Physics Questions and Answers

Answer: D

Because there is no air or other gas in the jar.

Sound is a mechanical wave and needs a material medium. With the jar evacuated, the bell can be seen striking but nothing carries the vibration to the glass. This experiment is the classic demonstration that sound, unlike light, cannot travel through a vacuum.

Answer: C

Because alcohol has a very low freezing point.

Ethanol freezes at about \(-115\) °C, so it stays liquid in the severest cold, whereas mercury freezes at \(-39\) °C and would be useless. Alcohol also expands more per degree, giving a more open scale, though it must be dyed to be visible.

Answer: B

It is equal to the body's weight.

By Newton's second law the rate of change of momentum equals the applied force, and for a body falling freely the only force acting is gravity: \[\frac{dp}{dt} = F = mg\] which is the weight.

Answer: C

An air bubble in water behaves like a concave lens.

What matters is the refractive index of the lens relative to its surroundings. Air is optically rarer than water, so a bubble that is convex in shape diverges light instead of converging it, and acts as a diverging - concave - lens.

Answer: A

The parallactic second is not a unit of time.

Despite the name, the parallactic second, or parsec, measures distance: it is the distance at which one astronomical unit subtends one arcsecond, about 3.26 light years. Microsecond, leap year and solar day all measure time.

Answer: B

A falling barometer indicates decreasing pressure.

The barometer measures atmospheric pressure directly, and a fall means a low-pressure system is approaching. Rising air in a low cools and condenses, so a falling glass generally foretells cloud, rain or, if the fall is rapid, a storm.

Answer: B

Raindrops are spherical because of surface tension.

Molecules at the surface are pulled inwards by their neighbours, so the drop contracts until its surface area is as small as possible for the volume it contains - and for a fixed volume the sphere is that shape. Large falling drops are flattened by air resistance.

Answer: B

Because metals contain free electrons.

The valence electrons in a metal are delocalised over the whole lattice rather than bound to individual atoms, and an applied field sets them drifting. The same electron sea carries heat, which is why good electrical conductors are also good thermal ones.

Answer: D

Light is slowest in glass.

Speed in a medium is \(c/n\), so the higher the refractive index the slower the light. Glass has \(n \approx 1.5\), water 1.33, air 1.0003 and vacuum exactly 1 - so the order of speeds runs the other way.

Answer: B

The amount of water vapour in the air is called humidity.

Absolute humidity is the mass of vapour per unit volume; relative humidity, the figure quoted in weather reports, is the ratio of the actual vapour pressure to the saturation value at that temperature. Dew forms when air is cooled until its relative humidity reaches 100 per cent.