Class nine science Chapter 9 NCERT Solution
Chapter 9: Force and Newton’s Laws
Intext Questions (PAGE 118)
1. 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 one rupee coin?
Ans. Mass of a body is the measure of its inertia, i.e., more the mass more is the inertia. Keeping this in mind :
(a) Stone has more inertia.
(b) The train has more inertia.
(c) Five rupees coin has more inertia.
2. 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.
Ans. The velocity of football changes four times.
The first time the velocity changes when the player applies force to kick the ball towards another player of his team.
The second time the velocity changes when the other player kicks the ball towards the goal.
The third time the velocity changes when the goalkeeper collects the ball by applying force in the direction opposite to the direction of the motion of the ball.
The fourth time the velocity changes when the goalkeeper kicks the ball towards the player of his own team by applying force.
3. Explain, why some of the leaves may get detached from a tree if we vigorously shake its branch.
Ans. When the branch is suddenly set in motion, the leaves attached to it tend to continue in their state of rest, on account of inertia of rest. Due to the unbalanced force acting on the leaves, some leaves may get detached from the branch.
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?
Ans. Before the application of the brakes, the bus and the persons in it are in the state of motion. When the brakes are suddenly applied, the bus, as well as our feet in contact with the floor of the bus, then comes to the state of rest, but the rest of our body on account of inertia of motion tends to continue in the state of motion. Thus, we fall in the forward direction.
Conversely, when the bus is in the state of rest, the passengers in it have the inertia of rest. When the bus accelerates suddenly, the body tends to continue in its state of rest and hence is left behind, relative to the position of the bus. Thus, we fall in the backward direction.
Intext Questions (PAGE 126)
1. If action is always equal to the reaction, explain how a horse can pull a cart?
Ans. It is true that when a horse pulls a cart with a certain force, then the cart pulls the horse with the same force and the system of horse and cart should not move. However, if we analyse the problem carefully, then the horse not only applies force on the cart but also pushes the earth backward with his feet. In this action of the horse, the earth reacts back and pushes it in the forward direction. Now, the force applied by the horse on the earth is insufficient to move the earth, but the force applied by the earth is sufficient to make the horse move in the forward direction. It is this unbalanced force applied by the earth, which makes the system of horse and cart to move in the forward direction.
2. Explain, why is it difficult for a fireman to hold a hose, which ejects a large amount of water at a high velocity.
Ans. The above observation can be explained on the basis of the law of conservation of momentum. When the system of the hose and water before ejecting water, the momentum is zero. When the water issues out from the hose with a high velocity, it has momentum in the forward direction. Thus, in order to conserve momentum, the hose tends to move in the backward direction and hence is difficult to hold.
3. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 ms–1. Calculate the initial velocity of the recoil of the gun.
Ans. Initial mass of rifle = 4 kg
Let the initial velocity of recoil of rifle = ν
∴ The initial momentum of the rifle = 4 kg × ν
The initial mass of bullet = 50 g
The initial velocity of bullet = 35 ms–1
∴ Initial momentum of bullet = 0.05 kg × 35 ms–1
By the law of conservation of momentum,
The momentum of the rifle = Momentum of bullet
⇒ 4 kg × ν = 0.05 kg × 35 ms–1
4. Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 m s–1 and 1 m s–1 respectively. They collide and after the collision, the first object moves with a velocity of 1.67 m s–1. Determine the velocity of the second object.
Let the 100 g and 200 g objects be A and B as shown in the diagram.
∴ Initial momentum of A = 100 g × 2 ms–1 = 200 g ms–1
Initial momentum of B = 200 g × 1 ms–1 = 200 g ms–1
∴ Total momentum of A and B before collision = (200 + 200) g ms–1 = 400 g ms–1
Let the velocity of B after collision = ν
∴ The momentum of B after collision = 200 g × ν
Also, the momentum of A after collision = 100 g × 1.67 ms–1 = 167 g ms–1
∴ The total momentum of A and B after collision = 200 g × ν + 167 g m s–1.
Now, by the law of conservation of momentum,
The momentum of A and B after collision = Momentum of A and B before the collision
200 g × ν + 167 g m s–1 = 400 g ms–1