Physics Questions and Answers

Answer: B

Newton's second law states that the rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction of the force.

\[F = \frac{dp}{dt} = \frac{d(mv)}{dt} = ma\] for constant mass. The law gives force its quantitative meaning and defines the newton as the force that gives 1 kg an acceleration of \(1\ \mathrm{m\,s^{-2}}\).

Answer: C

Work is the scalar.

It is the dot product of force and displacement, \(W = \vec{F}\cdot\vec{d} = Fd\cos\theta\), and a dot product of two vectors is a scalar. Force, velocity and acceleration all have direction as well as magnitude.

Answer: A

The primary colours of light are red, green and blue.

They match the three types of cone in the retina, and mixing them additively in the right proportions produces any colour, including white. Pigments mix subtractively, and their primaries are cyan, magenta and yellow.

Answer: C

A moving charge produces both an electric and a magnetic field.

Every charge has an electric field around it whether it moves or not; motion adds a magnetic field encircling the direction of travel, given by the Biot-Savart law. Whether a field looks electric or magnetic in fact depends on the observer's frame, which is one of the roots of special relativity.

Answer: C

Newton's third law states that to every action there is an equal and opposite reaction.

The two forces act on different bodies, which is why they never cancel each other: the gun pushes the bullet forward and the bullet pushes the gun back. The law is equivalent to the conservation of momentum.

Answer: A

A telescope has an objective of large focal length and an eyepiece of small focal length.

Its magnification is \(f_o/f_e\), so a long objective focal length helps. A microscope is the other way round: its objective has a very short focal length, of a few millimetres, to give a large magnified real image which the eyepiece then magnifies again.

Answer: B

False. Orbital speed does not depend on the satellite's mass.

Equating gravitational and centripetal force, \[\frac{GMm}{r^2} = \frac{mv^2}{r} \Rightarrow v = \sqrt{\frac{GM}{r}}\] The satellite's mass \(m\) cancels, so it depends only on the earth's mass and the radius of the orbit.

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: D

A ball pen works on capillarity and surface tension.

Ink is drawn along the narrow tube by capillary action and held at the tip by surface tension, so it does not run out. Rolling the ball picks up a film of ink and lays it on the paper.

Answer: B

Gravitational acceleration would increase at the equator.

Part of the earth's gravity is currently spent providing the centripetal force for rotation: \[g_{eff} = g - \omega^2 R\cos^2\lambda\] The correction is largest at the equator and zero at the poles. If rotation stopped, \(\omega = 0\) and the equatorial value would rise by about \(0.034\ \mathrm{m\,s^{-2}}\), while the poles would be unaffected.