Physics Questions and Answers

Answer: B

The water level will decrease.

In the boat, each rock displaces its own weight of water; sunk on the bottom, it displaces only its own volume. Since the rocks are denser than water, the volume is the smaller of the two, so the total displacement falls and the level drops.

Answer: A

The spectrum of an oil flame is a continuous emission spectrum.

Incandescent solids and liquids - the glowing soot particles in a flame, a lamp filament, molten metal - emit at every wavelength, so the spectrum shows no gaps. A gas at low pressure gives a line spectrum instead, characteristic of its atoms.

Answer: C

Because the pressure of the skate melts the ice by lowering its melting point.

Water is unusual in expanding when it freezes, so pressure favours the denser liquid phase and the melting point falls. A thin film of water forms under the blade and acts as a lubricant, refreezing once the skate has passed.

Answer: C

The sound comes from air rushing in to fill the vacuum inside the bulb.

The bulb is either evacuated or filled with gas at low pressure. When the glass breaks, atmospheric air rushes in at high speed and the sudden compression produces the pop - an implosion rather than an explosion.

Answer: C

The heating is due to both the heating effect of the current and hysteresis loss.

Copper loss is the \(I^2R\) dissipation in the windings; iron loss is hysteresis plus eddy currents in the core. Laminating the core cuts eddy currents, and a soft-iron core with a narrow hysteresis loop cuts the other - but neither can be eliminated.

Answer: C

A washing machine's spin cycle works by centrifugation.

The drum spins rapidly, and the water in the fabric, having no centripetal force to hold it in a circle, moves outward and escapes through the perforations while the clothes are held by the drum wall.

Answer: B

Rectifiers convert alternating current to direct current.

A diode conducts in one direction only, so it passes half of each AC cycle. Four diodes in a bridge give full-wave rectification, and a smoothing capacitor turns the pulses into steady DC. The reverse conversion is done by an inverter.

Answer: C

It depends on both the nature of the signal and the nature of the medium.

The signal's frequency and power determine how far it can carry and how much information it holds, while the medium fixes the attenuation, dispersion and noise it will meet. Neither alone decides the quality of transmission.

Answer: D

The gun recoils with the same momentum as the bullet.

Momentum is conserved, and the system was at rest, so \[m_b v_b = m_g v_g\] The gun's momentum is equal in magnitude and opposite in direction. Being much heavier, it moves back far more slowly.

Answer: C

Natural gas is a non-renewable resource.

It was formed over millions of years from buried organic matter and is consumed far faster than it is replaced, so the stock is finite. Wind, solar and hydrogen fuel are replenished continuously.