Physics Part 14

5. Cartesian diver.

6. Hydraulic ram.

7. Balloon.

8. Air brake.

9. Gas meter.

10. Vacuum cleaner.

Exercises

1. Explain why smoke settles to the ground before storms.

2. Why does the water rise in the suction pipe of a pump?

3. Why is it easier to float in water when the lungs are filled with air than when they are not filled?

4. Why is it easier to swim in salt water than in fresh water?

5. How are submarines made to sink? to rise to the surface?

6. How can a fish rise or sink in water?

7. Explain why a life preserver made of cork will enable a person to float.

8. Hold the open hand out flat with the fingers together. Place underneath the fingers a piece of paper. Blow between the first and second fingers against the paper. As long as you blow hard the paper will not fall but will stick to the hand. Explain.

9. Why does pressing the bulb of an atomizer force out the liquid in a fine spray?

10. Why is air that contains a large amount of water vapor lighter than air that only contains a small amount?

11. How are heights above sea-level ascertained by a barometer?

12. Oil floats on water but sinks in alcohol. Explain.

13. In a balloon the lower end is often open to the air. Why does not the gas escape and prevent the balloon from rising?

14. How long will a balloon continue to rise?

15. If the pressure against the 8-in. piston of an air brake is 70 lbs.

per square inch, how much force does the piston exert?

16. The capacity of a balloon is 40,000 cu. ft. The weight of the balloon, car, etc., is 600 lbs.; specific gravity of the gas used is 0.46 that of the air. Find how much weight the balloon can carry.

17. The so-called Magdeburg hemispheres were invented by Otto von Guericke of Magdeburg, Germany. When the hemispheres (see Fig. 54) are placed in contact and the air exhausted it is found very difficult to pull them apart. Explain.

18. Von Guericke's hemispheres had an inside diameter of 22 in. What force would be required to pull them apart if all the air were exhausted from them? (Find the atmospheric force on a circle, 22 in. in diameter.)

19. Von Guericke made a water barometer whose top extended through the roof of his house. On the top of the water in the tube was placed a wooden image. In fair weather the image appeared above the roof, but it descended before a storm. Explain.

20. The balloon "Goodyear" (Fig. 48), which won the International championship race at Paris in 1913, has a capacity of 80,000 cu. ft. The gas bag weighs 653 lbs., the net 240 lbs. and the basket 92 lbs. How large a load can it carry when filled with hydrogen specific gravity 0.069 (compared with air).

[Ill.u.s.tration: FIG. 54.--Magdeburg hemispheres.]

Review Outline: Liquids and Gases

Liquids: Force, pressure, and density. Floating and immersed bodies.

Laws: Liquid force, _F = A.h.d_, Pascal's, Archimedes. Ill.u.s.trations and Applications:

Specific gravity, _W_{a}/(W_{a} - W_{w})_, _(W_{a} - W_{l})/(W_{a} - W_{w})_, Boyle's, _PV = P'V'_

Devices: Hydraulic press, air cushion, barometer--mercurial and aneroid.

Pumps, lift, force, vacuum, compression, centrifugal, balloon, siphon, etc. Construction and action of each.

CHAPTER V

FORCE AND MOTION

(1) FORCE, HOW MEASURED AND REPRESENTED

=71. Force.=--We have been studying various forces, such as air pressure, pressure in liquids, and the force of elasticity in solids, and have considered them simply as pushes or pulls. A more formal study of forces in general and of devices for representing and measuring them will be helpful at this point of the course.

_A force is that which tends to cause a change in the size or shape of a body or in its state of motion._ In other words a force is a push or a pull. That is, force tends to produce distortion or change of motion in a body. Force itself is invisible. We measure it by the effect it produces. Forces are usually a.s.sociated with the objects exerting them.

Thus we speak of _muscular force_, _air pressure_, _liquid pressure_, the force of a spring, the force of the earth's attraction and so on.

Forces are cla.s.sified in various ways.

I. With respect to the _duration and steadiness_ of the force.

(a) Constant, as the earth's attraction. (b) Impulsive, as the stroke of a bat on a ball. (c) Variable, as the force of the wind.

II. With respect to the _direction_ of the force.

(a) Attractive, as the earth's attraction. (b) Repulsive, as air pressure, liquid pressure, etc.

=72. Methods of Measuring Force.=--Since forces are measured by their effects which are either distortion or change of motion, either of these effects may be used to measure them. For example, the force exerted by a locomotive is sometimes computed by the _speed_ it can develop in a train of cars in a given time, or the force of the blow of a baseball bat is estimated by the _distance_ the ball goes before it strikes the ground.

The more common method of measuring force, however, is by _distortion_, that is, by measuring the change of shape of a body caused by the force.

In doing this, use is made of Hooke's Law (Art. 32), in which it is stated that "within the limits of perfect elasticity," changes of size or shape are directly proportional to the forces employed. That is, twice as great a force will produce twice as great a change of shape and so on.