Physics Questions and Answers

Answer: A

The reading increases.

The water exerts an upward buoyant force on the iron; by Newton's third law the iron pushes down on the water with an equal force. That extra force is carried by the bucket and the string, so the balance reads more by exactly the weight of water displaced.

Answer: A

Because there is very little friction between ice and the feet.

Walking depends on pushing backwards against the ground and receiving a forward frictional reaction. Ice has a very low coefficient of friction, made lower still by a thin film of meltwater, so the foot slips instead of gripping.

Answer: A

A man jumping from a moving train is thrown forward.

His whole body shares the train's forward velocity. On landing, the ground brings his feet to rest at once while the upper body keeps moving by inertia, so he pitches forward. Running a few steps in the direction of motion is the way to avoid it.

Answer: A

Power is the scalar.

It is the rate of doing work, and work is itself a scalar, so power has magnitude only. Electric field, magnetic moment and average velocity all have direction as well.

Answer: A

The 25 W bulb will fuse.

Resistance from the rating: \[R = \frac{V^2}{P} \Rightarrow R_{25} = \frac{220^2}{25} = 1936\ \Omega,\quad R_{100} = \frac{220^2}{100} = 484\ \Omega\]

In series they carry the same current, and the voltage divides in proportion to resistance, so the 25 W bulb takes four-fifths of 440 V - about 352 V, far above its rating. In series, the lower-wattage bulb always gets the larger share.

Answer: A

Speed is not a vector.

It is the magnitude of velocity and carries no direction. Velocity, torque and displacement are all vectors, with both magnitude and direction.

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: A

The SI unit of pressure is the pascal.

\[1\ \mathrm{Pa} = 1\ \mathrm{N\,m^{-2}}\] It is a small unit - atmospheric pressure is about \(10^{5}\) Pa. The dyne and the newton are units of force, and the joule of energy.

Answer: C

Because diamond has a very high refractive index, 2.42.

That gives a critical angle of only 24.4°, so light entering the stone is totally internally reflected many times before it can escape, and emerges concentrated through the crown. The high dispersion also splits the light into colours, giving the fire. A cut that respects those angles is what makes the sparkle.

Answer: D

Air pressure is highest when the air is cool and dry.

Cool air is denser than warm air, so a column of it weighs more. Dry air is denser than moist air too, because a water molecule (18 u) is lighter than the nitrogen and oxygen molecules (28 and 32 u) it displaces. Cool and dry therefore gives the greatest pressure.