1.what is viscocity? 2.will a helium filled balloon rise indefinitely in the air

Question

1.what is viscocity?
2.will a helium filled balloon rise indefinitely in the air?explain.
3.what do you mean by spherical aberration?
4.does the focal length of a mirror change when it is immersed in water?
5.during winter,water pipes brust.why?would the mercury thermometer break if the temperature went below the freezing point of mercury?why or why not?
6.what is centripetal force?derive an expression for it.
7.derive the relation between volume of liquid entering into tube per second which depends on (i)pressure gradient(ii)radius of tube(iii)coefficient 0f viscosity

Explanation / Answer

1) Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction"

2) NO. Although, helium is lighter/less dense than normal air, but only at or close to sea level. As the balloon rises, the atmosphere becomes thinner (i.e., less dense), and at a certain point, the density of the balloon will match the density of the air, and it will hover (assuming it doesn't pop or lose any helium).

3) Spherical aberration is an optical effect observed in an optical device (lens, mirror, etc.) that occurs due to the increased refraction of light rays when they strike a lens or a reflection of light rays when they strike a mirror near its edge, in comparison with those that strike nearer the centre. It signifies a deviation of the device from the norm, i.e., it results in an imperfection of the produced image

4) Mirrors, spherical or otherwise, operate on the principle that the angle of reflection of a ray of light equals the angle at which it strikes the mirror's surface. That behavior is not affected by the medium in which the light travels, so the mirror's focal length would be no different in air or water.

5) During the winter, the cold air is surrounding the pipes, which has water. The air turns the water inside the pipes to ice. When water freezes, it expandes. The frozen water expanded just enough to cause the pipe to burst.

5**) Mercury freezes at -39.8° C (about -40° F.)

My guess would be that the thermometer doesn't break when the temperature gets too low, but when it starts to warm up again.

Once it starts to warm again after the mercury has frozen, it will expand as a solid (below -40°) before it changes state back to a liquid. When it does this, it can not flow through the small aperture from the bulb to the guage.

When that happens, the pressure increases in the bulb and the bulb breaks.

Another possible cause (going the other way) is that once the mercury freezes, if it continues to get colder the solid mercury contracts even more. This creates a vacuum in the bulb and the pressure differential between the air pressure and the vacuum breaks the glass.

6) Centripetal force  is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path

FOR DERIVATION: Consult http://en.wikipedia.org/wiki/Centripetal_force

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