Physics Questions and Answers

Answer: C

Because there is always water just beneath the ice.

Water is densest at 4 °C, so as a pond cools the coldest water rises rather than sinking, and ice forms at the top. Ice is a poor conductor and floats, insulating the water below, which stays near 4 °C - cold, but liquid.

Answer: C

So that the necessary centripetal force is supplied by the horizontal component of the reaction.

Leaning through an angle \(\theta\) tilts the normal reaction, and its horizontal component provides the inward force: \[\tan\theta = \frac{v^2}{rg}\] The faster the turn or the tighter the curve, the further he must lean.

Answer: B

The centre of gravity is shifted downwards.

An empty cylinder's centre of gravity is at its geometric centre. Adding liquid at the bottom puts extra mass low down, pulling the combined centre of gravity below the midpoint - and it is lowest when the cylinder is about half full, rising again as it fills.

Answer: C

The SI unit of luminous intensity is the candela.

It is one of the seven base units. The lumen measures luminous flux and the lux illuminance - one lux is one lumen per square metre - while the watt measures radiant power without regard to the eye's sensitivity.

Answer: C

The fuse is a safety device.

It is a short length of wire of low melting point in series with the circuit. If the current exceeds the rated value, the heat \(I^2Rt\) melts the wire and breaks the circuit before the wiring can overheat and start a fire.

Answer: A

Argon, copper and silver are diamagnetic.

A diamagnetic substance has all its electrons paired, so it has no permanent magnetic moment and is weakly repelled by a field. Oxygen has two unpaired electrons and is paramagnetic, and hydrogen atoms are paramagnetic though \(\mathrm{H_2}\) is not.

Answer: C

The total weight remains the same.

The balance reads the total weight of everything the man and his load contain. Moving the fish from one hand to the other, or into the bucket, moves mass about within that system but adds and removes nothing, so the reading does not change.

Answer: A

At minimum deviation, the angle of emergence equals the angle of incidence.

The ray passes symmetrically through the prism, parallel to its base, so \(i = e\) and \(r_1 = r_2\). That symmetry gives the prism formula \[n = \frac{\sin\left(\frac{A+D_m}{2}\right)}{\sin\left(\frac{A}{2}\right)}\]

Answer: B

The decibel measures the intensity of sound.

It is a logarithmic measure: \[\beta = 10\log_{10}\frac{I}{I_0}\ \mathrm{dB}\] where \(I_0 = 10^{-12}\ \mathrm{W\,m^{-2}}\) is the threshold of hearing. Every 10 dB therefore means a tenfold rise in intensity, matching the ear's own logarithmic response.

Answer: C

The sea looks blue because of scattering of sunlight by water molecules.

Scattering follows \(I \propto 1/\lambda^4\), so short blue wavelengths are scattered far more than red ones. Water also absorbs red light strongly within a few metres, which deepens the effect.