NCERT Class 11 Physics Solutions: Chapter 6 – Work, Energy, and Power Part 1

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Question 6.1:

The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative:

(a) work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket,

(b) work done by gravitational force in the above case,

(c) work done by friction on a body sliding down an inclined plane,

(d) work done by an applied force on a body moving on a rough horizontal plane with uniform velocity,

(e) work done by the resistive force of air on a vibrating pendulum in bringing it to rest.

Answer:

(a) work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket: Positive

Explanation:

In the given case, force and displacement are in the same direction. Hence, the sign of work done is positive. In this case, the work is done on the bucket.

(b) work done by gravitational force in the above case: Negative

Explanation:

In the given case, the direction of force (vertically downward) and displacement (vertically upward) are opposite to each other. Hence, the sign of work done is negative.

(c) work done by friction on a body sliding down an inclined plane: Negative

Explanation:

Since the direction of frictional force is opposite to the direction of motion, the work done by frictional force is negative in this case.

(d) work done by an applied force on a body moving on a rough horizontal plane with uniform velocity: Positive

Explanation:

Here the body is moving on a rough horizontal plane. Frictional force opposes the motion of the body. Therefore, in order to maintain a uniform velocity, a uniform force must be applied to the body. Since the applied force acts in the direction of motion of the body, the work done is positive.

(e) work done by the resistive force of air on a vibrating pendulum in bringing it to rest: Negative

Explanation:

The resistive force of air acts in the direction opposite to the direction of motion of the pendulum. Hence, the work done is negative in this case.

Question 6.1:

A body of mass initially at rest moves under the action of an applied horizontal force of on a table with coefficient of kinetic friction Compute the

(a) work done by the applied force in

(b) work done by friction in

(c) work done by the net force on the body in

(d) change in kinetic energy of the body in , and interpret your results.

Answer:

Mass of the body,

Applied force,

Coefficient of kinetic friction,

Initial velocity,

Time,

The acceleration produced in the body by the applied force is given by Newton’s second law of motion as:

Frictional force is given as:

The acceleration produced by the frictional force:

Total acceleration of the body:

The distance travelled by the body is given by the equation of motion:

(a) work done by the applied force ,

(b) work done by friction force,

(c) work done by the net force on the body,

Work done by the net force,

(d) change in kinetic energy of the body,

From the first equation of motion, final velocity can be calculated as:

Change in kinetic energy