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Question
The sign of work done by a force on a body is important to understand. State carefully if the following quantity is positive or negative:
Work done by the resistive force of air on a vibrating pendulum in bringing it to rest.
Solution
The work done by the resistive force of air on a vibrating pendulum in bringing it to rest is negative.This is because the resistive force's direction is opposite to the pendulum's motion. Thus, θ = 180°,
`W = Fdcosθ = F.dcos 180° = -Fd` (negative)
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