Solution of Force and Law of Motion (Page No - 57) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Question 51:
A plastic ball and a clay ball of equal masses, travelling in the same direction with equal speeds, strike against a vertical wall. From which ball does the wall receive a greater amount of momentum ?
Solution :
The wall will receive equal momentum from both the balls because both balls have equal mass and velocity.

Question 52:
A moving bicycle comes to rest after sometime if we stop pedalling it. But Newton’s first law of motion says that a moving body should continue to move for ever, unless some external force acts on it. How do you explain the bicycle case ?
Solution :
In this case, the bicycle has been compelled to change its state of motion by the external force of air resistance and friction. If there were no air resistance and no friction to oppose the motion of the bicycle, then according to the first law of motion, the bicycle would go on moving forever.

Question 53:
A man throws a ball weighing 500 g vertically upwards with a speed of 10 m/s.

1. What will be its initial momentum ?
2. What would be its momentum at the highest point of its flight ?

Solution :
Mass of ball = 500 g = 0.5 kg
Initial velocity = 10 m/s

1. Initial momentum = 0.5 x 10 = 5 kg.m/s
2. Velocity at the highest point = 0 m/s

Momentum at the highest point = 0.5 x 0 = 0 kg.m/s

Question 54:
A car is moving on a level road. If the driver turns off the engine of the car, the car’s speed decreases gradually and ultimately it comes to a stop. A student says that two forces act on the car which bring it to a stop. What could these forces be ? Which of these two forces contributes more to slow down and stop the car ?
Solution :
The two forces acting on the car are force of friction and air resistance. Force of friction contributes more to slow down and stop the car.

Question 55:
There are two types of forces X and Y. The forces belonging to type X can produce motion in a stationary object but cannot change the shape of the object. On the other hand, forces belonging to type Y cannot produce motion in a stationary object but can change the shape of the object. What is the general name of the forces such as (a) X, and (b) Y ?
Solution :
a) X are unbalanced force.
b) Y are balanced forces.