Physics Questions and Answers

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: C

Because the eye cannot separate two images shown less than about a sixteenth of a second apart.

An image persists on the retina for roughly 1/16 second after the light is removed - the persistence of vision. Frames projected faster than that blend into continuous motion, which is why film runs at 24 frames a second.

Answer: D

\(\mathrm{ML^2T^{-2}}\) is the dimensional formula of a couple, or torque.

Torque is force times perpendicular distance: \(\mathrm{MLT^{-2}} \times \mathrm{L} = \mathrm{ML^2T^{-2}}\). Work and energy share these dimensions, though torque is a vector and they are scalars. Moment of inertia is \(\mathrm{ML^2}\) and pressure \(\mathrm{ML^{-1}T^{-2}}\).

Answer: C

Newton's third law states that to every action there is an equal and opposite reaction.

The two forces act on different bodies, which is why they never cancel each other: the gun pushes the bullet forward and the bullet pushes the gun back. The law is equivalent to the conservation of momentum.

Answer: A

The lux is the SI unit of illuminance, the intensity of illumination.

\[1\ \mathrm{lx} = 1\ \mathrm{lm\,m^{-2}}\] The lumen measures luminous flux and the candela luminous intensity at the source; the lux measures how much of that flux lands on a surface.

Answer: A

Because the moon's periods of rotation and revolution are equal.

Both take about 27.3 days, so the moon turns once on its axis in exactly the time it takes to go once round the earth, and keeps the same face towards us. This synchronous or tidally locked rotation is the result of tidal friction over billions of years.

Answer: A

This is viscous force.

Adjacent layers of fluid moving at different speeds exert a tangential drag on each other: \[F = \eta A \frac{dv}{dx}\] Viscosity is in effect internal friction, and it is what makes honey pour more slowly than water.

Answer: C

An air bubble in water behaves like a concave lens.

What matters is the refractive index of the lens relative to its surroundings. Air is optically rarer than water, so a bubble that is convex in shape diverges light instead of converging it, and acts as a diverging - concave - lens.

Answer: A

This is Stefan's law, or the Stefan-Boltzmann law.

\[E = \sigma T^4\] with \(\sigma = 5.67\times10^{-8}\ \mathrm{W\,m^{-2}\,K^{-4}}\). The fourth power means a small rise in temperature increases radiation sharply - doubling the absolute temperature multiplies the emission by sixteen.

Answer: C

One joule equals \(10^{7}\) ergs.

\[1\ \mathrm{J} = 1\ \mathrm{kg\,m^2\,s^{-2}} = 10^{3}\ \mathrm{g}\times(10^{2}\ \mathrm{cm})^2\,\mathrm{s^{-2}} = 10^{7}\ \mathrm{g\,cm^2\,s^{-2}} = 10^{7}\ \mathrm{erg}\]