Page 1 [1] Test -03 (Laws of motion, Work, Power & Energy) 1. An object of mass 10 kg. moves at a constant speed of 10 ms -1 . A constant force, that acts for 4 sec on the object. Gives it is speed of 2 ms -1 in opposite direction. The force acting on the object is : (a) -3N (b) -30 N (c) 3 N (d) 30 N 2. A block of mass M is pulled along horizontal frictionless surface by a rope of mass m. Force P is applied at one end of rope. The force which the rope exerts on the block is : (a) m M P + (b) ) M m ( M P + (c) ) M m ( PM + (d) m M P - 3. A uniform rope of length L resting on a frictionless horizontal surface is pulled at one end by a force F. What is the tension in the rope at a distance ? from the end where the force is applied : (a) F (b) ) L / 1 ( F ? + (c) F/2 (d) ) L / 1 ( F ? - 4. A machine gun has a mass 5 kg. It fires 50 gram bullets at the rate of 30 bullets per minute at a speed of 400 ms -1 . What force is required to keep the gun in position : (a) 10 N (b) 5 N (c) 15 N (d) 30 N 5. A man weighing 80 kg is standing on a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley along the rails at a speed of one metre per second, then after 4 seconds, his displacement relative to the ground will be : (a) 5 metres (b) 4.8 metres (c) 3.2 metres (d) 3.0 metres 6. The elevator shown in fig. is descending with an acceleration of 2 m/s 2 . The mass of the block A = 0.50 kg. the force exerted by the block A on block B is : (a) 2 N (b) 4 N (c) 6 N (d) 8 N 7. A particle of mass 10 kg is moving in a straight line if its displacement x with time t is given by x = (t 3 - 2t - 10)m, then the force acting on it at the end of 4 seconds is : (a) 24 N (b) 240 N (c) 300 N (d) 1200 N 8. When force F 1 , F 2 , F 3 are acting on a particle of mass m such that F 2 and F 3 are mutually perpendicular. Then the particle remains stationary. If the force F 1 is now removed then the acceleration of the particle is : (a) m F 1 (b) 1 3 2 mF F F (c) m F F 3 2 - (d) m F 2 9. A body moves with uniform velocity on a rough surface, under the action of a force. The force F of dynamic friction is plotted against time as shown below. The graph will be : 10. A particle moves in a circle of radius 25 cm at two revolutions per seconds. The acceleration of the particle in metre per second is : (a) p 2 (b) 8 p 2 (c) 4 p 2 (d) 2 p 2 11. An automobile of mass m is crossing over a convex upwards over bridge with a speed v. If the radius of the bridge is r, The thrust on the bridge at the highest point will be : (a) mg + r mv 2 (b) r mv mg 2 - (c) r g v m 2 2 (d) r g v 2 12. The ratio of angular speeds of minute’s hand and hour’s hand of a watch is : (a) 1 : 12 (b) 6 : 1 (c) 12 : 1 (d) 1 : 6 13. A particle is moving along a circular path with a uniform speed. Through what angle does its angular velocity change when it completes half of the circular path : (a) 0 ° (b) 45 ° (c) 180 ° (d) 360 ° 14. Three identical blocks of masses m = 2 kg are drawn by a force F = 10.2 N on a frictionless surface, then what is the tension (in N) in the string between the blocks B and C ? Page 2 [1] Test -03 (Laws of motion, Work, Power & Energy) 1. An object of mass 10 kg. moves at a constant speed of 10 ms -1 . A constant force, that acts for 4 sec on the object. Gives it is speed of 2 ms -1 in opposite direction. The force acting on the object is : (a) -3N (b) -30 N (c) 3 N (d) 30 N 2. A block of mass M is pulled along horizontal frictionless surface by a rope of mass m. Force P is applied at one end of rope. The force which the rope exerts on the block is : (a) m M P + (b) ) M m ( M P + (c) ) M m ( PM + (d) m M P - 3. A uniform rope of length L resting on a frictionless horizontal surface is pulled