STD-IX, FORCE AND LAWS OF MOTION, MCQs

1. A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper to

(a) exert larger force on the ball

(b) Increase the force exerted by the balls on the hands

(c) increase the rate of change of momentum

(d) decrease the rate of change of momentum

ANS: decrease the rate of change of momentum

2. The inertia of an object tends to cause the object

(a) to increase its speed

(b) to decrease its speed

(c) to resist any change in its state of motion

(d) to decelerate due to friction

ANS: to resist any change in its state of motion

3. Inertia is the property of a body by virtue of which, it cannot change by itself

(a) its state of rest

(b) its steady state of uniform motion

(c) its direction of motion

(d) all of these

ANS: all of these

4. A ball is thrown vertically upward in a train moving with uniform velocity. The ball will

(a) fall behind the thrower

(b) fall ahead of the thrower

(c) return back to the thrower

(d) fall on the left of the thrower

ANS: return back to the thrower

5. A passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is

(a) accelerated

(b) uniform

(c) retarded

(d) along circular tracks

ANS: accelerated

6. An object of mass 2 kg is sliding with a constant velocity of 4 ms-1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is

(a) 32 N

(b) 0 N

(c) 2 N

(d) 8 N

ANS: 0 N

7. A water tanker filled up to 23 of its height is moving with a uniform speed. On a sudden application of brakes, the water in the tank would

(a) move backward

(b) move forward

(c) be unaffected

(d) rise upwards

ANS: move forward

8. When a balloon held between the hands is pressed, its shape changes. This happens because:

(a) Balanced forces act on the balloon

(b) Unbalanced forces act on the balloon

(c) Frictional forces act on the balloon

(d) Gravitational force acts on the balloon

ANS: balanced forces act on the balloon

9. When a number of forces acting simultaneously on a body bring about a change in its state of rest or of uniform motion in a straight line, then these forces acting on the body are said to be

(a) balanced forces

(b) equal forces

(c) unbalanced forces

(d) opposite forces

ANS: unbalanced forces

10. When a car at high speed makes a sharp turn, the driver in a car tends to get thrown to the side opposite to the turn. This is due to the

(a) inertia of motion

(b) inertia of time

(c) inertia of rest

(d) inertia of direction

ANS: inertia of direction

11. Which of the following is an incorrect statement?

(a) Mass is measure of inertia of a body.

(b) Newton’s first law of motion is the law of inertia.

(c) Unbalanced force produces constant velocity.

(d) Newton’s third law talks about the direction of the force

ANS: unbalanced force produces constant velocity

12. When we stop pedalling, the bicycle begins to slow down. This is because of the

(a) Frictional force acting along the direction of motion of bicycle

(b) Air resistance which is in the direction of motion

(c) Frictional force acting opposite to the direction of motion of bicycle by the road

(d) Nature of the bicycle to stop after some time.

ANS: frictional force acting opposite to the direction of motion of bicycle by the road

13. An athlete does not come to rest immediately after crossing the winning line due to the

(a) inertia of motion

(b) inertia of rest

(c) inertia of direction

(d) none of these

ANS: inertia of motion

14. The masses of two bodies are in ratio 5 : 6 and their velocities are in ratio 1 : 2. Then their linear momentum will be in the ratio

(a) 5 : 6

(b) 1 : 2

(c) 12 : 5

(d) 5 : 12

ANS: 5 : 12

15. Which of the following situations involves the Newton’s second law of motion?

(a) A force can stop a lighter vehicle as well as a heavier vehicle which are moving

(b) A force exerted by a lighter vehicle on collision with a heavier vehicle results in both the (vehicles coming to a standstill

(c) A force can accelerate a lighter vehicle more easily than a heavier vehicle which are moving

(d) A force exerted by the escaping air from a balloon in the downward direction makes the balloon to go upwards

ANS: A force can accelerate a lighter vehicle more easily than a heavier vehicle which are moving

16. If the mass of a body is doubled and its velocity becomes half, then the linear momentum of the body will

(a) remain same

(b) become double

(c) become half

(d) become four times.

ANS: remain same

17. The speed of a car weighing 1500 kg increases from 36 km/h to 72 km/h uniformly. What will be the change in momentum of the car?

(a) 15000 kg km/h                           

(b) 15000 kg m/s

(c) 54000 kg m/s                                              

(d) 54000 g m/s

ANS: 15000 kg m/s

18. Qualitative definition of force is given by,

(a) Newton’s First Law of motion

(b) Newton’s Second Law of motion

(c) Newton’s Third Law of motion

(d) Newton’s Law of Gravitation

ANS: Newton’s First Law of motion

19. Quantitative definition of force is given by,

(a) Newton’s First Law of motion

(b) Newton’s Second Law of motion

(c) Newton’s Third Law of motion

(d) Newton’s Law of Gravitation

ANS: Newton’s Second Law of motion

20. The seat belts are provided in the cars so that if the car stops suddenly due to an emergency braking, the persons sitting on the front seats are not thrown forward violently and saved from getting injured. Can you guess the law due to which a person falls in forward direction on the sudden stopping of the car?

