Class 9th Important NCERT Questions and answers pdf | Chapter 9-Forces and Laws of Motion
Forces And Laws Of Motion
Class 9th Important NCERT questions with answers PDF.
Very Short Questions & Answers:-
Question 1. Name the physical quantity, which changes the shape of a ball. Answer: Force.
Question 2. Define the term force. (CBSE 2011, 2012, 2014) Answer: A force is a physical quantity which causes or tends to cause a motion in an object at rest or changes or tends to change the direction of motion of a moving object or the shape or size of the object.
Question 3. Define balanced forces. (CBSE 2013)
State the meaning of balanced forces. (CBSE 2012) Answer: Two equal and opposite forces acting simultaneously on an object are known as balanced forces. The resultant force due to balanced forces is zero.
Question 4. Define unbalanced forces. (CBSE 2013) Answer: Two forces of unequal magnitudes and acting in opposite directions on an object simultaneously are known as unbalanced forces. The resultant force due to unbalanced forces is non zero.
Question 5. Name the force which is responsible for change in position or state of an object. (CBSE 2012, 2013) Answer: Unbalanced force.
Question 6. A ball is moving over a horizontal smooth surface with a constant velocity. What type of forces are acting on the ball ? Answer: Balanced forces are acting on the ball.
Question 7. An unbalanced force is acting on an object. List two possible effects that will be caused in the motion of object. (CBSE 2011) Answer:
Object may be accelerated or retarded,
Direction of motion of the object may change.
Question 8. What do you mean by inertia ? (CBSE 2010, 2012) Answer: The tendency of a body to oppose any change in its state of rest or uniform motion is called inertia of the body.
Question 9. Write the kinds of inertia. (CBSE 2012) Answer:
Inertia of rest,
Inertia of motion
Inertia of direction.
Question 10. Name the physical quantity that measures inertia. (CBSE 2010, 2012, 2015) Answer: Mass of the body measures its inertia.
Question 11. On which factor does inertia depend ? (CBSE 2013, 2015) Answer: It depends on the mass of the object.
Question 12. Which has more inertia : a man or a child ? Answer: A man has more inertia than a child.
Question 13. Which has highest inertia : solid made of aluminium, steel and wood oÇ same shape and same volume ? (NCERT Question Bank, CBSE 2011, 2012) Answer: Solid made of steel has the highest inertia because its mass is greater than aluminium and wooden solids.
Question 14. Name the property of bodies to resist a change in their velocity. (CBSE 2012, 2013, 2014) Answer: Inertia.
Question 15. In a tug of war, the rope does not move in any direction. Why ? (CBSE 2013) Answer: Because balanced forces act on the rope. Both the teams apply equal and opposite forces on the rope.
Question 16. Which one has greater inertia : a stone of mass 1 kg or a stone of mass 5 kg ? Answer: Inertia depends on the mass of the object. A stone of mass 5 kg has greater inertia than the stone of mass 1kg.
Question 17. What kind of force is required to accelerate the motion of an object ? Answer: Unbalanced force.
Question 18. A ball is moving over a horizontal smooth surface with a constant velocity. What type of forces are acting on the ball ? Answer: Balanced forces.
Question 19. Name the forces (type of forces) when their resultant force acting on a body is not zero. Answer: Unbalanced forces.
Question 20. Mention any two affects of force. Answer:
It changes the state of rest or motion of a body,
It changes the shape of the body.
Question 21. Is force a scalar or a vector quantity ? Answer: Force is a vector quantity.
Question 22. State Newtons first law of motion. Or Define the first law of motion. Or State the law of inertia. Answer: Everybody continues in its state of rest or uniform motion in a straight line unless some external force acts on it.
Question 23. Which law of motion defines the force ? Answer: First law of motion.
Question 24. Define momentum and give its SI unit. (CBSE 2010, 2011, 2012, 2014, 2015) Answer: Momentum = mass x velocity. SI unit of momentum is kg m s-1.
Question 25. Name the factors on which momemtum of a body depends. (CBSE 2012, 2013, 2015) Answer:
Mass of the body and
Velocity of the body.
Question 26. Define momentum of a body. (CBSE 2010, 2011) Answer: Momentum of a body is defined as the product of the mass and velocity qf the body. Momentum of body = m x v.
Question 27. Is momentum a scalar or a vector quantity ? (CBSE 2015) Answer: Momentum is a vector quantity.
