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The sign of work done by a force on a bo...

The sign of work done by a force on a body is important to understand. State carefully if the following quantities are poisitve 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 iniclined 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.

Text Solution

Verified by Experts

Work done `W=vecF. vecs=Fs cos theta` where `theta` is smaller angle between force `vec F` and displacement `vec s`.
(a) To lift the bucket, force equal to weight of bucket has to be applied vertically upwards and the bucket moves along the same direction i.e., `theta=0^(@)` therefore `W=F s cos 0^(@)=F s.` It si positive.
(b) The bucket moves in a direction opposite to the gravitational force which acts vertically downwards.
`:. theta=180^(@), W=F s cos 180^(@)=-Fs.` It is negative.
(c) Friction always opposes the relative motion `:. theta =180^(@), W=F s cos 180^(@)=-Fs`. It is negative.
(d) As the body moves along the direction of applied force `theta=0^(@), :. W=F s cos 0^(@)=Fs.`. It is positive.
(e) The direction of resistive force is opposite to the direction of motion of the bob, i.e., `theta=180^(@).`. Hence, work done, in this case, is negative.
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The sign of work done by a force is important to understant. 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 the 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 froce 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.

Work Done By Gravitational Forces

Knowledge Check

  • Work done by frictional force

    A
    Is always negative
    B
    Is always positive
    C
    Is zero
    D
    May be positive, negative or zero
  • Work done by a frictional force is

    A
    negative
    B
    positive
    C
    zero
    D
    all of the above
  • the work done by applied force is

    A
    10 J
    B
    50 J
    C
    100 J
    D
    150 J
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