at one end by a force F. What is the tension in the rope at a distance ? from the end where the force is applied : (a) F (b) ) L / 1 ( F ? + (c) F/2 (d) ) L / 1 ( F ? - 4. A machine gun has a mass 5 kg. It fires 50 gram bullets at the rate of 30 bullets per minute at a speed of 400 ms -1 . What force is required to keep the gun in position : (a) 10 N (b) 5 N (c) 15 N (d) 30 N 5. A man weighing 80 kg is standing on a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley along the rails at a speed of one metre per second, then after 4 seconds, his displacement relative to the ground will be : (a) 5 metres (b) 4.8 metres (c) 3.2 metres (d) 3.0 metres 6. The elevator shown in fig. is descending with an acceleration of 2 m/s 2 . The mass of the block A = 0.50 kg. the force exerted by the block A on block B is : (a) 2 N (b) 4 N (c) 6 N (d) 8 N 7. A particle of mass 10 kg is moving in a straight line if its displacement x with time t is given by x = (t 3 - 2t - 10)m, then the force acting on it at the end of 4 seconds is : (a) 24 N (b) 240 N (c) 300 N (d) 1200 N 8. When force F 1 , F 2 , F 3 are acting on a particle of mass m such that F 2 and F 3 are mutually perpendicular. Then the particle remains stationary. If the force F 1 is now removed then the acceleration of the particle is : (a) m F 1 (b) 1 3 2 mF F F (c) m F F 3 2 - (d) m F 2 9. A body moves with uniform velocity on a rough surface, under the action of a force. The force F of dynamic friction is plotted against time as shown below. The graph will be : 10. A particle moves in a circle of radius 25 cm at two revolutions per seconds. The acceleration of the particle in metre per second is : (a) p 2 (b) 8 p 2 (c) 4 p 2 (d) 2 p 2 11. An automobile of mass m is crossing over a convex upwards over bridge with a speed v. If the radius of the bridge is r, The thrust on the bridge at the highest point will be : (a) mg + r mv 2 (b) r mv mg 2 - (c) r g v m 2 2 (d) r g v 2 12. The ratio of angular speeds of minute’s hand and hour’s hand of a watch is : (a) 1 : 12 (b) 6 : 1 (c) 12 : 1 (d) 1 : 6 13. A particle is moving along a circular path with a uniform speed. Through what angle does its angular velocity change when it completes half of the circular path : (a) 0 ° (b) 45 ° (c) 180 ° (d) 360 ° 14. Three identical blocks of masses m = 2 kg are drawn by a force F = 10.2 N on a frictionless surface, then what is the tension (in N) in the string between the blocks B and C ? [2] (a) 9.2 (b) 8 (c) 3.4 (d) 9.8 15. Work is always done on a body when : (a) a force acts on it (b) it moves through a certain distance (c) it experience an increase in energy through a mechanical influence (d) none of the above 16. A weight hangs freely from the end of a spring. A body then slowly pushes the weight upwards until the spring becomes slack. The gain in gravitational potential energy of the weight during this process is equal to : (a) the workdone by the boy against the gravity force acting on the weight (b) the loss of stored energy by the spring minus the work done by the tension in the spring (c) the workdone on the weight by the boy plus the stored energy lost by the spring (d) the workdone on the weight by the boy minus the workdone by the tension in the spring plus the stored energy lost by the spring 17. A light and a heavy body have equal momentum. Which one has greater K.E. : (a) The light body (b) Both have equal K.E. (c) The heavy body (d) Data given is incomplete 18. A shell initially at rest explodes into two equal fragments A and B. Then : (a) A and B have collinear velocities in the same direction (b) A and B have equal speeds but opposite velocities (c) A and B have equal speeds but their velocities