Physics Questions and Answers

Answer: D

The answer is Dispersion and total internal reflection.

Sunlight entering a raindrop is refracted and split into its colours (dispersion), reflected from the back surface of the drop, and refracted again as it leaves. The textbook account names this internal reflection as total internal reflection, and the paper follows that account.

Dispersion alone would not send the light back towards an observer standing with the sun behind him, and reflection alone would not separate the colours, so each single process is incomplete. Scattering is what makes the sky blue; it does not form a rainbow.

Answer: D

Sound travels fastest in humid air at 30 degree centigrade.

Two effects raise the speed of sound in air. It increases with temperature, by about 0.6 m/s for every degree Celsius, because the molecules move faster. And it increases with humidity, because water vapour (molecular mass 18) is lighter than the nitrogen and oxygen it replaces, so humid air is less dense.

\[ v \approx 331 + 0.6\,T \ \text{m/s} \]

So the warmest and most humid air wins. Dry air at 0 degrees is the slowest of the four, and humid air at 0 degrees or dry air at 30 degrees each has only one of the two advantages.

Answer: B

Radiocarbon dating is used to estimate the age of fossils, that is, of once-living material.

Living organisms take in carbon-14 from the atmosphere. When they die the intake stops and the carbon-14 decays with a half-life of about 5,730 years. Measuring how much is left gives the time since death, up to roughly 50,000 years, so the method suits bones, wood, shells and other organic remains.

Rocks are dated by other radioactive clocks, such as uranium-lead or potassium-argon, because they contain no carbon from living things. Soil and buildings are not dated as such; only organic material found in them can be.

Answer: D

The rays produced in radioactive nuclear changes are Gamma rays.

When a nucleus decays by alpha or beta emission it is often left in an excited state, and it drops to its ground state by emitting a high-energy photon from the nucleus itself. That photon is a gamma ray.

X rays are also high-energy photons, but they come from the electron shells of atoms, for example when fast electrons strike a metal target, not from nuclear change. Infrared rays and visible light are emitted by atoms and molecules at much lower energies.

Answer: A

Only pair 2 is correctly matched.

A bird pushing air down, a swimmer pushing water back and a rocket ejecting gas all move because the pushed medium pushes back with an equal and opposite force: Newton's third law.

Dust leaving a beaten carpet is explained by inertia, Newton's first law; Kepler's first law is about elliptical planetary orbits. A fielder drawing back her hands increases the time over which the ball's momentum changes, reducing the force, which is Newton's second law (impulse); Bernoulli's theorem is about pressure in moving fluids.

Answer: D

All four statements are incorrect.

1. Kinetic energy and potential energy are both measured in joules; the watt is a unit of power.
2. It is the other way round: kinetic energy, \(\tfrac{1}{2}mv^{2}\), can never be negative, while work can be negative when force opposes displacement.
3. Kinetic energy is conserved only in an elastic collision; in an inelastic one some is turned into heat, sound and deformation.
4. Kinetic energy is proportional to \(v^{2}\), so halving it divides the speed by \(\sqrt{2}\):

\[ \begin{aligned} \tfrac{1}{2}mv'^{2} &= \tfrac{1}{2} \cdot \tfrac{1}{2}mv^{2} \\ v' &= \frac{v}{\sqrt{2}} \approx 0.71v \end{aligned} \]

The speed falls by about 29%, not 50%.

Answer: D

The SI unit of the power of a lens is the dioptre (diopter).

The power of a lens is the reciprocal of its focal length in metres, \(P = \frac{1}{f}\), so a lens of focal length 0.5 m has a power of 2 D. Convex lenses have positive power and concave lenses negative power.

The metre is the unit of focal length, not of power, the watt is the unit of electrical or mechanical power, and the Dobson unit measures the thickness of the ozone layer.

Answer: C

A convex mirror is used as a rear-view mirror in vehicles.

A convex mirror always forms an erect, diminished image and gives a wide field of view, so the driver can see a large area of traffic behind in a small mirror.

Shaving mirrors, dentists' mirrors and solar furnaces all use concave mirrors: the first two to form an enlarged, upright image of a nearby face or tooth, and the last to bring sunlight to a focus.

Answer: B

They possess kinetic energy.

Kinetic energy is the energy an object has because of its motion. The kinetic theory of matter holds that the particles of every substance are in constant motion, so they always have kinetic energy, and raising the temperature makes them move faster.

Potential energy is stored energy due to position or arrangement. The question asks about energy that comes from continuous movement, which is kinetic, so 'potential energy', 'both' and 'none of these' do not answer it.

Answer: A

If the surface area is increased, the rate of evaporation will increase.

Evaporation is a surface phenomenon: only molecules at the surface of a liquid can escape into the air. A larger surface exposes more molecules, so more escape per second. That is why wet clothes dry faster when spread out, and why tea cools faster in a saucer than in a cup.

Evaporation also increases with temperature and wind speed and decreases with humidity, but with everything else held constant a larger area always speeds it up, so 'decrease', 'remain the same' and 'cannot be determined' are wrong.