General Science Mixed Quiz: Set 12

The correct answer is B. The third law says that to every action there is an equal and opposite reaction, and the crucial point is that the two forces act on two different bodies. A swimmer pushes the water backward and the water pushes the swimmer forward; the first force acts on the water and the second on the swimmer.

Option A states the popular misunderstanding: forces can cancel only when they act on the same body, so if action and reaction acted on the same body nothing would ever move. Option C contradicts the law, which insists the two are equal in magnitude. Option D reverses the direction; they are exactly opposite. Other standard examples are the recoil of a gun, the movement of a rocket as gases are pushed out behind it, and the backward push of the ground felt when walking.

The correct answer is A, Archimedes' Principle. It states that the upthrust on a body dipped fully or partly in a fluid equals the weight of the fluid the body pushes aside. This is why a bucket of water feels lighter while it is still in the well, and why a ship of steel floats: its hollow shape displaces enough water to balance its own weight. The principle also gives the law of flotation and is used to find the purity of metals and the density of liquids with a hydrometer. B is wrong because Bernoulli's principle links the speed of a flowing fluid with its pressure, and explains lift on an aeroplane wing. C is wrong because the Beer–Lambert law deals with how much light a solution absorbs as it passes through. D is wrong because Pascal's law says pressure applied to an enclosed fluid is passed on equally in every direction, which is the basis of hydraulic brakes and lifts. Exam tip: Archimedes for buoyancy, Pascal for hydraulic press, Bernoulli for aerofoil lift.

The correct answer is B, mechanical energy into electrical energy. A dynamo, also called a generator, is turned by some mechanical agency such as falling water, steam or a bicycle wheel, and the rotation of a coil in a magnetic field produces an electric current by electromagnetic induction.

Option A describes an electric motor, which is the exact reverse of a dynamo and drives fans, pumps and mixers. Option C describes a cell or battery, in which a chemical reaction pushes a current through a circuit. Option D describes a solar cell or photovoltaic cell, which turns sunlight directly into electricity. Device and conversion pairs are asked every year, so also fix the microphone as sound to electrical and the loudspeaker as electrical to sound.

The correct answer is D, General theory of relativity. LIGO, the Laser Interferometer Gravitational-Wave Observatory in the USA, detected gravitational waves for the first time in September 2015 (announced in February 2016), and such waves were predicted by Einstein's general theory of relativity in 1916. General relativity treats gravity as the curving of space-time and says that huge accelerating masses, like two black holes spiralling into each other, send ripples through space-time at the speed of light. LIGO's two detectors at Hanford and Livingston measured a change in arm length smaller than the width of a proton. India is building a third detector, LIGO-India, at Hingoli in Maharashtra. A, String theory, has made no prediction that any experiment has yet confirmed. B, Special relativity, deals with motion at constant speed and does not include gravity. C, Quantum mechanics, describes atoms and particles, not gravitational waves. Exam tip: LIGO means gravitational waves, which means general relativity; the 2017 Physics Nobel went to Weiss, Barish and Thorne for it.

The correct answer is A, speed. The slope of any graph is the change on the vertical axis divided by the change on the horizontal axis; here that is distance divided by time, which is speed by definition.

Option B, acceleration, is the slope of a velocity-time graph, not of a distance-time graph, and this is the pair that examiners deliberately confuse. Option C, distance, is read from the vertical axis directly and is not a slope at all. Option D, momentum, cannot be obtained from such a graph without the mass. Two shapes are worth recognising at sight: a straight sloping line on a distance-time graph means uniform speed, and a line parallel to the time axis means the body is stationary, while a curve of increasing slope means the body is speeding up.

The correct answer is C, Stapedius. The stapedius, only about 1 millimetre long, is the smallest skeletal muscle in the human body. It lies in the middle ear and is attached to the stapes, which is itself the smallest bone of the body. When a very loud sound reaches the ear, the stapedius contracts and damps the vibration of the stapes, protecting the delicate inner ear; this is called the acoustic reflex. It is supplied by the facial nerve, so damage to that nerve can make ordinary sounds seem painfully loud. Option A is wrong because the biceps is a large muscle at the front of the upper arm that bends the elbow. Option B is wrong because the sartorius, which runs across the thigh, is the longest muscle of the body, not the smallest. Option D is wrong because the triceps at the back of the upper arm straightens the elbow and is a large muscle. Exam tip: smallest muscle stapedius, smallest bone stapes, longest muscle sartorius, largest muscle gluteus maximus.

The correct answer is B, kinetic energy. The work-energy theorem states that the net work done on a body equals the difference between its final and initial kinetic energies, which is why a push that speeds a body up does positive work while friction, which slows it, does negative work.

Option A, momentum, changes with impulse, that is force multiplied by the time for which it acts, a different relation altogether. Option C, potential energy, changes with the work done against a conservative force such as gravity, and the theorem is not stated in those terms. Option D, power, is a rate and cannot be equated with work, which is a quantity. A useful check is that a force doing no work, such as the centripetal force, leaves the speed and therefore the kinetic energy unchanged.

The correct answer is C, 90 degrees. Work equals force into displacement into the cosine of the angle between them, and the cosine of ninety degrees is zero, so a force at right angles to the motion does no work at all. This is why the centripetal force in circular motion does no work and why a person carrying a load on the head along a level road does none against gravity.

Option A, zero degrees, gives the maximum positive work, since the cosine of zero is one and force and displacement point the same way. Option B, forty five degrees, gives partial positive work, as the cosine is about 0.707. Option D, 180 degrees, gives the maximum negative work, the case of friction opposing motion. Learning the four cosine values, one, 0.707, zero and minus one, settles this whole family of questions.

