4) (a) At Niagara Falls, the water drops 50 m. If the change in potential energy goes into the internal energy of the water, compute the increase in its temperature. (b) Do the same for Yosemite Falls, where the water drops 740 m. (These temperature rises are not observed because the water cools by evaporation as it falls.)

a)

b)

5) A block of ice at C whose mass is initially 50.0 kg slides along a horizontal surface starting at a speed of 5.38 m/s and finally coming to rest after traveling 28.3 m. Compute the mass of ice melted as a result of the friction between the block and the surface.

6) A heat engine absorbs heat from a thermal reservoir at 400 K, but the engine's maximum efficiency is only 31 percent. To raise the efficiency to 38 percent, what must the temperature of the reservoir be?