Exercises

1. In the last chapter we learnt that a satellite could be placed in geostationary orbit above the Earth only at a certain height.
   (a) Why is it then that in the absence of wind, balloons can stay at stationary points above the earth at much lower altitudes?
   (b) Why is it that for the satellite problem we could ignore some of the physics issues that crop up for the balloon?
   (c) Why not use balloons as communication satellites?
   (d) Other than recreation, are there any useful uses of balloons?
   (e) If a helium ballon was released on the Moon, where would it go ?
2. There are plans to build a self-contained floating city, with permanent residents, that will encircle the globe every two years. The ``Freedom Ship" (Ref.[1]) will be about 1.3 km long, 200m wide and 100m tall. It will weigh about 2.7 million tons.
   (a) How much water will the ship displace?
   (b) How much of the ship will be under water?
   
3. (a) Waterproof watches have a maximum depth to which they can safely be taken while swimming. Why?
   (b) Why must tall dams be much thicker at their bases than their tops?
4. (a) What causes water to rise up a drinking straw?
   (b) Why can't you drink from a long straw if you're more than 10 m above the drink?
   (c) How do vacuum cleaners create the ``vacuum” and how does this allow them to draw in matter?
5. Water can be drained from a container using a siphon.
   (a) How does a siphon work?
   (b) The toilet tank uses a siphon, valve and float to operate and regulate the flush. Explain how this is done. (Hint: See Ref.[3])
6. Suppose that the water pressure at a fire-hydrant at street level is 500,000 Pa above atmospheric pressure. Firemen have connected a hose to the hydrant to use in battling a fire in a tall building.
   (a) How high can the firemen take the hose inside the building and still expect water to flow out of it?
   (b) If instead, the firemen stay at street level and spray the water up, how high will the water go?
   (c) One way to boost the water pressure is to send it through pumps in the fire-engines. If a single fire-engine is used to boost the pressure by 500,000 Pa, what is the result for the above two questions?
7. Describe the action of piston pumps such as those used in water pistols and bicycle pumps. (Hint: See p132 of Ref. [4].)
8. A car traveling at high-speed requires a large force to stop it.
   (a) How is it possible for unassisted (non-power) brakes to apply such a large force?
   (b) Is energy conserved? Where does it go?
   (c) Which moves a greater distance: The foot-pedal or the brake-pads? Why?
9. (a) Why are there water tanks at the top of tall buildings?
   (b) At what time of the day would the pumps be working to pump the water up? Why?
   (c) Would it be desirable for a skyscraper to have one tank at the top or several at different levels? Why?
   
10. Read about "Submarines" in Ref.[6] and then answer the following questions briefly.
    (a) How do submarines change their buoyancy under normal conditions to dive or surface?
    (b) What constrains the depth to which submarines can dive?
    (c) How far down can modern submarines go?
    (d) What pressure does water exert on a submarine submerged to the depth you answered in above?
    (e) How does a submerged submarine navigate under water?
    (f)Where does a submarine get its supply of fresh-water for long journeys?
    (g) How is the air inside a submarine kept breathable?
    (h) How is the moisture that is exhaled by the crew removed? Why must it be removed?
11. Consider the following statement: In general, older technologies tend to serve recreational or aesthetic roles once the replacement (by newer technologies) is complete. This was the case with fuel wood, sailing ships, horses, convertible (open) cars, and many other examples." -- N. Nakicenovic (1986).
    (a) Give another example in support of the statement.
    (b) Think of a counterexample. What causes are responsible for your counterexample?
    (c) Are there examples of older technologies co-existing with newer technologies that perform similar functions? How did this come about?
12. Read the article in Ref.[5] on the origin of the QWERTY keyboard for class discussion.
    (a) Did you know that a more efficient keyboard due to Dvorak was specifically designed to reduce finger movement, is faster to use and less tiring?
    (b) Compare the performance of the QWERTY and Dvorak keyboards at Ref.[7].
    (c) Did you know that you can easily reconfigure your keyboard to the Dvorak system? See Ref.[7].