## 03 June 2011

### ncert solution Force and Law of motio for class 9

Force and Laws of Motion (Science IX)

Which of the following has more inertia:  (a) a rubber ball and a stone of the same size?
(b) a bicycle and a train?  (c) a five-rupees coin and a one-rupee coin?

Inertia of an object id directly related the mass of the body.  The greater is the mass of the body; the greater is its inertia and vice-versa.

(a) Mass of a stone is more than the mass of a rubber ball for the same size.
Hence, inertia of the stone is greater than that of a rubber ball.

(b) Mass of a train is more than the mass of a bicycle.
Hence, inertia of the train is greater than that of the bicycle.

(c) Mass of a five rupee coin is more than that of a one-rupee coin. Hence, inertia of the five rupee coin is greater than that of the one-rupee coin.

In the following example, try to identify the number of times the velocity of the ball changes:
“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”.   Also identify the agent supplying the force in each case.

The velocity of the ball changes four times.
When a football player kicks the football, its speed changes from zero to a certain value. In this case, force applied by player helps to change the velocity of the ball. This at first  changes the velocity of the ball.
Another player kicks the ball towards the goal post. This changes the direction of the ball . Therefore, its velocity also changes. In this case, the player applied a force to change the velocity of the ball. The goalkeeper collects the ball. In other words, the ball comes to rest. Thus, its speed reduces to zero from a certain value. The velocity of the ball has changed 2nd time

The goalkeeper applied an opposite force to stop/change the velocity of the ball. Hence, its velocity changes third time.

The goalkeeper kicks the ball towards his team players. Hence, the speed of the ball increases from zero to a certain value. Hence, its velocity changes once again. In this case, the goalkeeper applied a force to change the velocity of the ball.

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

when we vigorously shake the branches of a tree , it moves a tree to and fro, but its leaves try to remain at rest and fall down. In this case the inertia of the leaves tend to resist the to and fro motion of tree .

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: In a moving bus, a passenger moves with the bus due to inertia of motion. As the driver applies brakes, the bus comes to rest. But, the passenger tries to maintain to inertia of motion. As a result, a forward force is exerted on him.

Similarly, the passenger tends to fall backwards when the bus accelerates from rest. This is because when the bus accelerates, the inertia of rest of the passenger tends to oppose the forward motion of the bus. Hence, the passenger tends to fall backwards when the bus accelerates forward.

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

A horse pushes the ground with foot in the backward direction. According to Newton’s third law of motion, a reaction force is exerted by the Earth on the horse in the forward direction. Hence  the cart moves forward.

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

It is difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity due to the backward force applied by ejected water

When a fireman holds a hose, which is ejecting large amounts of water at a high velocity, then a reaction force is exerted on him by the ejecting water in the backward direction. This is because of Newton’s third law of motion. As a result of the backward force, the stability of the fireman decreases. Hence, it is difficult for him to remain stable while holding the hose.

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.

Yes. Even when an object experiences a net zero external unbalanced force, it is possible that the object is traveling 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 due to inertia of motion.

To change the state of motion, a net non-zero external unbalanced force must be applied on the object.

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

When a carpet is beaten with a stick, then the carpet comes to motion. But, the dust particles try to resist their inertia of rest and the dust particles come out of the carpet.

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

When the bus accelerates and moves in forward direction. The luggage kept on the roof try to retain its state of rest and ultimately falls from the roof of the bus and vice versa. Hence, to avoid this situation , it is advised to tie any luggage kept on the roof of a bus with a rope.

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.

(c) A batsman hits a cricket ball, which then rolls on a level ground.there is frictional force on the ball opposing its motion.Hence, after covering a short distance, the ball comes to rest
You remember that frictional force always acts in the direction opposite to the direction of motion of object.
Two objects, each of mass 1.5 kg are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 m s−1 before the collision during which they stick together. What will be the velocity of the combined object after collision?

Mass of one of the objects, m1 = 1.5 kg
Mass of the other object, m2 = 1.5 kg
Velocity of m1 before collision, v1 = 2.5 m/s
Velocity of m2, moving in opposite direction before collision, v2 = −2.5 m/s
(Negative sign arises because mass m2 is moving in an opposite direction)
After collision, the two objects stick together.then Total mass  = m1 + m2
Velocity of the combined object = v
According to the law of conservation of momentum:
Total momentum before collision = Total momentum after collision
m1v1 + m2 v1 = (m1 + m2) v
1.5(2.5) + 1.5 (−2.5) = (1.5 + 1.5) v
3.75 − 3.75 = 3 v
v = 0
Hence, the velocity of the combined object after collision is 0 m/s.and they comes into rest.