Physics
(a) Calculate the work done when
I. a 20 kg weight is lifted 2 m vertically
II. a bike is moved on a rough road through 35 m against a frictional resistance of 20 N. (g = 9.8 ms-2 )
(b) What is the work done by a force when the force is:
I. normal to the displacement produced.
II. in the same direction as the displacement produced?
Work, Energy & Power
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Answer
(a) I. Given:
Mass (m) = 20 kg
Distance (d) = Height = 2 m
Force of gravity on mass of 1 kg = 9.8 ms-2
Work done (W) = ?
Force (F) = mg = 20 x 9.8 = 196 N
Work done = Force x distance = 196 x 2 = 392 J
II. Given,
Resisting force = 20 N
Distance = 35 m
Work done = 35 x 20 = 700 J
(b) We know that, W = Fs cos θ
I. when force is normal to the displacement, θ = 90° hence,
W = Fs cos 90° = 0 [∵ cos 90° = 0]
II. when force is in the same direction as the displacement, θ = 0° hence,
W = Fs cos 0° = Fs [∵ cos 0° = 1]
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