Physics Questions and Answers

Answer: A

You should remain in the car.

The metal body acts as a Faraday cage: a lightning strike travels over the outer shell to the ground, and the field inside a closed conductor is zero, so the occupants are unharmed. Standing under a tree is the most dangerous choice, since a tall isolated object attracts the strike.

Answer: D

A rainbow is due to refraction and reflection of sunlight by water droplets.

Light entering a raindrop is refracted and dispersed into colours, reflected once off the inside of the far surface, then refracted again on leaving. It emerges at about 42° to the incoming ray, which fixes the radius of the bow.

Answer: C

A choke is used to reduce the current in an AC circuit.

It is an inductor, whose reactance \(X_L = 2\pi f L\) opposes alternating current. Because the reactance is 90° out of phase with the current, it limits the current while dissipating almost no power - unlike a resistor, which would waste energy as heat.

Answer: A

One thousand microns is \(10^{-3}\) m.

\[1000 \times 10^{-6}\ \mathrm{m} = 10^{-3}\ \mathrm{m}\] which is one millimetre. The micron, now called the micrometre, is \(10^{-6}\) m.

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: A

Three non-zero, non-collinear vectors are needed.

Two non-collinear vectors can never cancel, since their sum has a component along each. Three can, if they close a triangle head to tail - which requires them to be coplanar.

Answer: A

The rain will appear to come from the east, that is from in front of him.

Relative velocity of rain with respect to the man is \(\vec{v}_{rain} - \vec{v}_{man}\). Subtracting his eastward velocity adds a westward component to the rain's downward velocity, so the apparent direction tilts towards the east - which is why one leans an umbrella forward when walking in rain.

Answer: D

Young's modulus is a property of solids only.

\[Y = \frac{\text{longitudinal stress}}{\text{longitudinal strain}}\] It measures resistance to a change in length, and only a solid has a definite length to change. Fluids have no shape of their own and so have no Young's modulus or rigidity modulus; they do have a bulk modulus.

Answer: A

Force and pressure do not share dimensions.

Force is \(\mathrm{MLT^{-2}}\) and pressure, being force per unit area, is \(\mathrm{ML^{-1}T^{-2}}\). The other pairs do match: work and energy, impulse and momentum, and weight, which is simply a force.

Answer: C

Accurate work uses platinum resistance thermometers.

Platinum's resistance varies with temperature in a smooth, highly reproducible way, and the metal does not oxidise or contaminate. The instrument covers roughly \(-260\) to \(960\) °C to about a thousandth of a degree, and defines the international temperature scale over much of that range.