Q#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.
Q#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?
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.
Q#3
Can the work by kinetic friction on an object be positive? Zero?
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 case 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.
Q#4
Can static friction do nonzero work on an object? If yes give an example? If no, give reason.
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.
Q#5
Can normal force do a nonzero work on an object? If yes, give an example. If no, give reason.
Answer:
Yes. For example, when a person is moving up or down in an elevator, the normal force on the person does nonzero work.
Q#6
Can kinetic energy of a system be increased without applying any external force on the system?
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.
Q#7
Is work energy theorem valid in non-inertial frames?
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.
Q#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?
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).
Post a Comment for "Answers to Problems on (Work and Energy) HC Verma's Questions for Short Answer (1-8)"