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NCERT Solutions for class 11 Physics | Chapter 5 - Laws of motion

(1) A force of 20 N moves a body , in its direction throught a distance of 2 m . The work done by the force is
[A] 22 J
[B] 40 J
[C] 10 J
[D] 18 J
Answer: 40 J
(2) A block of wood is placed on a flat horizontal surface . A force F is applied to move it parallel to the surface . If I is the frictional force developed then the resultant face acting on the body , when it is sliding is
[A] F + f
[B] F - f
[C] Ff
[D] F / f
Answer: F - f

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(3) A machine gun fires a bullet of mass 40 g with a velocity of 1200 ms-1 . The man holding it can exert a maximum force on 144 N on the gum . How many bullets can he fire per second at the most ?
[A] one
[B] four
[C] two
[D] three
Answer: three
(4) A passenger in a moving bus is thrown forward when the bus is suddenly stopped . This is explained
[A] by Newtons first law
[B] by Newtons second law
[C] by Newtons third law
[D] by the principle of conservation of momentum
Answer: by Newtons first law
(5) A body of mass 10 kg is acted upon by two perpendicular forces, 6N and 8N. The resultant acceleration of the body is
[A] 1 m s–2 at an angle of tan-1 (4 / 3) w.r.t. 6N force.
[B] 0.2 m s–2 at an angle of tan-1 (4 / 3) w.r.t. 6N force.
[C] 1 m s–2 at an angle of tan-1 (3 / 4) w.r.t. 8N force.
[D] 0.2 m s–2 at an angle of tan-1 (3 / 4) w.r.t. 8N force.
Answer: A, c
(6) Inside the nucleus , two protons are held together by a force which overcomes the repulsion . This force is called
[A] gravitationl force
[B] electrostatic force
[C] weak force
[D] strong force
Answer: strong force
(7) The force required to produce an acceleration of 2 m/s2 on a mass of 2 kg is
[A] 4 N
[B] 10 N
[C] 22 N
[D] 18 N
Answer: 4 N
(8) Out of the basic forces , gravitational force
[A] ranks first in strength
[B] ranks second in strength
[C] ranks thrid in strength
[D] ranks fourth in strength
Answer: ranks fourth in strength
(9) A heavy uniform chain lies on horizontal table top . If the coefficient of friction between the chain and the table surface is 0.25 , then the maximum fraction of the length of the chain that can hang over the edge of the table is
[A] 20%
[B] 25%
[C] 30%
[D] 35%
Answer: 20%
(10) The coefficient of kinetic friction is
[A] equal to the coefficient of static friction(t - 1.5t2)
[B] larger than the coefficient of static friction
[C] less than the coefficient of static friction
[D] independent of the value of the coefficient of static friction
Answer: less than the coefficient of static friction
(11) A spring balance is attached to the ceiling of a lift . A msn hangs his bag on the spring and the spring reads 49 N, when the lift is stationary . If the lift moves downwards with an acceleration of 5 m/s , the reding of the spring balance will be
[A] 15 N
[B] 24 N
[C] 49 N
[D] 74 N
Answer: 24 N
(12) A block of wood is placed on a surface . A force is applied parallel to the surface to move the body . The frictional force developed acts
[A] normal to the surface upwards
[B] normal to the surface downwards
[C] along the direction of the applied force
[D] opposite to the direction of the applied force
Answer: opposite to the direction of the applied force
(13) A passenger sitting in a bus moving at uniform speed, feels pushed backward whenever the bus is accelerated forward . This type of force is called
[A] Gravitational force
[B] real force
[C] fictitious force or pseudo force
[D] frictional force
Answer: fictitious force or pseudo force
(14) The mass of a body is 2 kg . It weight is
[A] 19 .6 N
[B] 20 N
[C] 30 N
[D] 40 N
Answer: 19 .6 N
(15) The frame of reference attached to a satellite of the earth is
[A] an inertial frame
[B] an absolute frame at rest with respect to the stars
[C] a non - inertial frame
[D] a gravitational frame
Answer: a non - inertial frame
(16) Two bodies of masses 4 kg and 5 kg are acted upon by the same force . If the acceleration of lighter body is 2 m/s2 , the acceleration of heavier body is
[A] 1 m/s2
[B] 1 . 2 m/s2
[C] 1 . 6 m/s2
[D] 1 . 8 m/s2
Answer: 1 . 6 m/s2
(17) A body of mass 5 kg is travelling with a uniform velocity of 2 m/s . Its momentum is
[A] 10 kg m/s
[B] 7 kg m/s
[C] 2 .5 kg m/s
[D] 3 kg m/s
Answer: 10 kg m/s
(18) Three forces acting at a point keep it in equilibrium . If the angle between two of the forces , 3 N and 3 N is 120o , then the third force is
[A] 3 N
[B] 5 N
[C] 1 N
[D] 12 N
Answer: 3 N
(19) Two forces 6 N and 8 N act at a point O . If the angle between the lines of action of the force is 90o , then their resultant is
[A] 14 N
[B] 12 N
[C] 10 N
[D] 48 N
Answer: 10 N
(20) A force of 12 N acts on a body of mass 2 kg . The acceleration produced on the body is
[A] 24 m/s2
[B] 6 m/s2
[C] 14 m/s2
[D] 10 m/s2
Answer: 6 m/s2

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