Physics Questions and Answers

Answer: A

Scalar quantities are physical quantities that have magnitude only and no direction.

Mass, time, temperature, work and energy are scalars, and they add by ordinary arithmetic. Vectors such as displacement and force need direction as well, and add by the triangle or parallelogram rule.

Answer: C

This is Kirchhoff's law of thermal radiation.

At a given temperature and wavelength, the emissivity of a surface equals its absorptivity, so a good absorber must be a good emitter. It is why a matt black surface both absorbs and radiates most strongly, and why a perfect absorber - a black body - is also the perfect emitter.

Answer: C

The particles are one alpha and two beta.

An alpha emission lowers the atomic number by 2 and a beta emission raises it by 1: \[Z \rightarrow Z-2+1+1 = Z\] So the nucleus returns to the same group of the periodic table, though its mass number has fallen by 4 - the daughter is an isotope of the original element.

Answer: C

Acceleration stays constant.

Near the earth's surface \(g\) is the same for all bodies at a given place and does not change as the body falls. Kinetic energy grows and potential energy falls as it descends; only their sum stays constant, and that was not among the options.

Answer: C

Henri Becquerel discovered natural radioactivity, in 1896.

He found that uranium salts fogged a wrapped photographic plate without any exposure to sunlight, so the radiation came from the element itself. He shared the 1903 Nobel Prize with Pierre and Marie Curie, who coined the word radioactivity.

Answer: D

This is the principle of conservation of linear momentum.

It follows from Newton's second and third laws: internal forces come in equal and opposite pairs and cancel, so with no external force \[\frac{d\vec{p}}{dt} = 0 \Rightarrow \vec{p} = \text{constant}\] It explains recoil, rocket propulsion and collisions alike.

Answer: A

The hydraulic brake works on Pascal's law.

Pressure applied to an enclosed fluid is transmitted undiminished throughout it, so a small force on the master cylinder produces a large force at the wide brake cylinders: \[\frac{F_1}{A_1} = \frac{F_2}{A_2}\] It also ensures all four wheels are braked equally.

Answer: C

Constantan is the ohmic conductor.

An ohmic conductor obeys \(V = IR\) with a constant resistance, so its V-I graph is a straight line. Constantan, a copper-nickel alloy, is prized because its resistance hardly changes with temperature. A transistor, an electrolyte and a thermistor are all non-ohmic.

Answer: A

Newton's first law is the law of inertia.

It states that a body remains at rest or in uniform motion in a straight line unless an external force acts on it, so it defines inertia and identifies the inertial frames in which the other two laws hold.

Answer: C

The bomb strikes the ground 3.33 km from A.

Vertically it starts with no downward velocity: \[t = \sqrt{\frac{2h}{g}} = \sqrt{\frac{2\times1960}{9.8}} = 20\ \mathrm{s}\]

Horizontally it keeps the aircraft's speed of \(166.7\ \mathrm{m\,s^{-1}}\): \[x = 166.7\times20 \approx 3.33\ \mathrm{km}\]