CBSE Class 9 Science Force and Laws of Motion Worksheet Set E

Short Answer Type Questions

Question. When a carpet is beaten with a stick, dust comes out of it. Explain.
Answer : Inertia of an object tends to resist any change in its state of rest or state of motion.
When a carpet is beaten with a stick, then the carpet comes to motion. But, the dust particles try to resist their state of rest. According to Newton's first law of motion, the dust particles stay in a state of rest, while the carpet moves. Hence, the dust particles come out of the carpet.

Question. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because
(a) the batsman did not hit the ball hard enough.
(b) velocity is proportional to the force exerted on the ball.
(c) there is a force on the ball opposing the motion.
(d) there is no unbalanced force on the ball, so the ball would want to come to rest.
Answer : (c) A batsman hits a cricket ball, which then rolls on a level ground. After covering a short distance, the ball comes to rest because there is frictional force on the ball opposing its motion.
Frictional force always acts in the direction opposite to the direction of motion.
Hence, this force is responsible for stopping the cricket ball.

Question. 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.
Answer : The velocity of the ball changes four times.
As a football player kicks the football, its speed changes from zero to a certain value. As a result, the velocity of the ball gets changed. In this case, the player applied a force to change the velocity of the ball. Another player kicks the ball towards the goal post. As a result, the direction of the ball gets changed.
Therefore, its velocity also changes.
In this case, the player applied a force to change the velocity of the ball. The goalkeeper collects the ball. In other words, the ball comes to rest. Thus, its speed reduces to zero from a certain value. The velocity of the ball has changed. In this case, the goalkeeper applied an opposite force to stop/change the velocity of the ball. The goalkeeper kicks the ball towards his team players. Hence, the speed of the ball increases from zero to a certain value. Hence, its velocity changes once again. In this case, the goalkeeper applied a force to change the velocity of the ball.

Question. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?
Answer : Due to the inertia of the passenger
Everybody tries to maintain its state of motion or state of rest. If a body is at rest, then it tries to remain at rest. If a body is moving, then it tries to remain in motion.
In a moving bus, a passenger moves with the bus. As the driver applies brakes, the bus comes to rest. But, the passenger tries to maintain his state of motion. As a result, a forward force is exerted on him.
Similarly, the passenger tends to fall backwards when the bus accelerates from rest. This is because when the bus accelerates, the inertia of the passenger tends to oppose the forward motion of the bus. Hence, the passenger tends to fall
backwards when the bus accelerates forward.

Question. If action is always equal to the reaction, explain how a horse can pull a cart.
Answer : A horse pushes the ground in the backward direction. According to Newton's third law of motion, a reaction force is exerted by the Earth on the horse in the forward direction. As a result, the cart moves forward.

Question. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s - '1. Calculate the initial recoil velocity of the rifle.
Answer : Mass of the rifle, m₁ = 4 kg
Mass of the bullet, m₂  = 50g = 0.05 kg
Recoil velocity of the rifle = v₁
Bullet is fired with an initial velocity, v₂ = 35 m/s
Initially, the rifle is at rest.
Thus, its initial velocity, v = 0
Total initial momentum of the rifle and bullet system = (m₁ + m₂ )v = 0
Total momentum of the rifle and bullet system after firing:
= m₁v₁ + m₂v₂ = 4(v₁) + 0.05×35 = 4v₁ + 1.75

According to the law of conservation of momentum:
Total momentum after the firing = Total momentum before the firing
4v₁ + 1.75 = 0
⇒ v₁ = – 1.75/4 = – 0.4375 m/s
The negative sign indicates that the rifle recoils backwards with a velocity of 0.4375 m/s.

 

13 The velocity - time graph of a ball moving on the surface of a floor is shown in figure. Find the force acting on the ball the mass of the ball is 100kg

 

14 A car of mass 200 kg moving at 36 km/h is brought to rest after it covered a distance of 10 m. Find the retarding force acting on the car.

 

15 A vehicle is moving with a velocity of 5 m/s. If the momentum of the vehicle is 5000 kgm/s , what is its mass?

 

16 An object gets 20 s to increase the speed from 10 m/s to 50 m/s .If the mass of the object is 1000 kg what is the force required to do so?

 

17 Car A of mass 1500 kg travelling at 25m/s collides with another car of mass 1000kg traveeliing at 15bm/s in the same direction. After collision the velocity of car A becomes 20 m/s , calculate the velocity of car B after collision.

 

18 Two spherical balls A and B of mass 2kg and 4kg respectively are moving with a velocity 2m/s and 4 m/s respectively. They collide and after collision the second ball moves with velocity of 1.50 m/ s. Find the final velocity of the first ball ( A: 7m/s)

 

19 The acceleration of two objects are 5 m/s2 and 20 m/s2 . If both the objects are combined and a force of 50 N is applied to it calculate the acceleration of the combined mass.

 

20 A mechanic strikes a nail with a hammer of mass 500g moving with a velocity of 20 m/ s . The hammer comes to rest in 0.02 s after striking the nail. Calculate the force exerted by the nail on the hammer.

 

Please click on below link to download CBSE Class 9 Science Force and Laws of Motion Worksheet Set E