Physics Questions and Answers

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.

Answer: C

The relation is velocity = frequency × wavelength.

\[v = f\lambda\] In a given medium the speed is fixed, so frequency and wavelength vary inversely. When a wave passes into another medium the frequency stays the same - it is set by the source - and the wavelength changes with the speed.

Answer: A

A mirage is due to unequal heating of different parts of the atmosphere.

Hot ground warms the air just above it, making it optically rarer, so a gradient of refractive index forms. Light from the sky curves gradually downward through these layers and is finally totally internally reflected, so it appears to come from the ground.

Answer: C

Because they produce perfectly homogeneous solutions.

Ultrasound sets up rapid pressure oscillations that create and collapse microscopic bubbles - cavitation - and the resulting microjets mix the liquid down to a very fine scale, far more thoroughly than mechanical stirring.

Answer: C

Infrared rays are electromagnetic.

They are oscillating electric and magnetic fields and travel through vacuum at \(3\times10^{8}\ \mathrm{m\,s^{-1}}\). Sound and ultrasonic waves are mechanical and need a medium; cathode rays are a stream of electrons, which are particles.

Answer: C

Water is densest at 4 °C.

Below that temperature the hydrogen-bonded open structure of ice begins to form, and the molecules move slightly further apart, so cooling from 4 °C to 0 makes water less dense. This anomalous expansion is why ice floats and why a lake freezes from the surface down, leaving fish alive beneath.

Answer: B

Rectifiers convert alternating current to direct current.

A diode conducts in one direction only, so it passes half the AC cycle and blocks the other. A bridge of four diodes gives full-wave rectification, and a capacitor smooths the pulses into steady DC. The reverse conversion is done by an inverter.