Physics Questions and Answers

Answer: A

It is an example of inertia of rest.

Inertia of rest is the tendency of a body at rest to stay at rest. When the bus starts suddenly, the passenger's feet, in contact with the floor, move forward with the bus, but the upper part of the body tends to remain where it was. So the passenger falls backwards, opposite to the bus's motion.

Inertia of motion is the opposite case: when a moving bus stops suddenly, passengers are thrown forwards. Inertia of direction is shown when a bus turns a corner and passengers lean outwards. 'Force' is not a type of inertia.

Answer: A

The force of gravity is least at the equator.

Two effects combine. The Earth bulges at the equator, so a point there is farther from the centre and gravitational attraction is weaker. And the Earth's rotation is fastest at the equator, so more of the pull is used up providing the centripetal force for the circular motion. Together they make the effective value of g lowest there, about 9.78 m/s².

At the poles both effects vanish, so g is greatest, about 9.83 m/s². Points in between take intermediate values.

Answer: C

The keyed answer is total internal reflection of light.

On a hot day the air near the ground is hotter and less dense than the air above, so its refractive index is lower. Light from the sky travelling downwards passes from denser to rarer layers and is bent more and more away from the normal, until near the ground it is totally internally reflected and travels upwards to the eye. The observer sees an inverted image of the sky that looks like a pool of water.

Refraction through the layers is part of the process, but the standard textbook answer names the final step, total internal reflection. Diffraction, the bending of light round obstacles, plays no part.

Answer: D

The phenomenon is called Dispersion.

Dispersion is the splitting of white light into its component colours. It happens in a prism because the refractive index of glass is slightly different for each wavelength, so violet light is bent the most and red the least, and the colours fan out.

The spectrum is the band of colours (VIBGYOR) that dispersion produces, not the phenomenon itself. Refraction is the bending that causes it, but refraction alone does not imply splitting, and reflection plays no part.

Answer: A

An electric fuse is based on the heating effect of current.

A fuse is a short piece of wire with a low melting point placed in series with a circuit. When the current rises above the safe limit, because of a short circuit or overloading, the heat produced in the wire, proportional to I²Rt, melts it. The circuit is broken and the wiring and appliances are protected.

The chemical effect of current is used in electroplating and the magnetic effect in electromagnets, motors and circuit breakers that trip magnetically, not in a fuse wire.

Answer: B

The atom bomb is based on nuclear fission.

In an atom bomb, heavy nuclei of uranium-235 or plutonium-239 split when struck by neutrons, releasing enormous energy and more neutrons, which split further nuclei in an uncontrolled chain reaction. The bombs dropped on Hiroshima and Nagasaki in 1945 were fission weapons.

Nuclear fusion, the joining of light nuclei, is the basis of the hydrogen bomb, which uses a fission bomb to trigger it. A chemical reaction releases far less energy and powers ordinary explosives.

Answer: C

The key's answer is Position and momentum.

Heisenberg's uncertainty principle (1927) states that the position and the momentum of a particle cannot both be known exactly at the same time: the product of their uncertainties is at least \(\hbar/2\). This is the pair the principle is defined by.

An analogous relation between energy and time is often quoted alongside it, but the principle's standard statement is about position and momentum. Mass and energy are linked by Einstein's \(E = mc^{2}\), and velocity and acceleration are not an uncertainty pair.

Answer: B

The key's answer is Copernicus.

Nicolaus Copernicus proposed the heliocentric model in De revolutionibus orbium coelestium (1543), placing the Sun rather than the Earth at the centre with the planets revolving around it. That is the sense in which he is credited with discovering the solar system.

Aristotle taught that the Earth was at the centre of the universe. Kepler came later and described the elliptical orbits of the planets. Aryabhata proposed that the Earth rotates on its axis, but did not set out a Sun-centred system.

Answer: D

Sound travels fastest in steel.

Sound is a mechanical wave and travels faster in media whose particles are more tightly bound. It moves at roughly 5,900 m/s in steel, about 1,500 m/s in water and about 343 m/s in air at room temperature.

In a vacuum there are no particles at all, so sound cannot travel there, which is why vacuum is the trap.

Answer: D

The unit of radioactivity is the Becquerel.

One becquerel (Bq) is an activity of one nuclear disintegration per second. It is the SI unit and is named after Henri Becquerel, who discovered radioactivity in 1896. The older unit, the curie, equals 3.7 × 10¹⁰ Bq.

The decibel measures sound intensity level, the tesla magnetic flux density, and the hertz frequency.