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WORK AND ENERGY:-- Questions for Short Answer
1. When you lift a box from the floor and put it on an almirah the potential energy of the box increases, but there is no change in its kinetic energy. Is it a violation of conservation of energy?
Answer:- System's energy is conserved when no external force is applied. Here external force has done work on the box to increase its potential energy. It is not a violation of conservation of energy.
2. A particle is released from the top of an incline of height h. Does the kinetic energy of the particle at the bottom of the incline depend on the angle of incline? Do you need any more information to answer this question in Yes or No?
3. Can the work by kinetic friction on an object be positive? Zero?
4. Can static friction do nonzero work on an object? If yes give an example? If no, give reason.
5. Can normal force do a nonzero work on an object? If yes, give an example. If no, give reason.
6. Can kinetic energy of a system be increased without applying any external force on the system?
7. Is work energy theorem valid in non-inertial frames?
8. A heavy box is kept on a smooth inclined plane and is pushed up by a force F acting parallel to the plane. Does the work done by the force F as the box goes from A to B depend on how fast the box was moving at A and B? Does the work by the force of gravity depend on this?
9. One person says that the potential energy of a particular book kept in an almirah is 20 J and the other says it is 30 J. Is one of them necessarily wrong?
10. A book is lifted from the floor and is kept in an almirah. One person says that the potential energy of the book is increased by 20 J and the other says it is increased by 30 J. Is one of them necessarily wrong?
11. In one of the exercises to strengthen the wrist and fingers, a person squeezes and releases a soft rubber ball. Is the work done on the ball positive, negative or zero during compression? During expansion?
12. In a tug of war, the team that exerts a larger tangential force on the ground wins. Consider the period in which a team is dragging the opposite team by applying a larger tangential force on the ground. List which of the following works are positive, which are negative and which are zero?
(a) work by the winning team on the losing team
(b) work by the losing team on the winning team
(c) work by the ground on the winning team
(d) work by the ground on the losing team
(e) total external work done on the two teams.
13. When an apple falls from a tree what happens to its gravitational potential energy just as it reaches the ground? After it strikes the ground?
14. When you push your bicycle up on an incline the potential energy of the bicycle and yourself increases. Where does this energy come from?
Answer:- Chemical energy (converted from food) stored in our muscle is converted to mechanical energy that increases the P.E. of myself with the bicycle. The energy in the food is received from sunlight.
15. The magnetic force on a charged particle is always perpendicular to its velocity. Can the magnetic force change the velocity of the particle? Speed of the particle?
16. A ball is given a speed v0 on a rough horizontal surface. The ball travels through a distance l on the surface and stops. (a) what are the initial and final kinetic energies of the ball? (b) what is the work done by the kinetic friction?
17. Consider the situation of the previous question from a frame moving with a speed v0 parallel to initial velocity of the block. (a) What are the initial and final kinetic energies? (b) what is the work done by the kinetic friction?
Answer:- No. The kinetic energy of the particle at the bottom of the incline does not depend on the angle of incline. It remains the same as the potential energy mgh is converted to kinetic energy. 'h' remains the same whatever be the angle of incline.
No more information is needed to answer the question.
No more information is needed to answer the question.
Answer:- Generally work done by the kinetic friction on an object is negative because the displacement is always opposite the friction force. But in some cases can be positive or zero.
For example, suppose one block is kept over other and lower block starts to move with some acceleration in such a way that upper block slides over the lower block. In this case, kinetic friction on the upper block acts along the direction of motion of lower block. Though upper block slides, even then it moves in the direction of the lower block with lesser velocity. So work done by kinetic fiction is positive.
Now suppose A block is moving over the ground. Kinetic friction acts between the block and the ground. On the block, it is acting backwards but on the ground, it is acting forwards. But there is no movement along this forward kinetic friction force, so work is zero.
For example, suppose one block is kept over other and lower block starts to move with some acceleration in such a way that upper block slides over the lower block. In this case, kinetic friction on the upper block acts along the direction of motion of lower block. Though upper block slides, even then it moves in the direction of the lower block with lesser velocity. So work done by kinetic fiction is positive.
Now suppose A block is moving over the ground. Kinetic friction acts between the block and the ground. On the block, it is acting backwards but on the ground, it is acting forwards. But there is no movement along this forward kinetic friction force, so work is zero.
Answer:- In above example suppose the upper block does not slide over the lower block. Now if the lower block moves, it applies static friction force on the upper block and the upper block moves in the direction of this static friction force. So nonzero work is done.
Answer:- Yes. For example, when a person is moving up or down in an elevator, the normal force on the person does nonzero work.
Answer:- Yes. Suppose two opposite charged particles are at rest at a separation of l. As soon as they are released the particles start to move towards each other under the force of attraction and their velocities start to increase. So the kinetic energy of the system of these two particles which was zero initially (since u = 0) begins to increase because their velocities are increasing (K.E.= ½mv²). No external force is applied to this system of two particles even then the kinetic energy increases.
