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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 friction on a body sliding down an inclined plane.
Solution
The work done by friction on a body sliding down an inclined plane is positive. Friction always operates in a direction that is opposite to the motion. Therefore, θ= 180°.
`W =F·dcosθ = F·dcos180° = -Fd`. (negative)
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