Physics Questions and Answers

Answer: C

It depends on the number of overtones.

Two instruments sounding the same note produce the same fundamental frequency, but each adds a different set of harmonics in different strengths. That mixture is the timbre, or quality, and it is why a flute and a violin playing middle C are instantly told apart.

Answer: B

The frequency is the same but the wavelength is smaller in water.

Frequency is fixed by the source and cannot change at a boundary. Since \(v = f\lambda\) and light slows to \(c/1.33\) in water, the wavelength must shrink in the same proportion - which is exactly why the ray bends.

Answer: A

Electric current is a scalar.

Although it has a sense of flow, current does not obey the vector law of addition: at a junction the currents add arithmetically, by Kirchhoff's rule, not by the parallelogram rule. Electric field, acceleration and linear momentum are all vectors.

Answer: C

A fuse must have a low melting point.

It is a thin wire of a tin-lead alloy in series with the circuit. When the current exceeds its rating, the heat \(I^2Rt\) melts the wire and breaks the circuit before the wiring can overheat. It is connected in series, not parallel, and its resistance must be low enough not to waste power in normal use.

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: D

Young's modulus is a property of solids only.

\[Y = \frac{\text{longitudinal stress}}{\text{longitudinal strain}}\] It measures resistance to a change in length, and only a solid has a definite length to change. Fluids have no shape of their own and so have no Young's modulus or rigidity modulus; they do have a bulk modulus.

Answer: A

Angular momentum remains constant.

Gravity acts along the line joining the satellite to the earth's centre, so it exerts no torque about that centre and \(\vec{L} = \vec{r}\times\vec{p}\) is conserved. In a circular orbit the speed is constant too, but the linear velocity is not, since its direction changes continuously.

Answer: A

The minimum distance is 4f.

With object distance \(u\) and image distance \(v\), the total separation is \(D = u+v\), and the lens formula gives \[\frac{1}{v} - \frac{1}{-u} = \frac{1}{f} \Rightarrow \frac{1}{u} + \frac{1}{v} = \frac{1}{f}\]

Minimising \(u+v\) subject to that condition gives \(u = v = 2f\), so \[D_{min} = 2f + 2f = 4f\]

Below this separation a convex lens cannot form a real image at all.

Answer: B

Force times velocity is power.

\[P = \frac{dW}{dt} = \frac{\vec{F}\cdot d\vec{s}}{dt} = \vec{F}\cdot\vec{v}\] Force times displacement gives work; force times time gives impulse.

Answer: D

Water has the highest specific heat of these.

It is \(4186\ \mathrm{J\,kg^{-1}\,K^{-1}}\), against about 2400 for alcohol and 2100 for kerosene - among the highest of any common substance, because much of the energy goes into breaking hydrogen bonds. This is why water is used as a coolant and why coastal climates are mild.