Identify the INCORRECT pair with respect to the substances and their specific heat capacity (J kg-1K-1) at room temperature and atmospheric pressure.

The correct answer is A, low coefficient of friction. Teflon is the trade name of polytetrafluoroethylene, a polymer whose surface is one of the most slippery among solids, with a coefficient of friction of about 0.04. A bearing has to let a shaft turn with as little rubbing as possible, so this slipperiness is the reason Teflon is chosen; it also lets many such bearings work without any oil or grease. The polymer was found by accident by Roy Plunkett in 1938, resists almost every chemical and holds up to about 260 degrees Celsius, which is why it also coats non-stick cookware and is wound as sealing tape on pipe threads. B is wrong because Teflon is a poor conductor of heat, so it is not chosen to carry heat away. C is wrong because saving space depends on the design of the bearing, not on the material. D is wrong because only the first reason holds. Exam tip: Teflon is PTFE, the non-stick polymer with the lowest friction.

The correct answer is B, Static friction > Kinetic friction > Rolling friction. For the same pair of surfaces the order of friction is always static, then kinetic, then rolling. Static friction is the largest because the surfaces interlock while at rest and that grip has to be broken to start motion. Once sliding begins the contact keeps breaking and remaking, so kinetic friction is smaller. Rolling friction is the smallest of the three, since a rolling wheel does not slide over the road but only presses and deforms it slightly at the point of contact. This is exactly why wheels, rollers and ball bearings are used to shift heavy loads. A and D are wrong because they place rolling friction at the top, though it is the least of the three. C is wrong because it puts rolling friction above kinetic friction, reversing the true order. Exam tip: static greater than kinetic greater than rolling; the smallest of them gave us the wheel.

The correct answer is A, The principle of conservation of energy is not violated. As the body falls its height shrinks, so the potential energy m g h falls, but exactly that much energy reappears as kinetic energy, half m v squared, because the body speeds up. The sum of the two stays the same at every point of the fall, so the total mechanical energy is conserved and energy is only changing form. At the moment of release the energy is all potential; just before the body strikes the ground it is almost all kinetic. When air resistance acts, the small shortfall leaves as heat and sound, so the energy is still accounted for. B is wrong because no energy is destroyed, only converted. C and D are wrong because gravity is a force, not a law that can be obeyed or broken, and neither statement answers what happens to the lost potential energy. Exam tip: for free fall, m g h plus half m v squared stays constant.

The correct answer is B, Class B. Fires are sorted into classes by what is burning, and a fire fed by flammable liquids such as petrol, diesel, kerosene, oil, paint or thinner is a class B fire. Such a fire is put out by smothering it with foam, carbon dioxide or dry chemical powder, which cuts off the oxygen. Water must never be thrown on burning oil, because the liquid floats on water and spreads the flames further. A is wrong because class D fires involve burning metals like magnesium, sodium and potassium, which need special dry powders. C is wrong because class C covers fires in live electrical equipment in common usage, and flammable gases under the Indian Standard. D is wrong because class A fires are the ordinary ones in solids like wood, paper and cloth, where water is the cheapest and best agent. Exam tip: A for solids, B for liquids, C for electrical or gas and D for metals.

The correct answer is D, Gustatory receptors. The receptors that pick up taste are called gustatory receptors, and they sit inside the taste buds on the tongue. They respond only to substances dissolved in saliva, which is why a dry tongue tastes little, and they report five basic tastes, sweet, sour, salty, bitter and umami. From the tongue the signal travels along cranial nerves to the brain, where taste and smell together give food its flavour. A is wrong because sensory receptor is the broad name for every receptor of the body, including those for light, sound, touch and pressure. B is wrong because chemoreceptor is the wider class that covers both taste and smell, so it does not name the taste receptor in particular. C is wrong because olfactory receptors lie in the lining of the nose and detect smell. Exam tip: gustatory means taste, olfactory means smell, auditory means hearing.