inclined to each other (d) A and B may have unequal speeds but opposite velocities 19. One end of a light spring of spring constant k is fixed to a wall and the other end is tied to a block placed on a smooth horizontal surface. In a displacement, the work done by the spring is 2 1 k x 2 . The possible cases are : (a) The spring was initially compressed by a distance x, was finally in its natural length (b) It was initially stretched by a distance x and was finally in its natural length (c) It was initially in its natural length and finally in a compressed position (d) Both (a) & (b) are correct 20. Choose the correct statement. Work done by a variable force : (a) it defined as S . F ? ? (b) Is independent of path (c) Is always dependent on the initial and final positions (d) None of these 21. Work done by friction : (a) Is always negative (b) Is always zero (c) May be negative zero or positive (d) Is always positive 22. The example (s) of conservative force is/are : (a) Gravitational force (b) Spring force (c) Electrostatic force (d) All the above 23. A particle describes a horizontal circle of radius 0.5 m with uniform speed. The centripetal force acting is 10 N. The work done in describing a semicircle is : (a) zero (b) 5 J (c) 5 pJ (d) 10 pJ 24. A boy pushes a toy box 2.0 m along the floor by means of a force of 10 N directed downward at an angle of 60 ° to the horizontal. The work done by the boy is : (a) 6 J (b) 8 J (c) 10 J (d) 12 J 25. Two masses m 1 = 5 kg and m 2 = 4.8 kg tied to a string are hanging over a light frictionless pulley. What is the acceleration of the masses what left free to move ? (g = 9.8 m/s 2 ) (a) 0.2 m/s 2 (b) 9.8 m/s 2 (c) 5 m/s 2 (d) 4.8 m/s 2 In every question a statement of ASSERTION is followed by a statement of REASON. Mark the correct answer out of the following choices: If both ASSERTION and REASON are true and reason is the correct explanation of the assertion. (a) If both ASSERTION and REASON are true but reason is not the correct explanation of the assertion. (b) If ASSERTION is true but REASON is false. (c) If ASSERTION is false but REASON is true. Page 3 [1] Test -03 (Laws of motion, Work, Power & Energy) 1. An object of mass 10 kg. moves at a constant speed of 10 ms -1 . A constant force, that acts for 4 sec on the object. Gives it is speed of 2 ms -1 in opposite direction. The force acting on the object is : (a) -3N (b) -30 N (c) 3 N (d) 30 N 2. A block of mass M is pulled along horizontal frictionless surface by a rope of mass m. Force P is applied at one end of rope. The force which the rope exerts on the block is : (a) m M P + (b) ) M m ( M P + (c) ) M m ( PM + (d) m M P - 3. A uniform rope of length L resting on a frictionless horizontal surface is pulled at one end by a force F. What is the tension in the rope at a distance ? from the end where the force is applied : (a) F (b) ) L / 1 ( F ? + (c) F/2 (d) ) L / 1 ( F ? - 4. A machine gun has a mass 5 kg. It fires 50 gram bullets at the rate of 30 bullets per minute at a speed of 400 ms -1 . What force is required to keep the gun in position : (a) 10 N (b) 5 N (c) 15 N (d) 30 N 5. A man weighing 80 kg is standing on a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley along the rails at a speed of one metre per second, then after 4 seconds, his displacement relative to the ground will be : (a) 5 metres (b) 4.8 metres (c) 3.2 metres (d) 3.0 metres 6. The elevator shown in fig. is descending with an acceleration of 2 m/s 2 . The mass of the block A = 0.50 kg. the force exerted by the block A on block B is : (a) 2 N (b) 4 N (c) 6 N (d) 8 N 7. A particle of mass 10 kg is moving in a straight line if its displacement x with time t is given