Physics
An object is thrown vertically up. It reaches the highest point and then comes down. The work done by the force of gravity on the object is:
- positive for both the way up and way down
- negative for both the way up and way down
- negative for the way up and positive for the way down
- positive for the way up and negative for the way down
Work, Energy & Power
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Answer
negative for the way up and positive for the way down
Reason — As, work done by an object is given by
W = FS cos θ
where,
- F is magnitude of force,
- S is magnitude of displacement, and
- cos θ is cosine of the angle θ between the directions of force F and displacement S .
Case 1 : When the object is thrown vertically up
When an object is thrown upwards then the displacement (upward) is opposite to the direction of force of gravity (downwards) i.e., θ = 180° so the work done on the body by the force of gravity is negative.
Case 2 : When the object is falling vertically down
When the object comes down, the force of gravity (downwards) is in the direction of displacement (downwards) i.e., θ = 0° and the work done by the force of gravity is positive.
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