Ncert Exercises & Solutions

Q. 24 A body is being raised to a height h from the surface of the earth. What is the sign of work done by

(a) applied force and

(b) gravitational force?

Hint: Work =

\vec{F .} \vec{dx}

(a)Step 1: Find work done by the applied force.

Force is applied to the body to lift it in an upward direction and displacement of the body is also in an upward direction, therefore, the angle between the applied force and displacement is $θ = 0 °$

$∴$ Work is done by the applied force

$W = Fs \cos θ = Fs \cos 0^{\circ} = Fs (∵ \cos 0^{\circ} = 1)$

i.e., W = positive

Step 2: Find work done by gravitational force.

(b) The gravitational force acts in the downward direction and displacement in the upward direction, therefore, the angle between them is $θ = 180 °$
$∴$ Work was done b the gravitational force

$W = Fs \cos 180 ° = - Fs (∵ \cos 180 ° = 1)$

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