Chapter - 4 Work and Energy

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Q
Work and Energy

Question:

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.

Answer:

Case (i): In this case, the applied force and the displacement both are perpendicular to each other, the angle 	heta = 90 degree  and the value of cos90 degree = 0.

We know, work done on an object is given by,

W = FScos	heta

So, work done  W = FS 	imes 0 = 0

Hence, the work done is zero.

Case (ii): In this case, the applied force and the displacement are in the same direction, the angle	heta = 0degree  and the value of cos0 degree = 1.

We know, work done on an object is given by,

W = FScos	heta

So, work done  W = FS 	imes 1 = FS

Hence, the work done is positive.

Case (iii): In this case, the applied force acts opposite to the direction of displacement, the angle 	heta = 180degree  and the value of cos180 degree = -1

We know, work done on an object is given by,

W = FScos	heta

So, work done  W = FS 	imes -1= -FS

Hence, the work done is negative.


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