Physics Questions and Answers

Answer: A

The observer on the ground sees a parabola.

The coin leaves the boy's hand already sharing the train's horizontal velocity, which stays constant, while gravity gives it a uniform downward acceleration. Constant horizontal velocity with uniform vertical acceleration traces a parabola. To the boy in the train it falls straight down.

Answer: B

Heat energy is the total energy of the molecules of the object.

It depends on how many molecules there are as well as how energetic each is, so a bucket of warm water holds more heat than a cup of boiling water. Temperature, by contrast, measures the average molecular kinetic energy.

Answer: D

Radio waves are reflected by the ionosphere.

Solar ultraviolet ionises the upper atmosphere above about 60 km, and the free electrons reflect radio waves back to earth, allowing transmission far beyond the horizon. Reflection is strongest at night, when the lower absorbing layers thin out - which is why distant stations come in after dark.

Answer: A

The brighter bulb has the smaller resistance.

Household bulbs are in parallel, so both have the same voltage across them: \[P = \frac{V^2}{R}\] With \(V\) fixed, power varies inversely with resistance, so the lower-resistance bulb dissipates more and glows brighter. In a series circuit the opposite would hold.

Answer: B

An object with constant speed might be accelerated.

Acceleration is the rate of change of velocity, which is a vector. A body moving in a circle at constant speed is changing direction continuously, so it is accelerating - towards the centre. Only constant velocity guarantees zero acceleration.

Answer: C

The maxwell is the unit of magnetic flux.

It is the CGS unit: one maxwell is one line of flux, and \(1\ \mathrm{Wb} = 10^{8}\ \mathrm{Mx}\). The SI unit is the weber.

Answer: A

The acceleration is constant in magnitude and directed radially inwards.

In uniform circular motion the speed does not change, so the magnitude \[a = \frac{v^2}{r}\] is fixed, but the direction always points at the centre and therefore turns with the body. It is constant in size and variable in direction - the reverse of what one might guess.

Answer: C

Lightning occurs when strong opposite charges in different clouds break down the resistance of the air between them.

Charge separation in a thundercloud builds a potential difference of hundreds of millions of volts. When the field exceeds about \(3\times10^{6}\ \mathrm{V\,m^{-1}}\), the air ionises and becomes conducting, and the accumulated charge discharges along that channel.

Answer: C

The temperature decreases slightly.

Evaporation removes the fastest molecules from the surface, lowering the average kinetic energy of those left behind, and the latent heat of vaporisation is drawn from the water itself. The drop is small because heat flows in from the surroundings to replace it.

Answer: A

This is explained by Bernoulli's principle.

Water forced through the narrow gap between the hulls speeds up, and where the speed rises the pressure falls: \[P + \tfrac{1}{2}\rho v^2 + \rho gh = \text{constant}\] The pressure on the outer sides then exceeds that between them, pushing the ships together. The same effect makes a passing express train tug at a person on the platform.