by x = (t 3 - 2t - 10)m, then the force acting on it at the end of 4 seconds is : (a) 24 N (b) 240 N (c) 300 N (d) 1200 N 8. When force F 1 , F 2 , F 3 are acting on a particle of mass m such that F 2 and F 3 are mutually perpendicular. Then the particle remains stationary. If the force F 1 is now removed then the acceleration of the particle is : (a) m F 1 (b) 1 3 2 mF F F (c) m F F 3 2 - (d) m F 2 9. A body moves with uniform velocity on a rough surface, under the action of a force. The force F of dynamic friction is plotted against time as shown below. The graph will be : 10. A particle moves in a circle of radius 25 cm at two revolutions per seconds. The acceleration of the particle in metre per second is : (a) p 2 (b) 8 p 2 (c) 4 p 2 (d) 2 p 2 11. An automobile of mass m is crossing over a convex upwards over bridge with a speed v. If the radius of the bridge is r, The thrust on the bridge at the highest point will be : (a) mg + r mv 2 (b) r mv mg 2 - (c) r g v m 2 2 (d) r g v 2 12. The ratio of angular speeds of minute’s hand and hour’s hand of a watch is : (a) 1 : 12 (b) 6 : 1 (c) 12 : 1 (d) 1 : 6 13. A particle is moving along a circular path with a uniform speed. Through what angle does its angular velocity change when it completes half of the circular path : (a) 0 ° (b) 45 ° (c) 180 ° (d) 360 ° 14. Three identical blocks of masses m = 2 kg are drawn by a force F = 10.2 N on a frictionless surface, then what is the tension (in N) in the string between the blocks B and C ? [2] (a) 9.2 (b) 8 (c) 3.4 (d) 9.8 15. Work is always done on a body when : (a) a force acts on it (b) it moves through a certain distance (c) it experience an increase in energy through a mechanical influence (d) none of the above 16. A weight hangs freely from the end of a spring. A body then slowly pushes the weight upwards until the spring becomes slack. The gain in gravitational potential energy of the weight during this process is equal to : (a) the workdone by the boy against the gravity force acting on the weight (b) the loss of stored energy by the spring minus the work done by the tension in the spring (c) the workdone on the weight by the boy plus the stored energy lost by the spring (d) the workdone on the weight by the boy minus the workdone by the tension in the spring plus the stored energy lost by the spring 17. A light and a heavy body have equal momentum. Which one has greater K.E. : (a) The light body (b) Both have equal K.E. (c) The heavy body (d) Data given is incomplete 18. A shell initially at rest explodes into two equal fragments A and B. Then : (a) A and B have collinear velocities in the same direction (b) A and B have equal speeds but opposite velocities (c) A and B have equal speeds but their velocities inclined to each other (d) A and B may have unequal speeds but opposite velocities 19. One end of a light spring of spring constant k is fixed to a wall and the other end is tied to a block placed on a smooth horizontal surface. In a displacement, the work done by the spring is 2 1 k x 2 . The possible cases are : (a) The spring was initially compressed by a distance x, was finally in its natural length (b) It was initially stretched by a distance x and was finally in its natural length (c) It was initially in its natural length and finally in a compressed position (d) Both (a) & (b) are correct 20. Choose the correct statement. Work done by a variable force : (a) it defined as S . F ? ? (b) Is independent of path (c) Is always dependent on the initial and final positions (d) None of these 21. Work done by friction : (a) Is always negative (b) Is always zero (c) May be negative zero or positive (d) Is always positive 22. The example (s) of conservative force is/are : (a) Gravitational force (b) Spring force (c) Electrostatic force (d) All the above 23. A particle describes a horizontal circle of radius 0.5 m with uniform speed. The centripetal force acting is 10 N. The work done in describing a semicircle is : (a) zero (b) 5 J (c) 5 pJ (d) 10 pJ 24. A boy pushes a toy box 2.0 m along the floor by means of a force of 10 N directed downward at an angle of 60 ° to the horizontal. The work done by the boy is : (a) 6 J (b) 8 J (c) 10 J (d) 12 J 25. Two masses m 1 = 5 kg and m 2 = 4.8 kg tied to a string are hanging over a light frictionless pulley. What is the acceleration of the masses what left free to move ? (g = 9.8 m/s 2 ) (a) 0.2 m/s 2 (b) 9.8 m/s 2 (c) 5 m/s 2 (d) 4.8 m/s 2 In every question a statement of ASSERTION is followed by a statement of REASON. Mark the correct answer out of the following choices: If both ASSERTION and REASON are true and reason is the correct explanation of the assertion. (a) If both ASSERTION and REASON are true but reason is not the correct explanation of the assertion. (b) If ASSERTION is true but REASON is false. (c) If ASSERTION is false but REASON is true. [3] 26. Assertion : When a car crashes head on into a heavy truck, the car experiences a greater change in momentum. Reason : Newton’s second law states that the change in momentum is equal to impulse. 27. Assertion : When a force acts on a body, it can change its momentum and kinetic energy Reason : A force produces acceleration 28. Assertion : No work is done when a man is whirling a stone tied to a rope in a circle with a constant speed. Reason : The centripetal force is perpendicular to the direction of displacement. 29. Assertion : Kilowatt hour (Kwh) is the unit of electric power. Reason : Watt hour has units of energy. 30. Assertion : A body builder does no work in holding up a weight. Reason : No work is done because the displacement moved is zero. Test -02 (Set - B) Medical C10 (Laws of motion, Work, Power & Energy) ANSWER KEY 1 B 6 B 11 B 16 C 21 C 26 D 2 C 7 B 12 C 17 A 22 D 27 A 3 D 8 A 13 A 18 B 23 A 28 A 4 A 9 B 14 C 19 D 24 C 29 D 5 C 10 C 15 C 20 C 25 A 30 A SOLUTIONS 1. Here, u = 10 ms - 1 , v = - 2ms - 1 ; t = 4 s, a = ? Using a = 2 s / m 3 4 10 2 t u v - = - - = - , Force, F = ma = 10 × ( -3) = - 30 N 2. Acceleration of block, a = mass total force = m M P + ? Force on block = mass × acceleration = Ma = m M MP + . 3. Let m be the mass per unit length of rope. Therefore, mass of rope = mL. Acceleration in the rope due to force F will be a = F/mL mass of rope of length (L - ) ? will be m (L - ) ? . Therefore, tension in the rope of length (L - ) ? , is equal to pulling fore on it = m(L - ) ? a = m(L - ) ? × F/mL = F(1 - ) L / ? 4. Force required = taken time momentum in change = N 10 60 400 ) 30 10 50 ( 3 = × × × - 5. (c) Due to conservation of momentum 80 × 1 = (80 + 320)V ? V = 5 1 = 0.2 m/s. Velocity of man wrt ground =- 1 – 0.2 = 0.8 m/s. ?Displacement of man ground = (0.8 m/s) × 4s = 3.2 m 6. F = mg - R = 0.5 × 10 - 0.5 × 2 = 5 - 1 = 4N 7. m = 10 kg, x = (t 3 - 2t - 10)m ? 2 t 3 v dt dx 2 - = = and t 6 a dt x d 2 2 = = At the end of 4 secs, a = 6 × 4 = 24 m/s 2 F = ma = 10 × 24 = 240 N. 8. The formula for force is given by F 1 = ma. Acceleration of the particle a = m F 1 9. Dynamic friction force is constant 10. (c) a = rw 2 = 4 1 × (2 p ×2) 2 = 4 1 × 4 p 2 = 4 p 2 11. Mg – N = r mv 2 ? N = mg - r mv 2 . 12. Angular speed of hour hand. ? 1 = ) 60 12 ( 2 × p . Angular speed of minute hand ? 2 = 60 2 p ? 