Physics Questions and Answers

Answer: A

Nuclear sizes are given in fermi.

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

Answer: B

Because the friction between sand and the feet is less than between concrete and the feet.

Walking depends on the backward push of the foot producing a forward frictional reaction. Loose sand shifts under the foot instead of gripping, so much of the effort goes into displacing grains rather than into forward motion.

Answer: A

Amplitude is independent of the others.

Velocity, wavelength and frequency are tied together by \(v = f\lambda\), so fixing two fixes the third. Amplitude is set by the source's energy and can be changed without affecting any of them - a loud note and a soft note of the same pitch differ only in amplitude.

Answer: B

Sound below 20 Hz is infrasonic.

The audible range for humans is roughly 20 Hz to 20 kHz; above that is ultrasonic. Elephants and whales communicate at infrasonic frequencies, which travel great distances, and earthquakes generate them too.

Answer: A

Sound travels fastest in steel.

Its speed is \(v = \sqrt{E/\rho}\), and a solid's elastic modulus is enormously larger than a liquid's or a gas's. Sound moves at about \(5,960\ \mathrm{m\,s^{-1}}\) in steel, \(1,500\) in water and \(343\) in air. In vacuum it cannot travel at all.

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: D

All of the above.

Radio waves pass through cloud, dust and the daylight sky, which stop or swamp visible light, so a radio telescope works around the clock and in any weather. It also detects sources - pulsars, quasars, cold hydrogen clouds - that emit little or no visible light at all.

Answer: A

C.V. Raman was awarded the Nobel Prize for scattering.

The Raman effect, announced in 1928, is the inelastic scattering of light: a small fraction of scattered photons come away with a shifted frequency, the shift matching a vibrational mode of the molecule. He received the Nobel Prize in Physics in 1930.

Answer: B

The light year is a unit of distance.

It is how far light travels in a year, about \(9.46\times10^{15}\) m. Despite the word 'year' in the name it measures length, not time.

Answer: C

Impurities are added to increase the semiconductor's electrical conductivity.

Doping with a pentavalent element such as phosphorus adds free electrons, giving n-type material; a trivalent one such as boron adds holes, giving p-type. Either raises the carrier concentration by orders of magnitude, and putting the two together makes the p-n junction.