(a) Newton’s first law of motion

(b) Newton’s second law of motion

(c) Newton’s third law of motion

(d) Newton’s law of gravitation

ANS: Newton’s first law of motion

21. In the absence of external force the velocity,

(a) vanishes

(b) remains constant

(c) changes continuously

(d) None of these

ANS: remains constant

22. Newton’s first law introduces the concept of _______.

(a) inertia

(b) momentum

(c) conservation of energy

(d) action & reaction

ANS: inertia

23. Newton’s second law introduces the concept of ________.

(a) momentum

(b) inertia

(c) conservation of energy

(d) action & reaction

ANS: momentum

24. Momentum of an object depends on ______.

(a) mass

(b) velocity

(c) mass & velocity

(d) force

ANS: mass and velocity

25. According to third law of motion, action and reactions happen at _______.

(a) same time

(b) different times

(c) action happens first and then reaction

(d) reaction happens first and then action

ANS: same time

26. A train of 20 wagons moves with constant velocity of 50 m/s for 3 minutes. The mass of each wagon is 10000 kg. Calculate the net force on each wagon.

(a) 500 N

(b) 1000 N

(c) 0 N

(d) Insufficient data to calculate force 

ANS: 0 N

27. If a bus starts suddenly, the passengers in the bus will tend to fall

(a) in the direction opposite to the direction of motion of bus

(b) in the same direction as the direction of motion of bus

(c) sideways

(d) none of the above

ANS: in the direction opposite to the direction of motion of bus

28. An athlete runs some distance before taking a long jump because, 

(a) he gains energy to take him through long distance

(b) it helps him to apply large force

(c) by running, action and reaction forces increase

(d) by running the athlete gives himself larger inertia of motion

ANS: by running the athlete gives himself larger inertia of motion

29. A rider on a horseback falls back when the horse starts running all of a sudden because

(a) rider is taken back

(b) rider is suddenly afraid of falling

(c) inertia of rest keeps the upper part of body at rest whereas the lower part of the body moves forward with the horse

(d) none of the above

ANS: inertia of rest keeps the upper part of body at rest whereas the lower part of the body moves forward with the horse

30. SI unit of force is,

(a) kg m/s

(b) Newton

(c) Dyne

(d) kg m2/s

ANS: Newton

31. A driver accelerates his car first at a rate of 1.8 m/s2 and then at a rate of 1.2 m/s2. The ratio of the two forces exerted by the engine in the two cases will be,

(a) 1:2

(b) 2:1

(c) 2:3

(d) 3:2

ANS: 3 : 2

32. An object will continue to accelerate until,

(a) the resultant force begins to decrease

(b) the resultant force on it is zero

(c) the velocity changes direction

(d) the resultant force on it is increased continuously

ANS: The resultant force on it is zero

33. A canon after firing recoils due to

(a) conservation of energy

(b) backward thrust of gases

(c) Newton’s third law of motion

(d) Newton’s first law of motion

ANS: Newton’s third law of motion

34. A rocket or jet engine works on the principle of

(a) conservation of energy

(b) conservation of momentum

(c) conservation of mass

(d) Newton’s second law of motion

ANS: Conservation of momentum

35. kg.m/s² is the unit of

(a) momentum

(b) speed

(c) acceleration

(d) force

ANS: Force

36. When an object undergoes acceleration

(a) its speed always increases 

(b) its velocity always increases

(c) it always falls towards the earth

(d) a force always acts on it

ANS: a force always acts on it

37. When a net force acts on an object, the object will be accelerated in the direction of force with acceleration proportional to 

(a) the force on the object

(b) the velocity of the object

(c) the mass of the object

(d) the inertia of the object

ANS: the force on the object

38. The action and reaction forces referred to in the Third law  

(a) must act on the same object

(b) must act on different objects

(c) need to be equal in magnitude and must have the same direction

(d) need not to be equal in magnitude but must have opposite direction

ANS: must act on different objects

39. 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.

ANS: there is a force on the ball opposing the motion.

