Physics Questions and Answers

Answer: B

The sphere reaches the bottom first.

For a body rolling without slipping, \[a = \frac{g\sin\theta}{1 + k^2/R^2}\] The ratio \(k^2/R^2\) is \(2/5\) for a solid sphere and \(1/2\) for a disc, so the sphere has the larger acceleration. Mass and radius cancel out entirely - only the shape matters.

Answer: C

An optical fibre works on total internal reflection.

The core's refractive index is slightly higher than the cladding's, so light striking the boundary beyond the critical angle is reflected entirely back into the core and zigzags along the fibre with almost no loss.

Answer: A

RADAR stands for Radio Detection and Ranging.

A pulse of radio waves is transmitted and the echo timed, so the range is \(d = ct/2\); the antenna's bearing gives the direction and the Doppler shift the target's speed.

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

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: D

This is the moment of a force, or torque.

\[\tau = F \times r_\perp\] It measures the turning effect about an axis, and depends on where the force is applied as well as how large it is - which is why a longer spanner loosens a nut more easily.

Answer: C

Above the Curie temperature a ferromagnet becomes paramagnetic.

Thermal agitation destroys the alignment of the magnetic domains, so the spontaneous magnetisation vanishes, though the atoms keep their individual moments and still respond weakly to an applied field. Iron's Curie point is 770 °C.

Answer: C

The erg second is the unit of Planck's constant.

Planck's constant has dimensions of energy times time, \(\mathrm{ML^2T^{-1}}\), so in the CGS system it is measured in erg seconds: \(h = 6.626\times10^{-27}\ \mathrm{erg\,s}\), or \(6.626\times10^{-34}\ \mathrm{J\,s}\) in SI.

Answer: B

There are seven base SI units.

They are the metre, kilogram, second, ampere, kelvin, mole and candela, for length, mass, time, electric current, temperature, amount of substance and luminous intensity. Every other SI unit is derived from these.

Answer: D

All of these are dimensionless.

Angle is arc divided by radius, so \(\mathrm{L/L}\); strain is change in length divided by original length, again \(\mathrm{L/L}\); and specific gravity is a ratio of two densities. Each is a pure number, which is why the radian is called a supplementary rather than a base unit.