Physics Questions and Answers

Answer: A

Venus is the planet nearest the earth.

At closest approach it comes within about 40 million km, nearer than Mars at about 56 million. Venus is also the most similar to the earth in size and mass, which is why it is called the earth's twin.

Answer: A

Persistence of vision is the principle behind cinema.

An image lingers on the retina for about a sixteenth of a second after the light is cut off, so still frames shown faster than that merge into apparently continuous motion. Film runs at 24 frames a second for this reason.

Answer: B

Growth of the baby in the uterus is followed by ultrasound.

High-frequency sound above 20 kHz is reflected at boundaries between tissues of different density, and the echoes are timed to build the image. Unlike X-rays and gamma rays it is non-ionising, so it does not harm the developing foetus.

Answer: A

Iron is used for electromagnets.

Soft iron has high permeability, so it concentrates the flux strongly, and very low retentivity, so it loses almost all its magnetism the moment the current stops - which is exactly what an electromagnet requires. Steel, which retains magnetism, is used for permanent magnets instead.

Answer: D

This is the principle of conservation of linear momentum.

It follows from Newton's second and third laws: internal forces come in equal and opposite pairs and cancel, so with no external force \[\frac{d\vec{p}}{dt} = 0 \Rightarrow \vec{p} = \text{constant}\] It explains recoil, rocket propulsion and collisions alike.

Answer: A

Because steam carries latent heat.

Steam and boiling water are both at 100 °C, but steam must give up its latent heat of vaporisation - about \(2260\ \mathrm{kJ\,kg^{-1}}\) - as it condenses on the skin, before it even begins to cool. That extra heat is what makes the burn so much worse.

Answer: D

It appears black and green.

A coloured surface shows the colour it reflects. In green light the green portion of the flag reflects green and looks green; the red portion has no red light to reflect and absorbs the green, so it looks black.

Answer: C

All of the above reasons apply.

Nichrome has high resistivity, so it dissipates a lot of heat for a moderate current; a high melting point of about 1,400 °C, so it survives being red-hot; and it forms a protective chromium oxide layer that stops it oxidising away in air.

Answer: D

Laser light is monochromatic.

All the photons come from the same atomic transition, so they share a single wavelength; they are also coherent, being in step in phase, and highly directional. Ordinary light contains many wavelengths with random phases.

Answer: C

Displacement is the shortest distance covered in a definite direction.

It is the straight-line vector from start to finish, so it has both magnitude and direction, and can be zero even after a long journey if the body returns to where it began. Distance is the scalar length of the actual path and can only increase.