Physics Questions and Answers

Answer: B

To reduce the friction between the wheel and the axle.

The balls turn the sliding contact of the axle in its housing into a rolling one, and rolling friction is smaller by a factor of a hundred or more. The point of contact is momentarily at rest, so the surfaces do not scrape.

Answer: A

The reaction is greater than the weight of the man.

The floor must support him and accelerate him upward as well: \[N - mg = ma \Rightarrow N = m(g+a)\] This is why one feels heavier as a lift starts up, and lighter as it starts down.

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: B

Because the forces of cohesion between the molecules are very large.

In a solid the intermolecular forces hold each molecule near a fixed lattice site, so it can only vibrate about that point. In liquids these forces are weaker and molecules can slide past one another, so a liquid takes the shape of its container.

Answer: A

Because the harmonium uses a tempered scale.

Its reeds are tuned to fixed, equally spaced semitones, so the intervals are slight compromises. Indian classical music uses just intonation with microtonal shrutis and continuous glides between notes - neither of which a fixed-pitch reed instrument can follow, which is why the sarangi and now the violin are preferred.

Answer: B

A siphon fails if the liquid levels in the two vessels are the same.

The flow is driven by the difference in height between the two surfaces, giving a pressure difference \(\rho g h\). With \(h = 0\) there is nothing to drive the flow. A siphon also fails in a vacuum, since atmospheric pressure is what pushes the liquid up the short limb.

Answer: C

Because the cohesion of mercury exceeds its adhesion to glass.

A liquid spreads when it is more attracted to the surface than to itself, as water is on clean glass. Mercury's cohesive forces are much stronger, so it draws itself into the shape of least surface area - a sphere - and its meniscus curves upward at the edges rather than down.

Answer: D

A mirage involves refraction of light and total internal reflection.

Air near hot ground is less dense and so optically rarer. Light from the sky bends gradually as it passes down through these layers, and when it meets the critical angle it is totally internally reflected upwards, so the observer sees the sky as a pool of water on the road.

Answer: A

Venus is the planet nearest the earth.

At closest approach it comes within about 40 million km, nearer than Mars at about 56 million. Venus is also the most similar to the earth in size and mass, which is why it is called the earth's twin.