1 12 60 60 12 1 2 = × = ? ? Page 4 [1] Test -03 (Laws of motion, Work, Power & Energy) 1. An object of mass 10 kg. moves at a constant speed of 10 ms -1 . A constant force, that acts for 4 sec on the object. Gives it is speed of 2 ms -1 in opposite direction. The force acting on the object is : (a) -3N (b) -30 N (c) 3 N (d) 30 N 2. A block of mass M is pulled along horizontal frictionless surface by a rope of mass m. Force P is applied at one end of rope. The force which the rope exerts on the block is : (a) m M P + (b) ) M m ( M P + (c) ) M m ( PM + (d) m M P - 3. A uniform rope of length L resting on a frictionless horizontal surface is pulled at one end by a force F. What is the tension in the rope at a distance ? from the end where the force is applied : (a) F (b) ) L / 1 ( F ? + (c) F/2 (d) ) L / 1 ( F ? - 4. A machine gun has a mass 5 kg. It fires 50 gram bullets at the rate of 30 bullets per minute at a speed of 400 ms -1 . What force is required to keep the gun in position : (a) 10 N (b) 5 N (c) 15 N (d) 30 N 5. A man weighing 80 kg is standing on a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley along the rails at a speed of one metre per second, then after 4 seconds, his displacement relative to the ground will be : (a) 5 metres (b) 4.8 metres (c) 3.2 metres (d) 3.0 metres 6. The elevator shown in fig. is descending with an acceleration of 2 m/s 2 . The mass of the block A = 0.50 kg. the force exerted by the block A on block B is : (a) 2 N (b) 4 N (c) 6 N (d) 8 N 7. A particle of mass 10 kg is moving in a straight line if its displacement x with time t is given by x = (t 3 - 2t - 10)m, then the force acting on it at the end of 4 seconds is : (a) 24 N (b) 240 N (c) 300 N (d) 1200 N 8. When force F 1 , F 2 , F 3 are acting on a particle of mass m such that F 2 and F 3 are mutually perpendicular. Then the particle remains stationary. If the force F 1 is now removed then the acceleration of the particle is : (a) m F 1 (b) 1 3 2 mF F F (c) m F F 3 2 - (d) m F 2 9. A body moves with uniform velocity on a rough surface, under the action of a force. The force F of dynamic friction is plotted against time as shown below. The graph will be : 10. A particle moves in a circle of radius 25 cm at two revolutions per seconds. The acceleration of the particle in metre per second is : (a) p 2 (b) 8 p 2 (c) 4 p 2 (d) 2 p 2 11. An automobile of mass m is crossing over a convex upwards over bridge with a speed v. If the radius of the bridge is r, The thrust on the bridge at the highest point will be : (a) mg + r mv 2 (b) r mv mg 2 - (c) r g v m 2 2 (d) r g v 2 12. The ratio of angular speeds of minute’s hand and hour’s hand of a watch is : (a) 1 : 12 (b) 6 : 1 (c) 12 : 1 (d) 1 : 6 13. A particle is moving along a circular path with a uniform speed. Through what angle does its angular velocity change when it completes half of the circular path : (a) 0 ° (b) 45 ° (c) 180 ° (d) 360 ° 14. Three identical blocks of masses m = 2 kg are drawn by a force F = 10.2 N on a frictionless surface, then what is the tension (in N) in the string between the blocks B and C ? [2] (a) 9.2 (b) 8 (c) 3.4 (d) 9.8 15. Work is always done on a body when : (a) a force acts on it (b) it moves through a certain distance (c) it experience an increase in energy through a mechanical influence (d) none of the above 16. A weight hangs freely from the end of a spring. A body then slowly pushes the weight upwards until the spring becomes slack. The gain in gravitational potential energy of the weight during this process is equal to : (a) the workdone by the boy against the gravity force acting on the weight (b) the loss of stored energy by the spring minus the work done by the tension in the spring (c) the workdone on the weight by the boy plus the stored energy lost by the spring (d) the workdone on the weight by the boy minus the