Physics Questions and Answers

Answer: C

This is streamline, or laminar, flow.

Every particle passing a given point follows the same path with the same velocity, so the streamlines never cross. Above a critical speed the flow becomes turbulent and the motion at each point varies irregularly with time.

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: C

In a static liquid, the pressure is the same at every point at the same horizontal level.

Pressure depends only on depth, \(P = P_0 + \rho gh\), so all points at one depth are at the same pressure whatever the shape of the vessel. If they were not, liquid would flow until they were - which is why connected vessels settle at a common level.

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: A

The blue zone is the hottest part of the flame.

There the air supply is plentiful and combustion is complete, so the temperature reaches about 1,500 °C. The luminous yellow region is cooler, its light coming from glowing unburnt carbon, and the innermost dark zone is unburnt gas and coldest of all.

Answer: D

The wavelength is the distance between crest and crest, or trough and trough.

It is the length of one complete cycle, so any two corresponding points in phase are a wavelength apart. Crest to the adjacent trough is only half a wavelength.

Answer: D

The minimum entry speed is \(\sqrt{5gR}\).

At the top of the loop, gravity must at least supply the centripetal force: \[mg = \frac{mv_{top}^2}{R} \Rightarrow v_{top}^2 = gR\]

Applying conservation of energy between bottom and top, a height \(2R\) apart: \[\tfrac{1}{2}mv^2 = \tfrac{1}{2}mv_{top}^2 + mg(2R)\]

\[v^2 = gR + 4gR = 5gR \Rightarrow v = \sqrt{5gR}\]

Answer: B

The amount of water vapour in the air is called humidity.

Absolute humidity is the mass of vapour per unit volume; relative humidity, the figure quoted in weather reports, is the ratio of the actual vapour pressure to the saturation value at that temperature. Dew forms when air is cooled until its relative humidity reaches 100 per cent.

Answer: B

This is the critical velocity.

Above it the flow turns turbulent. It follows from the Reynolds number: \[v_c = \frac{R_e\,\eta}{\rho D}\] Flow is laminar below about \(R_e = 2000\) and turbulent above about 3000.

Answer: A

Waterproofing depends on surface tension.

The fabric is treated so that water does not wet it: the contact angle becomes obtuse, cohesion within the drop exceeds its adhesion to the fibre, and surface tension pulls the drop into a bead that rolls off instead of soaking through the pores.