Physics Questions and Answers

Answer: B

A siphon fails if the liquid levels in the two vessels are the same.

The flow is driven by the difference in height between the two surfaces, giving a pressure difference \(\rho g h\). With \(h = 0\) there is nothing to drive the flow. A siphon also fails in a vacuum, since atmospheric pressure is what pushes the liquid up the short limb.

Answer: C

Because the cohesion of mercury exceeds its adhesion to glass.

A liquid spreads when it is more attracted to the surface than to itself, as water is on clean glass. Mercury's cohesive forces are much stronger, so it draws itself into the shape of least surface area - a sphere - and its meniscus curves upward at the edges rather than down.

Answer: D

A mirage involves refraction of light and total internal reflection.

Air near hot ground is less dense and so optically rarer. Light from the sky bends gradually as it passes down through these layers, and when it meets the critical angle it is totally internally reflected upwards, so the observer sees the sky as a pool of water on the road.

Answer: A

Venus is the planet nearest the earth.

At closest approach it comes within about 40 million km, nearer than Mars at about 56 million. Venus is also the most similar to the earth in size and mass, which is why it is called the earth's twin.

Answer: D

RADAR is used for detecting and locating objects such as aircraft.

The name is short for Radio Detection and Ranging. A pulse of radio waves is sent out and the echo timed; the range follows from \(d = ct/2\), and the direction from where the antenna was pointing. Radio waves do not penetrate sea water, so submerged submarines are found by sonar instead.

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: A

The primary colours of light are red, green and blue.

They match the three types of cone in the retina, and mixing them additively in the right proportions produces any colour, including white. Pigments mix subtractively, and their primaries are cyan, magenta and yellow.

Answer: B

Impulse is the vector quantity.

It is force multiplied by time, and force has direction, so impulse does too - it equals the change in momentum. Pressure, gravitational potential and the coefficient of friction are all scalars.

Answer: A

Because steam carries latent heat.

Steam and boiling water are both at 100 °C, but steam must give up its latent heat of vaporisation - about \(2260\ \mathrm{kJ\,kg^{-1}}\) - as it condenses on the skin, before it even begins to cool. That extra heat is what makes the burn so much worse.

Answer: C

Soda is a heterogeneous mixture.

Carbonated water holds undissolved carbon dioxide as visible bubbles, so it has two distinct phases and its composition is not uniform throughout. Air and brass are homogeneous mixtures, and table salt is a compound rather than a mixture at all.