# Extra Questions For Class 9 Science Chapter 9 Force and Laws of Motion

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#### Very Short Answer Type Questions (One Mark Each)

Q.1. Give the net force acting on a body at rest.

Answer Ans. Net force acting on a body at rest is zero.

Q.2. State the meaning of balanced force.

Answer Ans. Forces are said to be balanced force if their resultant force is zero.

Q.3. What kind of force is required to accelerate the motion of an object ?

Q.4. State two effects, a balanced force can produce in a nonrigid body fixed at a position.

Answer Ans. The balanced force may change the shape as well as size of the given nonrigid body fixed at a position.

Q.5. Rakesh tries to push a box on a rough floor but is unable to move it. Name the force which balances the force applied by him.

Answer Ans. The force of friction due to rough floor on the box.

Q.6. What is inertia ?

Answer Ans. Inertia of a body is its natural tendency to maintain its state of rest or of uniform motion along a straight line by itself.

Q.7. What is a measure of inertia ? State its SI unit.

Answer Ans. The mass of an object. Its SI unit is a kilogram (kg).

Q.8. State the law of inertia.

Answer Ans. An object continues to be in its state of rest or of uniform linear motion unless acted upon by some external unbalanced force.

Q.9. Raju has three solid blocks of same size and shape made up of steel, wood and plastic (or rubber). Which one of these has highest inertia ? Give reason for your choice.

Answer Ans. The steel block has highest inertia because its mass is maximum.

Q.10. Define momentum and give its SI unit.

Answer Ans. Momentum of an object = Mass x Velocity. Its SI unit is kg m/s

Q.11. If you jerk a piece of paper under a book quick enough, the book does not move. Why?

Answer Ans. On account of its inertia of rest.

Q.12. Define the first law of motion.

Answer Ans. Everybody continues in its state of rest or of uniform motion unless acted upon by an unbalanced external force.

Q.13. Name the property by virtue of which a body resists a change in its state of motion or of rest. Also name the physical quantity which is a measure of this property.

Q.14. A car and a motorbike both are running with the same speed. Which of the two has greater momentum and why?

Answer Ans. Momentum is given by the product of mass and speed. Speed of car and motorbike both is same but mass of car is more. Hence momentum of car is more.

Q.15. A ball is thrown vertically upwards. What is its momentum at the highest point ?

Answer Ans. Zero because velocity of ball is zero at the highest point of its motion.

Q.16. What is the momentum of a body of mass 5 kg moving with a velocity of 0.20 m/s.

Answer Ans. Momentum p = mass (m) x velocity (v)=5 x 0.20 = 1.0 kg m/s.

Q.17. Name the physical quantity which determines the direction of momentum.

Answer Ans. The direction of momentum of an object is same as the direction of its velocity.

Q.18. Name the physical quantity which is determined by the rate of change of momentum of an object.

Answer Ans. The force exerted on the object.

Q.19. State the relation between the momentum of a body and the force acting on it.

Answer Ans. Rate of change of momentum of a body is equal to the external unbalanced force acting on it.

Q.20. What is the relationship between force applied on a body and the resulting acceleration caused in it ?

Answer Ans. Force (F) = Mass (m) x Acceleration (a).

Q.21. Define one newton force.

Answer Ans. Force is said to be 1 N if it produces an acceleration of 1 m/s2 in an object of mass 1 kg.

Q.22. When a motorcar makes a sharp turn at a high speed, in what direction a passenger sitting in it experiences a push ? Why?

Answer Ans. In a radially outward direction due to its directional inertia.

Q.23. When an athlete comes running from a distance, he is able to jump longer. Why?

Answer Ans. He gains momentum which helps him in taking a longer jump.

Q.24. What does a bowler tend to gain during running from a long distance before he bowls from the take off line ?

Answer Ans. The bowler tries to gain momentum so that he can bowl the ball at a faster pace.

Q.25. What would happen if a fielder stops the fast moving ball suddenly ? Justify of ball in a short time and consequently the ball will exert a large force on the hands of fielder,

Answer Ans. If a fielder stops a fast moving ball suddenly, there is a large decrease in momentum due to which he may get his hands injured.

Q.26. A heavy person experiences more sideways push when a turns suddenly. Why?

Answer Ans. A heavy person experiences more sideways push when a moving vehicle turns suddenly because his inertia is more.

Q.27. Why are the athletes in a high jump event made to fall either on a bed or a sand bed ?

Answer Ans. We prefer a cushioned bed or a sand bed so that the athlete loses it momentum in a greater time so that force exerted by the bed on the athlete is very much reduced and he is safe.

Q.28. A bird hit the windscreen of a fast moving car and falls on the bonnet. Which of the two, car or the bird, suffers greater change in momentum ?

Answer Ans. Both of them suffer equal change in momentum.

Q.29. State Newton’s third law of motion.

Answer Ans. For every action there is an equal and opposite reaction.

Q.30. How is a swimmer able to swim forward in a tank, although he is pushing water backward ?

Answer Ans. A swimmer pushes water backward. In turn, water pushes the swimmer forward. Due to this force of the reaction the swimmer is able to swim forward in a tank.

Q.31. When action and reaction forces are equal and opposite, why don’t they cancel each other?

Answer Ans. Because they are acting on two different bodies.

Q.32. Which law of motion is involved in rocket propulsion ?

Answer Ans. The third law of motion.

Q.33. In which direction should a passenger run before trying to catch a train running slowly along a platform ? Why?

Answer Ans. In the direction of motion of train so as to gain momentum which helps him in getting in to the train.

Q.34. State conservation law of momentum.

Answer The total momentum of an isolated system remains conserved.
Or
If no external force acts on an object, then its total momentum remains conserved.

Q.35. Shyam throws a heavy stone out of his small boat. As a result, the boat moves in opposite direction. Why?

Answer Ans. When Shyam throws a heavy stone out of his small boat, he imparts some momentum to it. In accordance with conservation law of momentum, Shyam gets an equal momentum in reverse direction and so Shyam and his boat moves backward.

Q.36. When a boy jumps out of a boat, the boat moves backward. Why?

Answer Ans. Reason is same as in Q. 35.

Q.37. Why is the foot of a man injured when he hits a stone ?

Answer Ans. When a man hits a stone, he exerts a force on the stone. In turn, the stone exerts an equal and opposite force on the foot of that man, due to which the foot may get injured.

Q.38. Why does a gun recoil when it fires a bullet ?

Answer Ans. The gun exerts a forward force on the bullet at the time of firing. The bullet exerts an equal reaction force on the gun in backward direction, due to which the gun recoils.

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