Physics Questions and Answers

Answer: A

Three non-zero, non-collinear vectors are needed.

Two non-collinear vectors can never cancel, since their sum has a component along each. Three can, if they close a triangle head to tail - which requires them to be coplanar.

Answer: C

A washing machine's spin cycle works by centrifugation.

The drum spins fast, and 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: C

Stellar and galactic masses are expressed in solar masses.

One solar mass is \(2\times10^{30}\) kg. Ordinary stars run from about 0.08 to 100 solar masses, and the Milky Way is of the order of \(10^{12}\) - numbers that would be unwieldy in kilograms.

Answer: A

The choke in a tube light is there to induce a high voltage.

When the starter's contacts open, the sudden collapse of current in the inductor produces a large back emf, \[e = -L\frac{di}{dt}\] of several hundred volts, which strikes the arc. Once the tube is conducting, the same choke's reactance limits the current.

Answer: B

Frequency remains unchanged.

Frequency is set by the source and cannot change at a boundary - the wave arriving at the interface and the wave leaving must oscillate together. Since \(v = f\lambda\) and sound travels faster in water, the wavelength increases in the same proportion as the speed.

Answer: C

Silver is the best conductor.

Its resistivity is \(1.59\times10^{-8}\ \Omega\,\mathrm{m}\), just below copper's. Copper is used for wiring nonetheless because silver is far more expensive and tarnishes in air.

Answer: C

To reduce heat loss by radiation.

A polished surface has low emissivity, and by the Stefan-Boltzmann law the rate of radiation is \(E = e\sigma AT^4\). Keeping \(e\) small means the heat goes into the cloth rather than radiating away into the room.

Answer: C

Because they produce perfectly homogeneous solutions.

Ultrasound sets up rapid pressure oscillations that create and collapse microscopic bubbles - cavitation - and the resulting microjets mix the liquid down to a very fine scale, far more thoroughly than mechanical stirring.

Answer: A

A quartz watch works on the piezoelectric effect.

Quartz develops a voltage when mechanically stressed, and deforms when a voltage is applied. Placed in an oscillator circuit, a cut crystal vibrates at a highly stable 32,768 Hz - a power of two, so a chain of fifteen dividers gives exactly one pulse per second.

Answer: C

Glass wool is the best sound absorber here.

It is a porous, fibrous mat full of trapped air. Sound entering it sets the air and fibres vibrating, and the viscous friction converts the energy to heat. Stone, steel and glass are dense and rigid, so they reflect sound rather than absorbing it.