Question 28. What is the name given to the product of mass and velocity of a body ? (CBSE 2012) Answer: Linear momentum.
Question 29. State Newtons second law of motion. Give its mathematical expression. (CBSE 2010, 2012, 2013) Answer: The rate of change of momentum of a body is directly proportional to the unbalanced force acting on it and the change in momentum takes place in the direction of the unbalanced force. F = ma.
Question 30. Name the physical quantity which is determined by the rate of change of linear momentum. (CBSE 2010, 2012, 2013) Answer: Force.
Question 31. Define one unit of force. (CBSE 2010, 2011, 2013) Answer: A force is said to be unit force if it produces unit acceleration in a body of unit mass.
Question 32. Name the unit of force in SI. Answer: newton (N).
Question 33. Define SI unit of force. (CBSE 2011, 2014) Or Define one newton of force. (CBSE 2012) Answer: SI unit of force is newton. Force is said to be 1 newton if it produces 1 m s-2 acceleration in a body of 1 kg.
Question 34. What is the relationship between force and acceleration ? (CBSE 2012, 2013) Answer: F = ma, where m is the mass of a body.
Question 35. What is the name given to the product of mass and acceleration of a body ? Answer: Force (∴ F = ma)
Question 36. How much force acts on a body whose momentum is constant (i.e. p = constant) ? Answer: F = dp/dt. If p = constant, then no force acts on the body.
Question 37. Calculate the force required to produce an acceleration of 2.5 m s-2 in a body of 4 kg. Answer: F = ma = 4 x 2.5 = 10 N.
Question 38. A body of mass 30 kg has a momentum of 150 kg m s-1. Find its velocity. (CBSE 2013, 2014) Answer:
Question 39. An athlete always runs some distance before taking a jump. Why ? Answer: To gain the momentum so that he may jump higher.
Question 40. An object is thrown vertically upward. What is its momentum at the highest point ? (CBSE 2012) Answer: Momentum = mass x velocity. Since velocity of the object at the highest point is zero, so momentum of the object is zero at the highest point.
Question 41. What will happen to the momentum of a body whose velocity is doubled ? Answer: Momentum = mv. Therefore, momentum of a body is doubled if its velocity is doubled.
Question 42. Name the physical quantity whose SI unit is “newton”. Answer: Force.
Question 43. Which would require a greater force — accelerating a 1 kg mass at 10 m s or a 2 kg mass at 4 m s ? Answer: F = ma, F1 = 1 x 10 = 10 N and F2 = 2 x 4 = 8 N So, 1 kg mass accelerating at 10 m s-2 requires greater force than the 2 kg mass at 4 m s-2.
Which would require a greater force — accelerating a 2 kg mass at 5 m s-2 or a 5 kg mass at 3 ms-2 ? Answer: F1 = m1 x a1 =2 x 5 = 10 N, F2 = m2 x a2 = 5 x 3 = 15 N ∴ F2 > F1
Question 44. If the mass of a body is doubled, what will happen to its acceleration, if the applied force remains constant ? Answer: a = F/m. If mass(m) is doubled, acceleration becomes half of its original value, if the applied force remains m constant.
Question 45. State Newtons third law of motion. Answer: To every action, there is equal and opposite reaction.
Question 46. State action and reaction while a person walks on a ground. Answer: Action : Force exerted by person on ground. Reaction : Force exerted by ground on person.
Question 47. Do action and reaction forces act on same body or different bodies ? Answer: Action and reaction act on the different bodies.
Question 48. State action and reaction in the case when a person swims. Answer: Action : Force with which swimmer pushes the water in the backward direction with his hands. Reaction : The force with which water pushes the swimmer in the forward direction.
Question 49. State action and reaction in the case when a bullet is fired from the gun. (CBSE 2011) Answer: Action : Force exerted by a spring on the bullet. Reaction : Force exerted on the gun.
Question 50. State action and reaction when a bullet is fired from the gun. Answer: Force exerted on the bullet is action and the force exerted on the gun is reaction.
Question 51. When a man jumps out from a boat to the bank of a river, the boat moves backwards. Identify the action and reaction in this situation. (CBSE 2015) Answer: Action = force with which the foot of the man pushes the boat in the backward direction. Reaction = force exerted by the boat on the man.
Based on the law of conservation of momentum
Question 52. State law of conservation of momentum. (CBSE 2010, 2012, 2013, 2015) Answer: The total momentum of a system remains constant if no external force acts on the system.
