Physics Questions and Answers

Answer: A

Momentum and impulse share dimensions.

Impulse is force times time and momentum is mass times velocity: \[[\mathrm{Ft}] = \mathrm{MLT^{-2}\cdot T} = \mathrm{MLT^{-1}},\qquad [\mathrm{mv}] = \mathrm{M\cdot LT^{-1}} = \mathrm{MLT^{-1}}\] They must agree, since the impulse–momentum theorem says one equals the change in the other.

Answer: D

Total internal reflection needs light to go from a denser to a rarer medium, so it can happen from diamond to glass.

Diamond's refractive index is 2.42 and glass's about 1.5. The critical angle follows from \[\sin C = \frac{n_2}{n_1} = \frac{1.5}{2.42} \Rightarrow C \approx 38^\circ\] In the other three cases light passes into the denser medium, where no critical angle exists.

Answer: D

All of the above are true of the image in a convex mirror.

For a real object a convex mirror always gives a virtual, erect and diminished image lying between the pole and the focus, wherever the object is placed. That constant behaviour, and the wide field of view, is why convex mirrors are used as rear-view mirrors.

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: A

The answer is 300 m.

The wavelength of the carrier is \[\lambda = \frac{c}{f} = \frac{3\times10^{8}}{1000\times10^{3}} = 300\ \mathrm{m}\] An antenna must be comparable to the wavelength to radiate efficiently, which is why long-wave broadcasting needs very tall masts. In practice a quarter-wave antenna of 75 m is often used.

Answer: C

The iris cannot dilate the pupil quickly enough.

In bright light the pupil is constricted to a small aperture; on entering darkness the iris muscles need time to open it, and the retinal pigment rhodopsin, bleached by the bright light, takes several minutes to regenerate. Full dark adaptation takes about twenty to thirty minutes.

Answer: B

The apparent weight is more than the true weight.

For a lift accelerating upwards with acceleration \(a\), the floor must both support the body and accelerate it: \[N - mg = ma \Rightarrow N = m(g+a)\] The reading is that normal force, so it exceeds \(mg\). Going down with acceleration \(a\) it would read \(m(g-a)\).

Answer: B

This is the greenhouse effect.

Short-wavelength sunlight passes through the glass and is absorbed by the seats and dashboard, which then re-radiate at much longer infrared wavelengths. Glass is opaque to that infrared, so the heat is trapped, and the closed windows also prevent it escaping by convection.

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: C

The sound comes from air rushing in to fill the vacuum inside the bulb.

The bulb is either evacuated or filled with gas at low pressure. When the glass breaks, atmospheric air rushes in at high speed and the sudden compression produces the pop - an implosion rather than an explosion.