Physics Questions and Answers

Answer: A

Moment of inertia and moment of a force do not share dimensions.

Moment of inertia is \(\mathrm{ML^2}\) while moment of a force, or torque, is \(\mathrm{ML^2T^{-2}}\). The other three pairs do match: work and torque, angular momentum and Planck's constant \((\mathrm{ML^2T^{-1}})\), and impulse and momentum \((\mathrm{MLT^{-1}})\).

Answer: A

Radiocarbon is produced by collisions between fast neutrons and nitrogen nuclei in the upper atmosphere.

\[\mathrm{^{14}_{7}N + ^1_0n \rightarrow ^{14}_{6}C + ^1_1H}\] The neutrons come from cosmic rays. The carbon-14 oxidises to \(\mathrm{CO_2}\), enters the food chain, and decays with a half-life of 5,730 years once the organism dies - which is the basis of radiocarbon dating.

Answer: C

Silver is the best conductor.

Its resistivity is \(1.59\times10^{-8}\ \Omega\,\mathrm{m}\), just below copper's. Copper is used for wiring nonetheless because silver is far more expensive and tarnishes in air.

Answer: D

Because planets are nearer the earth and so present an extended disc rather than a point.

A star is effectively a point source, so the whole of its light is deflected together by a turbulent air cell and the brightness flickers. A planet's disc is a collection of many such points, and their random fluctuations average out, so the light appears steady.

Answer: D

Moment of inertia is a tensor.

For rotation about a fixed axis a single number will do, but in general the angular momentum is not parallel to the angular velocity and a \(3\times3\) inertia tensor is needed to relate them. Only about a principal axis do the two line up.

Answer: A

The observer on the ground sees a parabola.

The coin leaves the boy's hand already sharing the train's horizontal velocity, which stays constant, while gravity gives it a uniform downward acceleration. Constant horizontal velocity with uniform vertical acceleration traces a parabola. To the boy in the train it falls straight down.

Answer: A

The boiling point increases with pressure.

A liquid boils when its saturated vapour pressure equals the surrounding pressure, so a higher external pressure requires a higher temperature to reach it. A pressure cooker exploits this, raising the boiling point to about 120 °C; on a mountain, the lower pressure makes water boil below 100.

Answer: A

Both are types of mechanical wave.

Mechanical waves need a material medium to travel through, and they differ in the direction of particle motion: along the wave for longitudinal, at right angles for transverse. Electromagnetic waves are non-mechanical and need no medium.

Answer: D

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

A pulse of radio waves is transmitted and the echo timed, giving the range as \(d = ct/2\); the antenna's direction gives the bearing, and the Doppler shift gives the speed. Radio waves do not travel through sea water, so submerged submarines are found by sonar.

Answer: C

Because metals are good conductors of heat.

Both parts are at 15 °C, but the metal cap draws heat away from the finger far faster than the plastic body does, and it is that rate of heat loss the nerves register as cold. The sensation reports conductivity, not temperature.