A block moving on a horizontal surface experiences a force of friction of 15 N in the opposite direction. If the displacement of the block is 5 m, how much work is done by the force of friction?
- 75 J
Work done by a constant force is \(W = Fs\cos\theta\), where \(\theta\) is the angle between the force and the displacement. Friction acts opposite to the motion, so \(\theta = 180^\circ\) and \(\cos 180^\circ = -1\).
Substituting \(F = 15\) N and \(s = 5\) m gives \(W = 15 \times 5 \times (-1) = -75\) J. The negative sign shows that friction takes energy away from the block.
Check: the size of the work is \(15 \times 5 = 75\) J, and since friction acts against the displacement the work must be negative, so \(-75\) J is consistent with both the magnitude and the sign.
Hence, - 75 J.
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