Physics Questions and Answers

Answer: C

The level will remain the same.

Floating ice displaces its own weight of water; on melting it becomes exactly that weight of water, which occupies exactly the volume it had been displacing. This is why melting sea ice does not raise sea level, whereas melting land ice does.

Answer: B

Wilhelm Roentgen discovered X-rays, in 1895.

He noticed a fluorescent screen glowing while he worked with a covered cathode-ray tube, and called the unknown radiation X. He received the first Nobel Prize in Physics in 1901, and the first X-ray photograph was of his wife's hand.

Answer: A

Isotopes have the same number of protons but different numbers of neutrons.

The proton count fixes the element and its chemistry; the differing neutron count changes only the mass. Carbon-12, carbon-13 and carbon-14 are examples, all with six protons.

Answer: B

The energy of a body in motion is its kinetic energy.

\[KE = \tfrac{1}{2}mv^2\] Note that it varies as the square of the speed, so doubling the speed quadruples the energy - which is why stopping distances grow so sharply with speed.

Answer: C

Refractive index is the ratio of the speed of light in vacuum to its speed in the medium.

\[n = \frac{c}{v}\] It is always greater than one, since nothing travels faster than light in vacuum, and it measures how much the medium slows and hence bends the light. For water \(n = 1.33\) and for glass about 1.5.

Answer: B

He must point at a higher elevation than the bird.

The bullet begins to fall under gravity the moment it leaves the barrel, so it follows a parabola and drops below the line of sight. Aiming slightly high compensates for that fall over the flight time - which is why rifle sights are zeroed for a particular range.

Answer: B

Heat energy is the total energy of the molecules of the object.

It depends on how many molecules there are as well as how energetic each is, so a bucket of warm water holds more heat than a cup of boiling water. Temperature, by contrast, measures the average molecular kinetic energy.

Answer: B

Because the stone conducts heat away from the feet more rapidly.

Both the floor and the carpet are at room temperature. Stone is a good conductor and draws heat from the skin quickly, which the nerves register as cold; the carpet is a poor conductor and traps air, so it removes heat slowly and feels warm.

Answer: B

Six spectral lines appear.

An electron falling from level \(n\) can reach any lower level, so the number of possible transitions is \[\frac{n(n-1)}{2} = \frac{4\times 3}{2} = 6\] The six are 4→3, 4→2, 4→1, 3→2, 3→1 and 2→1.

Answer: D

Moment of inertia is a tensor.

For rotation about a fixed axis it can be treated as a single number, but in general the angular momentum is not parallel to the angular velocity, and the relation between them requires a \(3\times3\) inertia tensor. Only for rotation about a principal axis do the two vectors line up.