# NCERT Class 9 Science Solutions: Chapter 9 – Force and Laws of Motion Part 4

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Question 9

Q. What is the momentum of an object of mass m. moving with a velocity v?

(a)

(b)

(c)

(d)

**Answer:**

(d)

Mass of the object

Velocity

Momentum

Momentum

Question 10

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

**Answer:**

A force of 200 N is applied in the forward direction. Thus, from Newton’s third law of motion, an equal amount of force will act in the opposite direction. This opposite force is the fictional force exerted on the cabinet. Hence, a frictional force of 200 N is exerted on the cabinet.

Question 11

Q. 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 before the collision during which they stick together. What will be the velocity of the combined object after collision?

**Answer:**

Mass of one of the objects,

Mass of the other object,

Velocity of before collision,

Velocity of moving in opposite direction before collision,

(Negative sign arises because mass is moving in an opposite direction)

After collision, the two objects stick together.

Total mass of the combined object

Velocity of the combined object

According to the law of conservation of momentum:

Total momentum before collision total momentum after collision

Hence, the velocity of the combined object after collision is 0 m/s.

Question 12

Q. 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 truck has a large mass. Therefore, the static friction between the truck and the road is also very high. To move the car, one has to apply a force more than the static friction. Therefore, when someone pushes the truck and the truck does not move, then it can be said that the applied force in one direction is cancelled out by the frictional force of equal amount acting in the opposite direction.

Therefore, the student is right in justifying that the two opposite and equal cancel each other.

Question 13

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

**Answer:**

Mass of the hockey ball,

Hockey ball travels with velocity,

Initial momentum

Hockey ball travels in the opposite direction with velocity,

Final momentum

Change in momentum

Hence, the change in momentum of the hockey ball is 3 kg