck, kg

From Newton’s second law of motion:

Force,

Hence, the acceleration of the truck is 2 and the force acting on the truck is 14000 N.