CBSE Class 9 Physics Force And Laws Of Motion Worksheet Set B

Exam Questions NCERT Class 9 Science Chapter 9 Force and Laws of Motion

Question. What is the direction of momentum?
Answer. The direction of momentum is the same as that of velocity.

Question. Which type of force brings an object in motion?
Answer. Unbalanced force.

Question. Define force.
Answer. It is a push or pull on an object that produces acceleration in the body on which it acts.

Question. What is balanced force?
Answer. When forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.

Question. No force is required to move an object with a constant velocity. Why?
Answer. For an object moving with a constant velocity, a = 0,
so F = ma = 0

Question. Why do the passengers fall backward when a bus accelerates from rest?
Answer. The passengers fall backward when a bus accelerates from rest due to their inertia. The passengers tend to remain in their state of rest even when the bus starts moving.

Question. What are the changes possible on an object at rest if we apply on it?
(a) A balanced force?
(b) An unbalanced force?
Answer.
(a) Object may changes its shape or size.
(b) Objects changes its speed, velocity or accelerates.

Question. What happens when you shake a wet piece of cloth? Explain your observation.
Answer. When a wet piece of cloth is shaken, small droplets of water fall down. This is because in the beginning both water and the piece of cloth were at rest. When he cloth is moved, the water in it tends to remain in the state of rest due to inertia of rest. As a result, the droplets of water fall down and the clothes dry quickly.

Question. State all 3 Newton’s law of motion.
Answer. Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force. Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. Newton’s III law of
motion : To every action, there is
an equal and opposite reaction and they act on two different bodies.


Question. Write the effect of force.
or
What change will be in a body by force?
Answer. Force can produce three effects :
(a) It can change the magnitude of velocity of an object (i.e. to make the object move faster or slower).
(b) It can change the direction of motion of an object.
(c) It can change the shape and size of an object.

Question. What do you understand by the gravitational unit of force?
Answer.A gravitational unit of force is defined as that force which produces an acceleration equal to the acceleration due to gravity in a body of one unit mass. Gram-weight and kilogram-weight are the gravitational unit of force.

Question.Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupee coin and a one-rupee coin?
Answer.
Since inertia is dependent on the mass of the object, the object with the greater mass will hold greater inertia. The following objects hold greater inertia because of their mass.
Stone
Train
Five-Rupee coin

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.
Let the two football teams be team A and team B respectively. Initially, the football is at rest. Now, the football is kicked by a player on team A to another player of team A. (The velocity of the ball has changed 1 time so far). This change is brought on by the force applied by the player who kicked the ball.
The football is now kicked by the other team A player towards the goal. (The velocity of the ball has changed 2 times so far). This change is also brought on due to the force applied by the team A player who kicked the ball.
The goalkeeper of team B stops the ball, bringing it to rest. (the velocity of the ball has changed 3 times so far). This change is brought on by the force applied by the goalkeeper of team B to stop the ball.
Finally, the goalkeeper of team B kicks the ball towards another player of team B. the velocity of the ball changes for a final time, bringing the total count to 4. This change is also brought on by the goalkeeper of team B, who applies the force to kick the ball forward.
Therefore, the velocity of the ball changes 4 times in this example.

Question. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Answer.
When the branch of the tree is shaken, the branch moves in a to-and-fro motion. However, the inertia of the leaves in attached to the branch resists the motion of the branch. Therefore, the leaves that are weakly attached to the branch fall off due to inertia whereas the leaves that are firmly attached to the branch remain attached.

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.
Initially, when the bus accelerates in a forward direction from a state of rest, the passengers experience a force exerted on them in the backward direction due to their inertia opposing the forward motion.
Once the bus starts moving, the passengers are in a state of motion in the forward direction. When the brakes are applied, the bus moves towards a position of rest. Now, a force in the forward direction is applied on the passengers because their inertia resists the change in the motion of the bus. This causes the passengers to fall forwards when the brakes are applied.

Question. If action is always equal to the reaction, explain how a horse can pull a cart.
Answer.
When the horse walks in the forward direction (with the cart attached to it), it exerts a force in the backward direction on the Earth. An equal force in the opposite direction (forward direction) is applied on the horse by the Earth. This force moves the horse and the cart forward.
The velocity at which the horse can move by applying a force on the earth depends on the mass of the horse (and the cart attached to it). The heavier the cart, the slower the motion of the horse (for a given amount of force applied by the horse on the Earth). If the cart is too heavy, the force exerted by the horse on the Earth will be insufficient to even overcome the force of inertia. In this case, the horse will not be able to pull the cart.

