Physics Questions and Answers

Answer: C

The water level remains the same.

A floating ice cube displaces its own weight of water. When it melts it becomes exactly that weight of water, which occupies exactly the volume it was displacing, so the level does not change. This is why melting sea ice does not by itself raise sea level, whereas melting land ice does.

Answer: B

Both acquire the same momentum.

By the impulse-momentum theorem \(Ft = \Delta p\), so equal forces acting for equal times give equal changes in momentum whatever the masses. Their velocities, accelerations and kinetic energies all differ.

Answer: C

Red has the longest wavelength of the visible colours.

The visible range runs from about 400 nm at the violet end to 700 nm at the red end, so red has the longest wavelength and, since \(E = hc/\lambda\), the lowest energy. White is not a spectral colour but a mixture of all of them.

Answer: A

\(\mathrm{C_2H_6O}\) - ethanol - is the non-electrolyte.

It dissolves in water as whole molecules, producing no ions, so the solution does not conduct. HCl ionises completely, and \(\mathrm{Na_2CO_3}\) and NaCl are ionic solids that dissociate on dissolving.

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.