Solution of Force and Law of Motion (Page No - 75 (Part - 2)) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Question 34:
Draw a diagram to show how a rocket engine provides a force to move the rocket upwards. Label the diagram appropriately.
Solution : 

Question 35:
Name the laws involved in the following situations :
(a) the sum of products of masses and velocities of two moving bodies before and after their collision remains the same.
(b) a body of mass 5 kg can be accelerated more easily by a force than another body of mass 50 kg under similar conditions
(c) when person A standing on roller skates pushes another person B (also standing on roller skates) and makes him move to the right side, then the person A himself gets moved to the left side by an equal distance.
(d) if there were no friction and no air resistance, then a moving bicycle would go on moving for ever.
Solution :
a)Law of conservation of momentum
b)Newton’s second law of motion
c) Newton’s third law of motion
d)Newton’s first law of motion

Question 36:
(a) State and explain Newton’s second law of motion.
(b) A 1000 kg vehicle moving with a speed of 20 m/s is brought to rest in a distance of 50 metres :
(i) Find the acceleration.
(ii) Calculate the unbalanced force acting on the vehicle.
Solution :
(a) According to Newton’s second law of motion: The rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction in which the force acts.
Consider a body of mass m having initial velocity u. the initial momentum of this body will be mu. Suppose a force F acts on this body for time t causing the final velocity to be v. The final momentum of the body will be mv. Now the change in momentum is mv < mu and the time taken for this change is t. So according to Newton’s second law of motion,

but

, represents changes in velocity with time i.e acceleration 'a'. so by replacing  with a in the above relation, we get

thus the force acting on a body is directly proportional to the product of mass and acceleration produced in the body by the action of the force.thus newton second law gives the relationship between force and aceeleration.

Question 37:
(a) Explain why, a cricket player moves his hands backwards while catching a fast cricket ball.
(b) A 150 g ball, travelling at 30 m/s, strikes the palm of a player’s hand and is stopped in 0.05 second. Find the force exerted by the ball on the hand.
Solution :
a) A player moves his hands backwards while catching a fast ball because a fast moving ball has a large momentum and in stopping this ball, its momentum has
to be reduced to zero. Now, when a cricket player moves back his hands on catching the fast ball, then the time taken to reduce the momentum of the ball is increased. So, the rate of change of momentum of ball is decreased and hence a small force is exerted on the hands of the player and the hands of the player do not get hurt.
b)Mass of ball = 150 g = 0.15 kg
Initial velocity u = 30 m/s
Final velocity v = 0 m/s
Time t = 0.05 s 

Question 38:
(a) State Newton’s third law of motion and give two examples to illustrate the law.
(b) Explain why, when a fireman directs a powerful stream of water on a fire from a hose pipe, the hosepipe tends to go backward. 

Solution :
a) According to Newton’s third law of motion: Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the first body. In other words, to every action, there is an equal and opposite reaction. Two examples to illustrate this law-
When a man jumps out from a boat, the boat moves backward. This is due to the fact that to step out of the boat, the man presses the boat with his foot in the backward direction. The push of the man on the boat in action. The boat exerts an equal force in the forward direction and since the boat is not fixed and is floating, it moves backward due to the action force exerted by the man.
Gunman gets a jerk on firing a bullet from his gun. This is because when a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backwards but due to high mass of the gun, it moves only a little distance backward giving a jerk to the gunman.
b) When a fireman directs a powerful stream of water on a fire, the hosepipe tends to go backward due to the reaction force of the water rushing through it in the forward direction at a great speed.