Question 80: Work, Energy and Power & Circular Motion

Question

Identify the correct statements from the following :
A. Work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket is negative.
B. Work done by gravitational force in lifting a bucket out of a well by a rope tied to the bucket is negative.
C. Work done by friction on a body sliding down an inclined plane is positive.
D. Work done by an applied force on a body moving on a rough horizontal plane with uniform velocity is zero.
E. Work done by the air resistance on an oscillating pendulum is negative.
Choose the correct answer from the options given below :

Accepted Answer

When a man lifts a bucket out of a well using a rope, work is done by the man and the gravitational force. The work done by the man is positive as he has to exert an upward force to lift the bucket. The work done by the gravitational force is negative because the direction of the force is opposite to the direction of displacement.
Therefore, the
statement (A)
"Work done by a man in lifting a bucket out of a well by means of rope tied to the bucket is negative." is
incorrect
.
Therefore, the
statement (B)
"Work done by gravitational force in lifting a bucket out of a well by a rope tied to the bucket is negative." is
correct
.
Work is defined as the product of force and displacement in the direction of the force. When a body slides down an inclined plane, the force of friction acts against the motion of the body, opposing its descent.
The direction of the force of friction is opposite to the direction of the displacement of the body, which is downwards. Hence, the work done by the force of friction is negative.
Therefore, the
statement (C)
"Work done by friction on a body sliding down an inclined plane is positive" is
incorrect
.
If the body is moving on a rough horizontal plane, there will be friction present, which will act in the opposite direction to the applied force. The force of friction will oppose the motion of the body, reducing its velocity. As a result, the net work done on the body will not be zero, as the force of friction and the applied force will not cancel each other out completely.
Therefore, the
statement (D)
"Work done by an applied force on a body moving on a rough horizontal plane with uniform velocity is zero." is
incorrect
.
Statement E:
"Work done by the air resistance on an oscillating pendulum is negative."
This statement refers to the work done by the air resistance on an oscillating pendulum, which is a physical system that swings back and forth under the influence of gravity.
As the pendulum oscillates, it experiences air resistance, which opposes its motion and slows it down. The direction of the air resistance force is opposite to the direction of the displacement of the pendulum, which is back and forth.
Hence, the work done by the air resistance force is negative, as the direction of the force and the displacement are opposite.
Therefore, the
statement (E)
"Work done by the air resistance on an oscillating pendulum is negative" is
correct
.

Answer

A and C only

Answer

B and D only

Answer

B, D and E only

Answer

B and E only

Work, Energy and Power & Circular Motion

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