Physics Questions and Answers

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

Blotting paper works by capillary action.

Its loosely packed cellulose fibres leave fine pores between them. Ink wets the fibres, so adhesion exceeds cohesion and surface tension draws the liquid into the pores - the narrower the pore, the stronger the pull.

Answer: D

Because ultraviolet light falls on the white coating inside the tube.

The discharge through mercury vapour emits chiefly in the ultraviolet at 254 nm, which is invisible. The phosphor coating absorbs it and re-emits in the visible range - fluorescence - which is why the tube is called fluorescent.

Answer: B

Because the earth rotates from west to east.

The apparent motion of the sky is the reflection of that rotation, so the stars rise in the east and set in the west just as the sun does. The whole sky appears to turn once in a sidereal day of 23 hours 56 minutes.

Answer: A

Because there is very little friction between ice and the feet.

Walking depends on pushing backwards against the ground and receiving a forward frictional reaction. Ice has a very low coefficient of friction, made lower still by a thin film of meltwater, so the foot slips instead of gripping.

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: A

Scalar quantities are physical quantities that have magnitude only and no direction.

Mass, time, temperature, work and energy are scalars, and they add by ordinary arithmetic. Vectors such as displacement and force need direction as well, and add by the triangle or parallelogram rule.

Answer: C

This is Kirchhoff's law of thermal radiation.

At a given temperature and wavelength, the emissivity of a surface equals its absorptivity, so a good absorber must be a good emitter. It is why a matt black surface both absorbs and radiates most strongly, and why a perfect absorber - a black body - is also the perfect emitter.

Answer: C

The particles are one alpha and two beta.

An alpha emission lowers the atomic number by 2 and a beta emission raises it by 1: \[Z \rightarrow Z-2+1+1 = Z\] So the nucleus returns to the same group of the periodic table, though its mass number has fallen by 4 - the daughter is an isotope of the original element.

Answer: C

Acceleration stays constant.

Near the earth's surface \(g\) is the same for all bodies at a given place and does not change as the body falls. Kinetic energy grows and potential energy falls as it descends; only their sum stays constant, and that was not among the options.