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A block with mass $m$ slides a distance $x$ down a ramp with angle $\theta$. The coefficient of kinetic friction between the block and the ramp is $\mu$.
How much work $W$ is done on the block by friction?
$W = -\mu mgx \cos\theta$
$W = \mu mgx\cos\theta$
$W = \mu mgx\cos^2\theta$
$W = -\mu mgx$