Question. Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
Answer.
For the hose to eject water at high velocities, a force must be applied on the water (which is usually done with the help of a pump or a motor). Now, the water applies an equal and opposite force on the hose. For the fireman to hold this hose, he must apply a force on it to overcome the force applied on the hose by the water. The higher the quantity and velocity of the water coming out of the hose, the greater the force that must be applied by the fireman to hold it steady.

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.
Given, the Bullet’s mass (m₁) = 50 g
The rifle’s mass (m₂) = 4kg = 4000g
Initial velocity of the fired bullet (v₁) = 35 m/s
Let the recoil velocity be v₂.
Since the rifle was initially at rest, the initial momentum of the rifle = 0
Total momentum of the rifle and bullet after firing = m₁v₁ + m₂v₂
As per the law of conservation of momentum, the total momentum of the rifle and the bullet after firing = 0 (same as initial momentum)
Therefore, m₁v₁ + m₂v₂ = 0

Therefore, the recoil velocity of the rifle is 0.4375 meters per second in the direction opposite to the trajectory of the bullet (backward direction).

Question. Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 ms^–1 and 1 ms^–1, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 ms^–1. Determine the velocity of the second object.
Answer.
Assuming that the first object is object A and the second one is object B, it is given that:
Mass of A (m₁) = 100g
Mass of B (m₂) = 200g
Initial velocity of A (u₁) = 2 m/s
Initial velocity of B (u₂) = 1 m/s
Final velocity of A (v₁) = 1.67 m/s
Final velocity of B (v₂) =?
Total initial momentum = Initial momentum of A + initial momentum of B
= m₁u₁ + m₂u₂
= (100g) × (2m/s) + (200g) × (1m/s) = 400 g.m.sec^-1
As per the law of conservation of momentum, the total momentum before collision must be equal to the total momentum post collision.

v₂ = 1.165 m/s
Therefore, the velocity of object B after the collision is 1.165 meters per second.

More Question

1. Give an example in each case where:
(a) Force causes a motion in stationary object.
(b) Force stops a moving body.
(c) Force changes the direction of a moving body.
(d) Force changes the speed of a moving body.
(e) Force changes the shape of a moving body.
2. Distinguish between balanced and unbalanced forces.
3. Give two examples to show that greater the mass, greater is inertia of a body.
4. Give reasons:
(a) A passenger in a bus tends to fall backward when it starts suddenly.
(b) A bullet fired against a glass window pane makes a hole in it without cracking it.
(c) An athlete runs a certain distance before taking a long jump.
5. What is the relationship between: (a) acceleration and mass of a body (b) force and acceleration.
6. Give reasons:
(a) A karate player can break a pile of tiles in a single blow.
(b) A car driver prefers to hit something soft (say hay stock) than a wall if his car goes out of control while driving.
(c) Shockers are provided in vehicles.
7. Two bodies A & B of the same mass are moving with velocities v and 3v respectively. Compare their (a) inertia (b) Momentum (c) the force needed to stop them in the same time.
8. Explain with reasons:
a. When a shot is fired from a gun the gun recoils.
b. A jet aeroplane releases a lot of hot gases before taking off.
c. We press the ground while walking forward.
d. A rubber ball rebounds when struck against a hard floor.
9. Why do we get hurt by falling on a concrete structure than on a sand track?
10. You are hurt when you kick a stone. Why?
11. Explain why a bicycle stops if we stop pedalling.
12. If we take out a piece of paper from under a book with a jerk, the book will not move. Explain.
13. A force acts on a body of mass m1 and produce an acceleration a1. The same force when acting on a mass m2 produces acceleration a2. Define the relation between m1, m2, a1, a2.
14. Could a body of mass ma have a weight equal to zero?
15. If Newton’s 3rd law of motion is written in the form F21 = F12; what is the mistake?

Students must free download and practice these worksheets to gain more marks in exams. CBSE Class 9 Physics Worksheet - Force And Laws Of Motion