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Question

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.

Solution
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As we know,

Workdone=F.S

Workdone=FScosθ

case(1)ifθ00

workdone=FS(1)=FSmax

case(2)ifθ900

workdone=FS(0)=0

case(3)ifθ1800

workdone=FS(1)=FSmini

Work done is considered positive when the displacement is in the same direction as that of the applied force and negative when the displacement is in the opposite direction of applied force.

(a)Positive : 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) Negative : 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) As force and displacement are in opposite directions, work done is negative.

(d) As force and displacement are in the same direction, work done is positive.

(e) As force and displacement are in opposite directions, work done is negative.

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Similar Questions
Q1

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.

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Q2
Statement (A): Work done by a man in lifting a bucket out of well, by means of a rope tied to the bucket, is positive.
Statement (B): In the above case, work done by the gravitational force is negative.
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Q3

Work done by gravitational force on a man, in lifting a bucket out of the well by rope tied to the bucket is


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Q4
In which of the following cases is the work done positive or zero or negative?
a) Work done by the porter on a suitcase in lifting it from the platform on to his head.
b) Work done by the force of gravity on suitcase as the suitcase falls from porter's head.
c) Work done by the porter standing on platform with suitcase on his head.
d) Work done by force of gravity on a ball thrown up vertically up into the sky.
e) Work done by force applied by hands of a man swimming in a pond.
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Q5

A body of mass 2 kg initially at rest moves under the action of an applied horizontal force of 7 N on a table with coefficient of kinetic friction = 0.1.

Compute the

(a) work done by the applied force in 10 s,

(b) work done by friction in 10 s,

(c) work done by the net force on the body in 10 s,

(d) change in kinetic energy of the body in 10 s,

and interpret your results.

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