Physics Questions and Answers

Answer: A

Newton's first law states that every body maintains its state of rest or of uniform motion in a straight line unless acted on by an external force.

It defines inertia and identifies the inertial frames in which the second and third laws hold. It is not a special case of \(F = ma\) but a statement about which frames the second law applies in.

Answer: C

Because the water that oozes out does not evaporate quickly, the air being already humid.

An earthen pot cools by evaporation of the water seeping through its pores, and evaporation draws latent heat from what remains. On a rainy day the air is close to saturation, so evaporation is slow and little heat is carried away.

Answer: D

Einstein received the Nobel Prize for the photoelectric effect.

The 1921 prize cited his services to theoretical physics and especially his discovery of the law of the photoelectric effect, \[h\nu = \phi + \tfrac{1}{2}mv^2_{max}\] Relativity was still thought too speculative by the committee to be honoured.

Answer: B

Sodium is the softest of these.

It rates about 0.5 on the Mohs scale and can be cut with a knife. Alkali metals are soft because each atom contributes only one electron to the metallic bond, and the bond weakens further as the atom grows; sodium is softer than lithium for that reason.

Answer: C

A solar eclipse occurs when the moon comes between the sun and the earth.

The moon's shadow falls on the earth, and only those in the narrow path of the umbra see totality. It can happen only at new moon, and not at every new moon, because the moon's orbit is tilted about 5° to the ecliptic.

Answer: D

It loses heat by conduction, convection and radiation.

Conduction carries heat into the metal table, convection carries it away in rising currents of warm air, and radiation carries it off as infrared. All three act together whenever a hot body sits in air on a solid surface.

Answer: C

One watt-hour equals \(3.6\times10^{3}\) J.

\[1\ \mathrm{Wh} = 1\ \mathrm{W}\times 3600\ \mathrm{s} = 3600\ \mathrm{J}\] A kilowatt-hour, the unit on an electricity bill, is a thousand times this, \(3.6\times10^{6}\ \mathrm{J}\).

Answer: C

II and III - refraction and internal reflection.

Sunlight entering a raindrop is refracted and dispersed into its colours, reflects once off the far inner surface, and is refracted again on leaving. It emerges at about 42° to the incoming ray, which is why the bow is an arc of fixed angular radius. Diffusion plays no part.

Answer: D

All of the above.

Radio waves pass through cloud, dust and the daylight sky, which stop or swamp visible light, so a radio telescope works around the clock and in any weather. It also detects sources - pulsars, quasars, cold hydrogen clouds - that emit little or no visible light at all.

Answer: B

Planets are non-luminous heavenly bodies.

They shine only by reflecting sunlight, unlike stars, which generate their own light by fusion. Planets also appear as small discs rather than points, which is why they shine steadily while stars twinkle.