A 0.75-kg block slides with a uniform velocity down a 20° inclined plane (▼ Figure 5.6). (a) How much work is done by the force of friction on the block as it slides the total length of the plane? (b) What is the net work done on the block? (c) Discuss the network done if the angle of incline is adjusted so that the block accelerates down the plane.
THINKING IT THROUGH. (a) The length of the plane can be found using trigonometry, so this part boils down to finding the force of friction. (b) The network is the sum of all the work done by the individual forces. (Note: Since the block has a uniform, or constant, velocity, the net force on it is zero. This observation should tell you the answer, but it will be shown explicitly in the solution.) (c) If there is acceleration, Newton’s second law applies, which involves a net force, so there may be a network.

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1-20 L-12m A FIGURE 5.6 Total or net work See Example 5.3 for description. g sin Mecse Free-body diagram