Physics Questions and Answers

Answer: C

One nanometre equals \(10^{-9}\) m.

The prefix nano- means one thousand-millionth. For scale, an atom is a few tenths of a nanometre across and visible light has wavelengths of 400 to 700 nm.

Answer: B

Friction acts opposite to the direction of motion.

It always opposes relative motion, or the tendency to it, between surfaces in contact. That is why it converts kinetic energy to heat and never adds to a body's speed - though it can drive a car forward, because there the tyre's tendency is to slip backwards.

Answer: B

The water cools most in the tumbler painted rough black.

Kirchhoff's law says a good absorber is a good emitter, and a rough black surface has emissivity near 1, so it radiates fastest. By the Stefan-Boltzmann law the rate is \(E = e\sigma A T^4\), and roughness raises the effective area as well.

Answer: B

Raindrops are spherical because of surface tension.

Molecules at the surface are pulled inwards by their neighbours, so the drop contracts until its surface area is as small as possible for the volume it contains - and for a fixed volume the sphere is that shape. Large falling drops are flattened by air resistance.

Answer: A

Speed is not a vector.

It is the magnitude of velocity and carries no direction. Velocity, torque and displacement are all vectors, with both magnitude and direction.

Answer: C

The highest temperature is reached in a valley below sea level.

With the valve open the cooker is simply an open vessel, so water boils at the temperature at which its vapour pressure equals the surrounding pressure. Atmospheric pressure is greatest at the lowest altitude, so the boiling point - and the internal temperature - is highest there.

Answer: B

Because metals contain free electrons.

The valence electrons in a metal are delocalised over the whole lattice rather than bound to individual atoms, and an applied field sets them drifting. The same electron sea carries heat, which is why good electrical conductors are also good thermal ones.

Answer: C

So that the necessary centripetal force is obtained from the horizontal component of the normal reaction.

Banking tilts the reaction inward: \[\tan\theta = \frac{v^2}{rg}\] The rails then do not have to supply the force by pressing sideways on the flanges, which would cause wear and risk derailment.

Answer: A

This is explained by Bernoulli's principle.

Water forced through the narrow gap between the hulls speeds up, and where the speed rises the pressure falls: \[P + \tfrac{1}{2}\rho v^2 + \rho gh = \text{constant}\] The pressure on the outer sides then exceeds that between them, pushing the ships together. The same effect makes a passing express train tug at a person on the platform.

Answer: D

It appears black and green.

A coloured surface shows the colour it reflects. In green light the green portion of the flag reflects green and looks green; the red portion has no red light to reflect and absorbs the green, so it looks black.