# NCERT Class 9 Science Solutions: Chapter 9 – Force and Laws of Motion Part 7

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Question 4

Q. A motorcar of mass 12kg is moving along a straight line with a uniform velocity of 90. Its velocity is slowed down to 18 in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.

**Answer:**

Mass of the motor car,

Initial velocity of the motor car,

Final velocity of the motor car,

Time taken,s

According to the first equation of motion:

Change in momentum

Acceleration of the motor car

Change in momentum of the motor car

Hence, the force required to decrease the velocity is 6000 N.

Question 5

A large truck and car, both moving with a velocity of magnitude v, have a head-on collision and both of them come to a half after that. If the collision lasts for 1 s:

Q. (a) which vehicle experiences the greater force of impact?

**Answer:**

Let the mass of the truck be M and that of the car be m.

Thus,

Initial velocity of both vehicles, v

Final velocity of both vehicles, (Since the vehicles come to rest after collision) Time of impact,

From Newton’s second law of motion, the net force experienced by each vehicle is given by the relation:

Since the mass of the truck is greater than that of the car, it will experience a greater force of impact.

Q. (b) which vehicle experiences the greater change in momentum?

**Answer:**

Let the mass of the truck be M and that of the car be m.

Thus,

Initial velocity of both vehicles, v

Final velocity of both vehicles, (Since the vehicles come to rest after collision) Time of impact,

Initial momentum of the car

Final momentum of the car

Change in momentum

Initial momentum of the truck

Final momentum of the truck

Change in momentum

Since the mass of the truck is greater than that of the car, it will experience a greater change in momentum.

Q. (c) Which vehicle experience the greater acceleration?

**Answer:**

Let the mass of the truck be M and that of the car be m.

Thus,

Initial velocity of both vehicles, v

Final velocity of both vehicles, (Since the vehicles come to rest after collision) Time of impact,

From the first equation of motion, acceleration produced in a system is independent of the mass of the system. The initial velocity, the final velocity, and the time of impact remain the same in both cases. Hence, both the car and the truck experience the same amount of acceleration.

Q. (d) Why is the car likely to suffer more damage than the truck?

**Answer:**

Let the mass of the truck be M and that of the car be m.

Thus,

Initial velocity of both vehicles, v

Final velocity of both vehicles, (Since the vehicles come to rest after collision) Time of impact,

According to Newton’s third law of motion, for every action there is an equal and opposite reaction that acts on different bodies. Since the truck experiences a greater force of impact (action), this larger impact force is also experienced by the car (reaction). Thus, the car is likely to suffer more damage than the truck.