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Force And Laws Of Motion Class 9 Science Assignment Pdf
Class 9 Science students should refer to the following printable assignment in Pdf for Force And Laws Of Motion in standard 9. This test paper with questions and answers for Grade 9 Science will be very useful for exams and help you to score good marks
Class 9 Science Assignment for Force And Laws Of Motion
CBSE Class 9 Science Force and Laws of Motion. Students are advised to refer to the attached assignments and practise them regularly. This will help them to identify their weak areas and will help them to score better in examination. Parents should download and give the assignments to their children for practice.
Chapter 9th: Force and Laws of Motion Page No: 118
1. 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-rupees coin and a one-rupee coin?
Answer: Inertia is the measure of the mass of the body. The greater is the mass of the body; the greater is its inertia and vice-versa
a) Mass of a stone is more than the mass of a rubber ball for the same size. Hence, inertia of the stone is greater than that of a rubber ball.
(b) Mass of a train is more than the mass of a bicycle. Hence, inertia of the train is greater than that of the bicycle.
(c) Mass of a five rupee coin is more than that of a one-rupee coin. Hence, inertia of the five rupee coin is greater than that of the one-rupee coin.
2. 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.
The velocity of football changes four times. First, when a football player kicks to another player, second when that player kicks the football to the goalkeeper. Third when the goalkeeper stops the football. Fourth when the goalkeeper kicks the football towards a player of his own team.
Agent supplying the force:
→ First case – First player
→ Second case – Second player
→ Third case – Goalkeeper
→ Fourth case – Goalkeeper
3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Some leaves of a tree get detached when we shake its branches vigorously because branches comes in motion while the leaves tend to remain at rest due to inertia of rest.
Exam Questions NCERT Class 9 Science Chapter 9 Force and Laws of Motion
Question. Give any three examples in daily life which are based on Newton’s third law of motion.
Ans : Three examples based on Newton’s third law are : Swimming : We push the water backward to move forward.
(i) Action – water is pushed behind
(ii) Reaction – water pushes the swimmer ahead Firing gun : A bullet fired from a gun and the gun recoils.
(i) Action – gun exerts force on the bullet
(ii) Reaction – bullet exerts an equal and opposite force on the gun Launching of rocket :
(i) Action – hot gases from the rocket are released
(ii) Reaction – the gases exert upward push to the rocket
Question. How are safety belts helpful in preventing any accidents?
Why do you think it is necessary to fasten your seat belts while travelling in your vehicle?
Ans : While travelling in a moving car, our body remains in the state of rest with respect to the seat. But when driver applies sudden breaks in the car, our body tendsto continue in the same state of motion because of its inertia. Therefore, this sudden break may cause injury to us by impact or collision. Hence, safety belt exerts a force on our body to make the forward motion slower.
Question. If a ball is thrown up in a moving train, it comes back to the person’s hands. Why?
This is because no horizontal force acts on it. It moves with the same horizontal speed.
Question. (a) What is friction?
(b) What are the advantages of friction?
Why is friction necessary?
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :
(i) The nails and screws hold the wooden boards together due to friction.
(ii) It is due to the friction between the ground/road and the soles of our shoes that we are able to walk.
(iii) The friction between the road and the surface of the tyres permits safe driving.
(iv) We are able to write on paper because of the friction between the pen/pencil and the paper.
That is why, it is very difficult to write on a glazed/waxed paper.
(v) The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.
Question. The fruits fall off the branches when a strong wind blows. Give reason.
Ans : Fruits tend to continue in the state of rest on account of inertia while branches suddenly come into motion.
Question. Name the physical quantity which is determined by the rate of change of linear momentum.
Ans : Force.
Question. Two similar trucks are moving with the same velocity on a road. One of them is loaded while the other one is empty. Which of the two will require a larger force to stop it?
Ans : The loaded truck will require a larger force to stop because it has greater momentum.
Question. Name the scientist who introduced the property of momentum.
Ans : Newton.
Question. (i) What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?
Describe some method for reducing friction.
The friction rises due to the following factors :
1. Due to the force of attraction between the molecules of the two surfaces in contact. This iscalled the force of adhesion.
2. Due to the interlocking of the surface irregularities.
3. Thus, friction is due to the roughness of the two surfaces in contact.
4. The friction between two surfaces can be reduced by following methods :
(i) By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction.
(ii) By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.
Question. What is the relation between Newton’s three laws of motion?
Ans : (i) Newton’s first law explains about the unbalanced force required to bring change in the position of the body.
(ii) Second law explains about the amount of force required to produce a given acceleration.
(iii) While Newton’s third law explains how these forces acting on a body are interrelated.
Question. Why does a ball rebound after striking against a floor?
Ans : When a ball strikes against a floor, it exerts a force on the floor. According to Newton’s third law of motion,
the floor exerts an equal and opposite force on the ball. Due to this reaction, the ball rebounds.
Question. How do we swim?
Ans : While swimming, a swimmer pushes the water backward with his hands. The reaction offered by the water to the swimmer pushes him forward.
Question. An iron sphere of mass 10 kg is dropped from a height of 80 cm. If the downward acceleration of the ball is 10 ms–2, calculate the momentum transferred to the ground by the ball.
Here, initial velocity of sphere, u = 0
Distance travelled, s = 80 cm = 0.8 m
Acceleration of sphere, a = 10 ms–2
Step 1. Final velocity of sphere when it just reaches the ground can be calculated using
v2 – u2 = 2as
u2 – 0 = 2 × 10 ms–2 × 0.8 m
= 16 m2s–2
or v = √16 m2s-2 = 4 ms–1.
Momentum of the sphere just before it touches the ground = mv = 10 kg × 4 ms–1 = 40 kg ms–1
Step 2. On reaching the ground, the iron sphere comes to rest, so its final momentum = 0
According to the law of conservation of momentum,
Momentum transferred to the ground = momentum of the sphere just before it comes to rest = 40 kg ms–1.
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