Answer:- Since the dynamics of a body cannot be explained in a non-inertial frame with real forces only so work-energy theorem is not valid but if we consider pseudo forces then Newton's laws can be applied and non-inertial frame can be treated as an inertial frame and work-energy theorem is valid.
Answer:- Forces acting on the box are force F, Normal force N and weight of the box mg. Work done by the individual force F does not depend on how fast the box was moving at A and B (velocities) neither the work done by the individual force of gravity (mg). Only the work done by the resultant force on the box depends on the velocities at A and B (equal to the change in kinetic energy of the box).
Answer:- No. One is free to choose the zero potential energy configuration as the origin is chosen arbitrarily. Hence the potential energy depends on this chosen configuration.
Answer:- Yes. Because in this case difference in potential energy depends on the height of the almirah which is fixed. So only one person is right and the other is necessarily wrong.
Answer:- Since the movement is along the force during compression the work done is positive. During expansion movement is opposite so the work done on the ball is negative.
(a) work by the winning team on the losing team
(b) work by the losing team on the winning team
(c) work by the ground on the winning team
(d) work by the ground on the losing team
(e) total external work done on the two teams.
Answer:-
(a) Positive
(b) Negative
(c) Zero (Assuming that work by ground means work by frictional force by ground, Also assuming the winning team does not step backwards)
(d) Negative (Assuming that work by ground means work by frictional force by ground, Losing team is dragged)
(e) Zero (Taking rope, both teams and ground as a system, no external force is on the system, so no external work done)
(a) Positive
(b) Negative
(c) Zero (Assuming that work by ground means work by frictional force by ground, Also assuming the winning team does not step backwards)
(d) Negative (Assuming that work by ground means work by frictional force by ground, Losing team is dragged)
(e) Zero (Taking rope, both teams and ground as a system, no external force is on the system, so no external work done)
Answer:- Since mechanical energy is conserved when no external force is applied, in this case, the gravitational potential energy is converted to kinetic energy just as it reaches the ground. After it strikes the ground, a part of this energy is used to deform the apple and other is converted to sound and heat energy.
Answer:- Chemical energy (converted from food) stored in our muscle is converted to mechanical energy that increases the P.E. of myself with the bicycle. The energy in the food is received from sunlight.
Answer:- Since the magnetic force on the particle is perpendicular to its velocity, it can not change the speed of the particle but it can change its direction. Due to change in the direction, the velocity changes.
Answer:- (a) Initial Kinetic energy = ½mv0²
Final Kinetic Energy = 0
(b) Forces on the ball are weight, normal force and kinetic friction. Since weight and normal force are equal and opposite, the resultant force on the ball is equal to kinetic friction. Work done by the resultant force is equal to change in kinetic energy, so work done by kinetic friction = 0-½mv0² = - ½mv0²
Answer:- (a) In this moving frame initial velocity becomes zero, so initial K.E. =0, Final velocity = -v0 , So final K.E. = ½mv0²
(b) work done by kinetic friction = ½mv0²
(b) work done by kinetic friction = ½mv0²
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Links for the chapter -
ALL LINKS
CHAPTER- 10 - Rotational Mechanics
Questions for Short Answers
OBJECTIVE - I
OBJECTIVE - II
EXERCISES - Q-1 TO Q-15
EXERCISES - Q-16 TO Q-30
CHAPTER- 10 - Rotational Mechanics
Questions for Short Answers
OBJECTIVE - I
OBJECTIVE - II
EXERCISES - Q-1 TO Q-15
EXERCISES - Q-16 TO Q-30
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
HC Verma's Concepts of Physics, Chapter-8, WORK AND ENERGY
Click here for → Question for Short AnswersClick here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Exercises (11-20)
Click here for → Exercises (21-30)HC Verma's Concepts of Physics, Chapter-7, Circular Motion
Click here for → Questions for Short Answer
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
HC Verma's Concepts of Physics, Chapter-6, Friction
Click here for → Friction OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → Friction - OBJECTIVE-II
Click here for → Questions for Short Answer
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
For more practice on problems on friction solve these "New Questions on Friction" .
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
For more practice on problems on friction solve these "New Questions on Friction" .
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HC Verma's Concepts of Physics, Chapter-5, Newton's Law's of Motion
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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HC Verma's Concepts of Physics, Chapter-4, The Forces
"Questions for short Answers"
Click here for "The Forces" - OBJECTIVE-I
Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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Sir in question 12 part (e) if we don't consider ground as a part of system then work done by external force should be negative due friction ,right sir?
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteSir, in question 16 and 17, the initial speed is given as v instead of v0.
ReplyDeleteBecause of this the answer to the question 17 may change