Physics Questions and Answers

Answer: B

Heat energy is the total energy of the molecules of the object.

It depends on how many molecules there are as well as how energetic each is, so a bucket of warm water holds more heat than a cup of boiling water. Temperature, by contrast, measures the average molecular kinetic energy.

Answer: B

Because the stone conducts heat away from the feet more rapidly.

Both the floor and the carpet are at room temperature. Stone is a good conductor and draws heat from the skin quickly, which the nerves register as cold; the carpet is a poor conductor and traps air, so it removes heat slowly and feels warm.

Answer: B

Six spectral lines appear.

An electron falling from level \(n\) can reach any lower level, so the number of possible transitions is \[\frac{n(n-1)}{2} = \frac{4\times 3}{2} = 6\] The six are 4→3, 4→2, 4→1, 3→2, 3→1 and 2→1.

Answer: D

Moment of inertia is a tensor.

For rotation about a fixed axis it can be treated as a single number, but in general the angular momentum is not parallel to the angular velocity, and the relation between them requires a \(3\times3\) inertia tensor. Only for rotation about a principal axis do the two vectors line up.

Answer: C

The bomb strikes the ground 3.33 km from A.

Vertically it starts with no downward velocity, so \[t = \sqrt{\frac{2h}{g}} = \sqrt{\frac{2\times1960}{9.8}} = \sqrt{400} = 20\ \mathrm{s}\]

Horizontally it keeps the aircraft's speed, \(600\ \mathrm{km/h} = 166.7\ \mathrm{m\,s^{-1}}\): \[x = 166.7 \times 20 = 3334\ \mathrm{m} \approx 3.33\ \mathrm{km}\]

Answer: B

Channel capacity is measured by its bandwidth.

Bandwidth is the range of frequencies the channel can carry, and by the Shannon-Hartley theorem the maximum data rate is \[C = B\log_2\!\left(1 + \frac{S}{N}\right)\] so capacity rises directly with bandwidth. Baud rate counts signal changes per second, which is a different thing.

Answer: A

True. This is Kepler's first law, the law of orbits.

Each planet moves in an ellipse with the sun at one of the two foci. The orbits of the planets are very nearly circular - the earth's eccentricity is only 0.017 - which is why the departure from a circle went unnoticed until Kepler analysed Tycho Brahe's observations of Mars.

Answer: B

It will reach a particular height and remain floating.

The balloon rises while the buoyant force exceeds its weight. As it climbs, the air thins and the upthrust \(\rho_{air}Vg\) falls, until it just balances the weight; there the balloon settles and floats.

Answer: D

This property is inertia.

Newton's first law states that a body continues in its state of rest or of uniform motion in a straight line unless acted on by an external force. Mass is the measure of inertia: the greater the mass, the harder it is to change the state of motion.

Answer: D

Ignition in a diesel engine is produced by compression.

Air alone is compressed to about a twentieth of its volume, which raises its temperature above 500 °C. Fuel injected into that hot air ignites of itself, so a diesel needs no spark plug - hence the name compression-ignition engine.