workdone by the tension in the spring plus the stored energy lost by the spring 17. A light and a heavy body have equal momentum. Which one has greater K.E. : (a) The light body (b) Both have equal K.E. (c) The heavy body (d) Data given is incomplete 18. A shell initially at rest explodes into two equal fragments A and B. Then : (a) A and B have collinear velocities in the same direction (b) A and B have equal speeds but opposite velocities (c) A and B have equal speeds but their velocities inclined to each other (d) A and B may have unequal speeds but opposite velocities 19. One end of a light spring of spring constant k is fixed to a wall and the other end is tied to a block placed on a smooth horizontal surface. In a displacement, the work done by the spring is 2 1 k x 2 . The possible cases are : (a) The spring was initially compressed by a distance x, was finally in its natural length (b) It was initially stretched by a distance x and was finally in its natural length (c) It was initially in its natural length and finally in a compressed position (d) Both (a) & (b) are correct 20. Choose the correct statement. Work done by a variable force : (a) it defined as S . F ? ? (b) Is independent of path (c) Is always dependent on the initial and final positions (d) None of these 21. Work done by friction : (a) Is always negative (b) Is always zero (c) May be negative zero or positive (d) Is always positive 22. The example (s) of conservative force is/are : (a) Gravitational force (b) Spring force (c) Electrostatic force (d) All the above 23. A particle describes a horizontal circle of radius 0.5 m with uniform speed. The centripetal force acting is 10 N. The work done in describing a semicircle is : (a) zero (b) 5 J (c) 5 pJ (d) 10 pJ 24. A boy pushes a toy box 2.0 m along the floor by means of a force of 10 N directed downward at an angle of 60 ° to the horizontal. The work done by the boy is : (a) 6 J (b) 8 J (c) 10 J (d) 12 J 25. Two masses m 1 = 5 kg and m 2 = 4.8 kg tied to a string are hanging over a light frictionless pulley. What is the acceleration of the masses what left free to move ? (g = 9.8 m/s 2 ) (a) 0.2 m/s 2 (b) 9.8 m/s 2 (c) 5 m/s 2 (d) 4.8 m/s 2 In every question a statement of ASSERTION is followed by a statement of REASON. Mark the correct answer out of the following choices: If both ASSERTION and REASON are true and reason is the correct explanation of the assertion. (a) If both ASSERTION and REASON are true but reason is not the correct explanation of the assertion. (b) If ASSERTION is true but REASON is false. (c) If ASSERTION is false but REASON is true. [3] 26. Assertion : When a car crashes head on into a heavy truck, the car experiences a greater change in momentum. Reason : Newton’s second law states that the change in momentum is equal to impulse. 27. Assertion : When a force acts on a body, it can change its momentum and kinetic energy Reason : A force produces acceleration 28. Assertion : No work is done when a man is whirling a stone tied to a rope in a circle with a constant speed. Reason : The centripetal force is perpendicular to the direction of displacement. 29. Assertion : Kilowatt hour (Kwh) is the unit of electric power. Reason : Watt hour has units of energy. 30. Assertion : A body builder does no work in holding up a weight. Reason : No work is done because the displacement moved is zero. Test -02 (Set - B) Medical C10 (Laws of motion, Work, Power & Energy) ANSWER KEY 1 B 6 B 11 B 16 C 21 C 26 D 2 C 7 B 12 C 17 A 22 D 27 A 3 D 8 A 13 A 18 B 23 A 28 A 4 A 9 B 14 C 19 D 24 C 29 D 5 C 10 C 15 C 20 C 25 A 30 A SOLUTIONS 1. Here, u = 10 ms - 1 , v = - 2ms - 1 ; t = 4 s, a = ? Using a = 2 s / m 3 4 10 2 t u v - = - - = - , Force, F = ma = 10 × ( -3) = - 30 N 2. Acceleration of block, a = mass total force = m M P + ? Force on block = mass × acceleration = Ma = m M MP + . 