Question 53. A fast moving truck loses momentum equal to 2 kg m s-1 after colliding with a stationary car. Will the car lose or gain momentum ? How much momentum is lost or gained by the car if the truck comes to the rest after collision ? Answer: The car will gain momentum. Momentum gained by the car = 2 kg m s-1
Question 54. Name the principle on which a rocket works. (CBSE 2012, 2013) Answer: Law of conservation of linear momentum.
Question 55. What do you mean by recoil velocity of a gun ? Answer: The velocity with which a gun moves backward after firing a bullet is called the recoil of the gun.
Question 56. Is momentum a scalar or a vector quantity ? (CBSE 2015) Answer: Momentum is a vector quantity.
Short Questions & Answers :-
Question 1. When are the forces acting on a body said to be balanced ? Give an example. What type of change can the balanced forces bring about in an object ? (CBSE 2010, 2015) Answer: Two forces of equal magnitude but acting in opposite directions on a body are said to be balanced forces. When we push a wall but the wall does not move at all, then balanced forces are acting on the wall. When balanced forces are -acting on an object, there is no change in the state of the object.
Question 2. What are the changes that a force can bring about on a body? Give examples. (CBSE 2010, 2011)
Explain any three effects that can be produced by force with examples. (CBSE 2012, 2013, 2014) Answer: Force can change
the shape of the body,
direction of motion .of the body,
the speed of the body and
size of the body
The shape of a ball lying on a floor can be changed by pressing it.
The direction of motion of moving ball can be changed by hitting it with a bat.
A ball at rest can be set in motion if force is applied on it.
The length of a spring tied at one end can be increased by pulling it.
Question 3. Two balls A and B of masses m and 2m are in motion with velocities ‘2V’ and ‘V’ respectively. Compare (i) their inertia, (ii) their momentum and (iii) the force needed to stop them in the same time. Answer:
(iii) Since momentum of both the balls is same, so same amount of force is needed to stop them in the same time.
Question 4. Property of a body by virtue of which it opposes any change in state is called (a) momentum (b) energy (c) inertia (d) acceleration. Answer: (c)
Question 5. If A and B are two bodies with masses 10 kg and 50 kg respectively, then which body has more inertia ? Also calculate the ratio of the inertia of two bodies. Answer: Inertia of body B is more than the inertia of body A. body A 10 kg and body B 50 kg. If the time taken to bring a ball to rest from a certain velocity V is reduced to half, what will be the change in values of
Question 6. Look at the diagrams given (Figure A) and answer the following questions. In which case will the object move and in which direction ? Give reason in support of your answer.
Answer: In case (ii) object moves in the direction of force of 25 N because in this case the net force acting on the object = 25N-15N=10N In case (i), net force acting on the object is zero.
Question 7. In the given experimental set-up (Figure B) a student gave the card a sharp, fast horizontal flick with a finger.
What will happen to the coin ?
Write reason for your answer.
The coin will fall into the glass.
The card will move, when a fast horizontal flick with a finger is given to it. On the other hand, coin remains at rest due to inertia of rest and hence falls into the glass.
Question 8. State the effect of force in each of the following cases,
A spring is stretched,
A hockey player hits an incoming ball
A football lying on the ground is kicked.
Length of spring increases and hence its shape is changed.
The direction of the ball is changed.
Football comes in motion from rest state.
Question 9. (a) Initial and final momentum (b) Change of momentum (c) Rate of change of momentum. Answer: (a) Initial and final momentum remain the same as they do not depend on time. (b) Change in momentum remains the same as change in momentum = Final momentum-initial momentum. (c)
therefore, rate of change of momentum is Time taken doubled if time is reduced to half.
Question 10. While riding on the bicycle if we stop paddling, why does the bicycle begin to slow down ? Answer: Bicycle begins to slow down as its motion is opposed by the unbalanced force known as force of friction between the types of bicycle and the road.
Based on Newton’s laws of motion
Question 11. What happens when a wet piece of cloth is shaken ? (CBSE 2011) Answer: When a wet piece of cloth is shaken, fibres of the cloth come in motion and water droplets in the cloth fall down due to inertia of rest.
Question 12. A person getting down from the moving bus falls in the direction of the bus. Explain, why ?
A passenger has to run along with moving bus in the same direction of the bus while getting down from the bus. Explain.
It is is dangerous to jump out of a moving bus. Why ? (CBSE 2011, 2013) Answer: As soon as the passenger gets down from a moving bus, his foot comes to rest but upper portion of his body remains in motion in the direction of the moving bus. He may fall down if he does not run along with the bus.
