Physics Questions and Answers

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: A

X-rays are produced in a Coolidge tube.

A heated tungsten filament emits electrons, which are accelerated through tens of kilovolts onto a tungsten target. Their sudden deceleration gives bremsstrahlung, and knocked-out inner-shell electrons give the characteristic lines. The hot cathode lets the tube's current and voltage be controlled independently, which the older gas tubes could not do.

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.

Answer: C

The bomb strikes the ground 3.33 km from A.

Vertically it starts with no downward velocity: \[t = \sqrt{\frac{2h}{g}} = \sqrt{\frac{2\times1960}{9.8}} = 20\ \mathrm{s}\]

Horizontally it keeps the aircraft's speed of \(166.7\ \mathrm{m\,s^{-1}}\): \[x = 166.7\times20 \approx 3.33\ \mathrm{km}\]

Answer: A

The rain will appear to come from the east, that is from in front of him.

Relative velocity of rain with respect to the man is \(\vec{v}_{rain} - \vec{v}_{man}\). Subtracting his eastward velocity adds a westward component to the rain's downward velocity, so the apparent direction tilts towards the east - which is why one leans an umbrella forward when walking in rain.

Answer: B

Three unequal vectors can give a zero resultant.

Two vectors sum to zero only if they are equal in magnitude and opposite in direction, so unequal ones cannot. Three can, provided they can be placed head to tail as the sides of a closed triangle and are coplanar.

Answer: A

The volume of the cavity will increase.

On heating, every linear dimension of the solid grows by the same factor, and the cavity scales with the rest - it expands exactly as if it were filled with the same metal. This is why a hole in a heated plate gets bigger, not smaller.

Answer: D

The colours arise from multiple reflection and interference.

Light reflected from the front surface of the film and light reflected from the back travel different distances and interfere. The path difference depends on both thickness and wavelength, so different colours cancel or reinforce at different points, giving the shifting bands.

Answer: D

When a body is accelerated, its speed may or may not change.

Acceleration is the rate of change of velocity, which is a vector. A body in uniform circular motion is accelerating continuously while its speed stays constant, because only the direction changes.

Answer: C

Nuclear radii are expressed in fermi.

One fermi, or femtometre, is \(10^{-15}\) m, and nuclear radius follows \(R = R_0A^{1/3}\) with \(R_0 \approx 1.2\) fm. The angstrom, \(10^{-10}\) m, is a hundred thousand times larger and suits atoms.