Physics — PPSC previous year questions

A freely falling body is acted on only by gravity, so it moves with a constant (uniform) acceleration g ≈ 9.8 m/s² directed towards the earth. Because the acceleration is constant, the velocity and speed keep increasing (v = gt), so they are not uniform. Weight (mg) stays the same but it is a force, not a description of the motion.

The Sun's gravitational force supplies the centripetal pull that keeps the planets moving in their elliptical orbits, as described by Newton's law of universal gravitation and Kepler's laws. Gravity is the weakest of the four fundamental forces but has infinite range and always attracts, so it dominates at astronomical scales. Electromagnetic and nuclear forces do not act between the Sun and planets in this way, and friction is not a fundamental force at all.

Statement (b) is wrong because in fusion, just as in fission, the products weigh less than the starting nuclei; the missing mass is released as energy (E = mc²). The other statements are true: geothermal energy is tapped by drilling wells for steam or hot water, biomass stores solar energy captured by photosynthesis, and islands and mountain passes have strong steady winds that make wind power profitable.

Pitch is the characteristic of sound that decides how shrill or grave it is. It depends on the frequency of vibration. A high-frequency sound is shrill. A low-frequency sound is grave. Loudness (intensity) depends on amplitude. Quality (timbre) is what lets us tell apart two sounds of the same pitch and loudness, such as a violin and a flute.

Light travels along an optical fibre by total internal reflection: it strikes the wall of the glass core at an angle greater than the critical angle and bounces back inside instead of escaping, so it follows the fibre even round bends.

Radiocarbon dating measures the carbon-14 left in material that was once alive. Its half-life is about 5,730 years. So the method dates fossils and other organic remains up to roughly 50,000 years old. Rocks and minerals are dated instead by uranium-lead or potassium-argon methods, whose half-lives run into hundreds of millions of years.

Sound is a mechanical wave and needs a material medium — solid, liquid or gas — whose particles can vibrate to carry it. A vacuum has no particles, so sound cannot travel through it; that is why the ringing bell inside an evacuated jar becomes inaudible. Sound travels fastest in solids such as steel, then in water and slowest in gases.

The inside of a camera is painted black so that stray light is absorbed. Otherwise it would be reflected back onto the film or sensor. That would fog the image and reduce contrast. Black surfaces absorb light, so absorption is what the coating provides, not what it avoids. Scattering and total internal reflection are not the issue.

In a gas the speed of sound is v = √(γP/ρ). At a fixed temperature, raising the pressure raises the density in the same proportion, so P/ρ and the speed stay unchanged. The other three do matter: the speed rises by about 0.61 m/s for every 1 °C, it is lower in a denser gas, and it is slightly higher in moist air because water vapour is lighter than dry air.

An earthen pot is porous, so a little water keeps seeping out to the surface and evaporating. Evaporation draws the latent heat of vaporisation from the water inside, which cools. Heat capacity, radiation or absorption by the clay cannot keep the water below room temperature.