3. Let m be the mass per unit length of rope. Therefore, mass of rope = mL. Acceleration in the rope due to force F will be a = F/mL mass of rope of length (L - ) ? will be m (L - ) ? . Therefore, tension in the rope of length (L - ) ? , is equal to pulling fore on it = m(L - ) ? a = m(L - ) ? × F/mL = F(1 - ) L / ? 4. Force required = taken time momentum in change = N 10 60 400 ) 30 10 50 ( 3 = × × × - 5. (c) Due to conservation of momentum 80 × 1 = (80 + 320)V ? V = 5 1 = 0.2 m/s. Velocity of man wrt ground =- 1 – 0.2 = 0.8 m/s. ?Displacement of man ground = (0.8 m/s) × 4s = 3.2 m 6. F = mg - R = 0.5 × 10 - 0.5 × 2 = 5 - 1 = 4N 7. m = 10 kg, x = (t 3 - 2t - 10)m ? 2 t 3 v dt dx 2 - = = and t 6 a dt x d 2 2 = = At the end of 4 secs, a = 6 × 4 = 24 m/s 2 F = ma = 10 × 24 = 240 N. 8. The formula for force is given by F 1 = ma. Acceleration of the particle a = m F 1 9. Dynamic friction force is constant 10. (c) a = rw 2 = 4 1 × (2 p ×2) 2 = 4 1 × 4 p 2 = 4 p 2 11. Mg – N = r mv 2 ? N = mg - r mv 2 . 12. Angular speed of hour hand. ? 1 = ) 60 12 ( 2 × p . Angular speed of minute hand ? 2 = 60 2 p ? 1 12 60 60 12 1 2 = × = ? ? [4] 13. There is no change in the angular velocity, when speed is constant. 14. a = 2 3 2 . 10 × ms - 1 = 6 2 . 10 = 1.7 ms - 2 ; T = 2 × 1.7N = 3.4 N 15. (C) 16. (C) 17. (A) 18. Apply law of conservation of momentum. 19. Stored elastic potential energy of spring = 2 1 kx 2 where , x is compression or elongation of spring from its natural length. In this position the spring can do work on the block tied to it, which is equal to 2 1 kx 2 , so both option (a) and (b) are correct. 20. The work done by a variable force is defined as W = ds . F ? ? . It may or may not depend on the path followed. It is always dependent on the initial and final positions. 21. The work done by friction may be positive, negative or zero. Imaging a situation in which a block of mass M is pulled horizontally by a force F on a rough horizontal surface with coefficient of friction µ. If F < µ Mg, then work done by friction force is zero. If F > µ Mg, then work done by friction force is positive. 22. All the mentioned forces are the examples of conservation force. 23. W = F s cos 90 ° = zero 24. W = F s cos ? = 10 × 2 cos 60 ° = 10 J 25. a= 2 1 2 1 m m m m + - g = 8 . 4 5 8 . 4 5 + - × 9.8ms - 2 = 0.2 ms - 2 26. (d) Assertion is false and reason is true. Since the car has smaller mass than the truck, the magnitude of its acceleration will be more than that of the truck. According to Newton’s third law, both the vehicle experience the same magnitude of force and the same impulse. Therefore, the change in momentum for the car and the truck is same. 27. (a) Both assertion and reason are true and reason is the correct explanation. When a force produces acceleration, it changes the velocity of the body and hence there is change in kinetic energy and momentum. 28. (a) Both assertion and reason are true and reason is the correct explanation of the assertion. ? = ? F . ? d = Fd cos90 = 0 ? here ? F is towards centre of circle & ? d is perpendicular to ? F . 29. (d) Assertion is false and reason is true. Kilowatt (1000 watt) is the unit of power. Kilowatt hour is the power consumed in one hour = power × time = energy. One watt is equal to Joule per second. Hence watt × time has units of energy. 30. (d) Both assertion and reason are true and reason is the correct explanation of the assertion. W = ? ? d . F =Fd cos ? = 0 because d = 0.Read More

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