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Chapter 7: Q32P (page 318)
A certain motor is capable of doing 3000 J of work in 11 s . What is the power output of this motor?
The power output of this motor is 272.2 W.
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A sky diver whose mass is 90 kg, is falling at a terminal speed of 60 m/s. What is the magnitude of the force of the air on the sky diver?
Question: Design a “bungee jump” apparatus for adults. A bungee jumper falls from a high platform with two elastic cords tied to the ankles. The jumper falls freely for a while, with the cords slack. Then the jumper falls an additional distance with the cords increasingly tense. You have cords that are10m long, and these cords stretch in the jump an additional 24mfor a jumper whose mass is 80kg, the heaviest adult you will allow to use your bungee jump (heavier customers would hit the ground). You can neglect air resistance. (a) Make a series of five simple diagrams, like a comic strip, showing the platform, the jumper, and the two cords at various times in the fall and the rebound. On each diagram, draw and label vectors representing the forces acting on the jumper, and the jumper’s velocity. Make the relative lengths of the vectors reflect their relative magnitudes. (b) At what instant is there the greatest tension in the cords? How do you know? (c) What is the jumper’s speed at this instant? (d) Is the jumper’s momentum changing at this instant or not? (That is, isnonzero or zero?) Explain briefly. (e) Focus on this instant, and use the principles of this chapter to determine the spring stiffnessfor each cord. Explain your analysis. (f) What is the maximum tension that each cord must support without breaking? (g) What is the maximum acceleration (in g’s) that the jumper experiences? What is the direction of this maximum acceleration? (h) State clearly what approximations and estimates you have made in your design.
of water whose temperature isare added to a thin glass containing 800 g of water at(about room temperature). What is the final temperature of the water? What simplifying assumptions did you have to make in order to determine your approximate result?
You observe someone pulling a block of mass 43kg across a low-friction surface. While they pull a distance of 3m in the direction of motion, the speed of the block changes from 5 m/s to 7 m/s. Calculate the magnitude of the force exerted by the person on the block. What was the change in internal energy (chemical energy plus thermal energy) of the person pulling the block?
Question: In the Niagara Falls hydroelectric generating plant, the energy of falling water is converted into electricity. The height of the falls is about 50m. Assuming that the energy conversion is highly efficient, approximately how much energy is obtained from one kilogram of falling water? Therefore, approximately how many kilograms of water must go through the generators every second to produce a megawatt of power 1 X 106W?
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