40. What options are true about momentum?

(a) Momentum is a vector quantity

(b) The Unit of Momentum is kgm/s

(c) Momentum is a scalar quantity

(d) When two bodies acts on each other and no external force is acting on the system, then the momentum remains constant

ANS: a, b, d

41. An object of mass 100kg is accelerated uniformly from a velocity of 4 m/s to 8 m/s in 2 seconds. Calculate the initial and final momentum of the object

(a) 400 kgm/s,800 kgm/s

(b) 100 kgm/s,400 kgm/s

(c) 200 kgm/s,400 kgm/s

(d) 400 kgm/s,400 kgm/s

ANS: 400 kg.m/s, 800 kg.m/s

Initial momentum =mV_i=100×4=400 kgm/s

Final Momentum = mV_f=100×8=800 kg/s

42. Which of the followings works on the principle of momentum?

(a) A gun recoils after firing

(b) Rocket

(c) The case of hose pipe

(d) All the above

ANS: all the above

43. A boy of mass 50 Kg running at 5 m/s jumps on to a 20 Kg trolley travelling in the same direction at 1.5 m/s. What is the common velocity?

(a) 4 m/s

(b) 3 m/s

(c) 3.5 m/s

(d) None of the above

ANS: 4 m/s

Since no external force, law of conservation of momentum can be applied

Initial Momentum

P_I=50×5+20×1.5=280pi=50×5+20×1.5=280 kgm/s

Final momentum P_F=70×v=70vpf=70×v=70v where v is the common velocity

Now we know that

Initial momentum=Final momentum

280=70v280=70v

V=4m/s

44. In the following example, try to identify the number of times the velocity of 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.”

(a) three times

(b) four times

(c) five times

(d) six times

ANS: four times

The velocity of the ball changes four times.

a. When a football player kicks the football, its speed changes from zero to a certain value.In this case, force applied by player helps to change the velocity of the ball.This at first changes the velocity of the ball.

b. Another player kicks the ball towards the goal post. This changes the direction of the ball. Therefore, its velocity also changes. In this case, the player applied a force to change the velocity of the ball. The velocity of the ball has changed 2nd time

c. The goalkeeper collects the ball. In other words, the ball comes to rest. Thus, its speed reduces to zero from a certain value. The goalkeeper applied an opposite force to stop/change the velocity of the ball.Hence, its velocity changes third time.

d. 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.

45. There will be a change in the speed or in the direction of motion of a body when it is acted upon by

(a) zero force

(b) balanced force

(c) unbalanced force

(d) uniform force

ANS: unbalanced force

46. Force required in accelerating a 3 kg mass at 5 m/s2 and a 4 kg mass at 4 m/s2, will be

(a) zero in both cases

(b) same in both the cases

(c) greater for 3 kg mass at 5 m/s2

(d) greater for 4 kg mass at 4 m/s2

ANS: greater for 4 kg mass at 4 m/s2

47. A bullet of mass 40 gm is fired from a gun of mass 8 kg with a velocity of 800 m/s, calculate the recoil velocity of gun

(a) 1 m/s

(b) -1 m/s

(c) 2 m/s

(d) -4 m/s

ANS: -4 m/s

48. A hammer of mass 300 g, moving at 40m/s, strikes a nail. The nail stops the hammer in a very short time of 0.02 s. The force of the nail on the hammer is

(a) 600 N

(b) 1200 N

(c) 2000 N

(d) 2400 N

ANS: 600 N

49. Linear momentum of an object is 250 g cm/s. If the velocity of the object is 5 m/s, then the mass of the object is _____.

(a) 5 Kg

(b) 0.5 g

(c) 5 mg

(d) 0.5 mg

ANS: 0.5 g

50. In SI system, the gravitational unit of force is ________.

(a) kg m s-2

(b) kgf 

(c) m/s

(d) metre 

ANS: kgf

51. Identify the force which cannot act without any physical contact.

(a) frictional force

(b) gravitational force

(c) electrostatic force

(d) magnetic force

ANS: frictional force

52. The frictional force that is dependent on the area of surfaces in contact is __________friction.

(a) static

(b) rolling

(c) sliding

(d) kinetic

ANS: rolling

53. Which one of the following is not a vector quantity?

(a) Speed

(b) Momentum

(c) Impulse 

(d) Force 

ANS: speed

54. Which of the following objects has greater inertia? (consider that all these objects are of the same size)

(a) a thermocol ball

(b) a plastic ball

(c) a paper ball

(d) a solid iron ball

ANS: a solid iron ball

55. 1 kgf is equal to _________.

(a) 9.8 N

(b) 980 dynes

(c) 9800 kg m s-2

(d) 105 dynes 

ANS: 9.8 N

56. Which of the following statements is true about force?

(a) It cannot be seen.

(b) It can move a stationary body.

(c) It can change the shape of a body.

(d) All of the above 

ANS: All of the above

57. A body of mass 3 kg moves with a velocity of 500 cm/s, the momentum of the body is _______.

(a) 150 kg m/s 

(b) 15 x 109 kg m/s

(c) 15 x 10-9 kg m/s

(d) 15 kg m/s

ANS: 15 kg m/s