Physics Questions and Answers

Answer: C

Its light takes 4.2 years to reach us.

Alpha Centauri is 4.2 light years away, and a light year is precisely the distance light covers in a year. So we see the system as it was more than four years ago - and there is no way to see it as it is now.

Answer: C

It depends on the number of overtones.

Two instruments sounding the same note produce the same fundamental frequency, but each adds a different set of harmonics in different strengths. That mixture is the timbre, or quality, and it is why a flute and a violin playing middle C are instantly told apart.

Answer: C

Earthquake magnitude is measured on the Richter scale.

Devised by Charles Richter in 1935, it is logarithmic: each whole number is a tenfold rise in wave amplitude and about a thirty-twofold rise in energy released. The Mercalli scale, by contrast, records the intensity of the shaking felt at a place rather than the energy at the source.

Answer: C

The ball was thrown at 30 m/s.

The two crossings are symmetric about the highest point, so the ball takes 2 s to go from 25 m up to the apex. Its speed at 25 m is therefore \[v = gt = 9.8\times 2 = 19.6\ \mathrm{m\,s^{-1}}\]

Applying \(v^2 = u^2 - 2gh\) from the ground to that level: \[u^2 = v^2 + 2gh = 384 + 2(9.8)(25) = 874\]

\[u = 29.6 \approx 30\ \mathrm{m\,s^{-1}}\]

Answer: A

Momentum is a vector.

It is mass times velocity, and velocity has direction, so momentum points the same way. Pressure, energy and work are all scalars - work is a dot product of two vectors, which yields a scalar.

Answer: C

A washing machine's spin cycle works by centrifugation.

The drum spins fast, and water in the fabric, having no centripetal force to hold it in a circle, moves outward and escapes through the perforations while the clothes are held by the drum wall.

Answer: B

Alternating current is not used in electroplating.

Electroplating needs the metal ions to travel consistently to one electrode and deposit there. Alternating current would reverse the direction every half cycle, plating and stripping in turn, so direct current is essential.

Answer: D

Light is slowest in glass.

Speed in a medium is \(c/n\), so the higher the refractive index the slower the light. Glass has \(n \approx 1.5\), water 1.33, air 1.0003 and vacuum exactly 1 - so the order of speeds runs the other way.

Answer: D

Laser light is monochromatic.

All the photons come from the same atomic transition, so they share a single wavelength; they are also coherent, being in step in phase, and highly directional. Ordinary light contains many wavelengths with random phases.

Answer: B

The water will not rise above the top of the tube, and it does not overflow.

If the tube is shorter than the calculated capillary height, the liquid rises to the top and stops. The radius of curvature of the meniscus adjusts itself so that \(hr\) stays constant, so the meniscus simply becomes flatter and the water stays put.