Question 13. A stone tied to a string whirls in a horizontal circle. It flies off tangentially when the string breaks suddenly. Answer: The stone flies off tangentially due to inertia of direction of motion.
Question 14. What happens to a person travelling in a bus when the bus takes a sharp turn ? Explain.(CBSE 2011)
A passenger in a moving car slips to one side of the seat when the car takes a sharp turn. Give reason for it. (CBSE 2012) Answer: When car or bus takes a sharp turn, passenger slips to one side of the seat due to the inertia of motion.
Question 15. It is difficult to balance our body when we accidendy step on a peel of banana. Explain, why ?(CBSE 2011) Answer: When we walk on the ground, our foot pushes the ground in the backward direction (Action). On the other hand, ground pushes our foot in the forward direction (Reaction). This reaction of the ground helps us to move in the forward direction. But, when our foot falls on a peel of banana, then our foot slips on the peel of banana and cannot push the ground in the backward direction. Consequently, no reaction force acts on our foot. Hence we lose the balance and fall down.
Question 16. Why are the wheels of vehicle provided with mud gaurds ? (CBSE 2012) Answer: The rotating wheels of a vehicle throw out mud sticking to it tangentially due to inertia of direction. The mud gaurds stop this mud to fall on another vehicle just moving behind the vehicle.
Question 17. All the cars are provided with seat belts. Give reason. (CBSE 2012, 2013) Answer: When a car stops suddenly, driver and passengers continue to move due to inertia of motion. Hence, they may be hurt due to the collision between them and the body of the car. Therefore, seat belts are provided to prevent their forward motion in case of an accident.
Question 18. A heavy leather ball and a light tennis ball are kept on the floor of a moving train. When train is suddenly stopped, they are set in motion. In which direction will they move ? Which one of them will attain higher velocity ? Answer: They will move in the direction of the motion of the train due to inertia of motion. The mass, of leather ball is greater than that of tennis ball, so leather ball will resist more than the tennis ball to change its state of motion. Hence, tennis ball will attain higher velocity.
Question 19. A bullet fired on a glass window makes a fine hole while a stone smashes when hits it. State the reason. (CBSE 2015) Answer: A bullet fired from a gun makes a small hole in the window pane while passing through it, but the stone striking the window pane breaks it into pieces. When a bullet strikes the window pane, only a small portion of the window pane where the bullet strikes comes in motion because the bullet makes contact with the pane for a very short time due to its high speed. On the other hand, the remaining portion of the window pane remains at rest due to inertia of rest. Thus, a small hole is made by the bullet in the window pane. The speed of the stone is very small compared to the speed of the bullet. So the stone makes contact with the window pane for longer period of time. During this-longer time, the whole window pane comes in motion and hence breaks into pieces.
Question 20. If the net external force acting on a body is zero, then total momentum of the body is (a) Zero (b) Variable (c) Constant (d) Information not sufficient. Answer:
Question 21. A body of mass ‘m is moving with a velocity u. When a force is applied on it for time t, its velocity increases to ‘v’. Write expressions for (a) Initial and final momentum, (b) Change in momentum (c) Rate of change of momentum. Also write SI unit for each (CBSE 2015) Answer: (a) Initial momentum = mu Final momentum = mv (b) Change in momentum = final momentum – initial momentum = (mv – mu) = m(v – u)
SI unit of initial momentum, final momentum and change of momentum is kg m s-1 SI unit of rate of change of momentum is kg m s-2 or N.
Question 22. If the time taken to bring a ball to rest from a certain velocity v is reduced to half, what will be the change in values of : (a) Initial and final momentum (b) Change of momentum (c) Rate of change of momentum. Answer: (a) Initial and final momentum remain the same as they do not depend on time. (b) Change in momentum remains the same as change in momentum = Final momentum – initial momentum (c)
∴ Rare of change of momentum is doubled if time is reduced to half.
Question 23. Derive First law of motion mathematically from the mathematical expression of Second law of motion. Answer: According to First law of motion, a body continues to move with uniform velocity or zero acceleration if no external force acts on it. Second law of motion is given by F= ma. If F = 0, a = 0. This is the mathematical expression of first law of motion.
Question 24. Why, no force is required to move an object with a constant velocity ? (CBSE 2011) Answer: We know, F = ma When, velocity is constant, then acceleration, a = 0 Hence, F = 0