The correct answer is B, its mass. Inertia is the tendency of a body to resist any change in its state of rest or of uniform motion, and the heavier the body the harder it is to start or to stop. For this reason mass is described as the measure of inertia.

Option A, velocity, does not alter inertia: a stationary truck and a moving truck of the same mass are equally hard to accelerate. Option C, shape, changes air resistance and the distribution of matter but not the inertia associated with straight line motion. Option D, the applied force, decides the acceleration produced, not the inertia the body already possesses. A useful everyday comparison is that a loaded truck is much harder to push into motion, and much harder to bring to a halt, than a bicycle of the same make of wheel.

The correct answer is A, The pressure in blood vessels exceeds outside pressure. At great heights the air is thin, so the atmospheric pressure acting on the body is much lower than at sea level. The blood inside our vessels keeps pushing outwards at nearly its usual pressure. In the nose the blood vessels are very fine and lie close to the surface, so when the pressure inside them becomes greater than the pressure outside, some of them burst and the nose bleeds. The cold, dry mountain air also makes the lining of the nose crack more easily. The same fall in air pressure is why water boils below 100 °C on mountains and food takes longer to cook. B is wrong because the pressure inside the vessels becomes more than, not less than, the outside pressure. C is wrong because equal pressures would give no push to burst the vessels. D is wrong because blood does not become thick enough at a height to cause nose bleeding. Exam tip: higher altitude, lower air pressure, so the pressure in blood vessels is relatively higher and the nose can bleed.

The correct answer is A, zero. The force he applies to support the load acts vertically upward, while his displacement along the platform is horizontal, so the angle between force and displacement is ninety degrees and the work done against gravity is zero, however tired he may feel.

Option B, maximum, would apply only if the force and the displacement were in the same direction, as when he climbs a staircase with the load. Option C confuses force with work; the weight of the load is a force in newton, whereas work would be that force multiplied by a vertical displacement. Option D, negative, applies when the force opposes the motion, as friction does. This question is the classic example of the difference between the ordinary and the scientific sense of the word work.

The correct answer is C, 277. Water is densest at about 4 degrees Celsius, and 4 plus 273 gives roughly 277 kelvin. Most liquids keep shrinking as they cool, but water behaves oddly: below 4 degrees Celsius it starts to expand again because its molecules begin forming the open, hexagonal pattern of ice. This anomalous expansion is why ice is lighter than water and floats, and why a lake freezes from the top while the 4-degree water sinks to the bottom, letting fish survive the winter. Its density at that point is very close to 1 gram per cubic centimetre. D is wrong because 273 kelvin is 0 degrees Celsius, the freezing point, where water is already less dense than at 4 degrees. A is wrong because 0 kelvin is absolute zero, where no liquid water can exist. B is wrong because 227 kelvin is about minus 46 degrees Celsius, far below freezing. Exam tip: maximum density of water at 4 degrees Celsius equals 277 K; freezing at 273 K, boiling at 373 K.

The correct answer is B. Whether the hands are moved or not, the momentum of the ball must be brought down to zero. By the second law of motion the force is the change of momentum divided by the time taken, so stretching the catch over a longer time means a smaller force on the hands, and the catch does not hurt.

Option A is wrong because the fielder is removing momentum, not adding it. Option C is impossible; the mass of the ball cannot change. Option D confuses the issue, since the fielder reduces the rate at which the velocity changes rather than increasing it. The same idea explains a number of familiar safety measures: helmets and padded dashboards lengthen the time of impact, an athlete lands in a sand pit rather than on hard ground, and a person jumping down bends the knees on landing.

The correct answer is A, 20 Hz – 20000 Hz. A healthy human ear can hear sound waves with frequencies from about 20 hertz to 20,000 hertz (20 kHz); this band is called the audible range. Sounds below 20 Hz are called infrasonic, and animals such as elephants and whales use them to communicate over long distances. Sounds above 20 kHz are called ultrasonic; bats, dogs and dolphins can hear them, and ultrasound is used in medical scans, in cleaning delicate parts and in SONAR. Children under five and some animals can hear up to about 25 kHz, and the upper limit falls as people grow older. Option B is wrong because 80 to 100 Hz is only a tiny slice of the audible range. Option C is wrong because 2 lakh to 4 lakh Hz, that is 200 to 400 kHz, lies far inside the ultrasonic range. Option D is wrong because 0 to 20 Hz is the infrasonic range, which humans cannot hear. Exam tip: infrasonic below 20 Hz, audible from 20 Hz to 20 kHz, ultrasonic above 20 kHz.

The correct answer is B, potential energy. A stone at rest at a height has no motion and therefore no kinetic energy, but it possesses gravitational potential energy equal to its mass into the acceleration due to gravity into the height of the tower.

Option A, kinetic energy, is what that potential energy becomes as the stone falls, and it is greatest at the instant the stone reaches the ground. Option C, an equal share of both, holds only at the point where it has fallen through half its height, ignoring air resistance. Option D, heat energy, appears only after the impact, when the mechanical energy is dissipated. Through the fall the sum of the two mechanical forms stays constant, which is the law of conservation of energy in its simplest illustration.

The correct answer is A, it is dipolar in nature. Water is called the universal solvent because its molecule is polar: the oxygen end carries a partial negative charge and the two hydrogen ends carry partial positive charges. The molecule is bent at an angle of about 104.5°, so these charges do not cancel out. When a salt such as sodium chloride is put in water, the negative oxygen ends surround the sodium ions and the positive hydrogen ends surround the chloride ions, pulling the crystal apart; this is called hydration. Water also forms hydrogen bonds with polar substances such as sugar and alcohol, which is why they dissolve easily, while non-polar oil does not. Option B is wrong because water is in fact a poor conductor of heat, and heat conduction has nothing to do with dissolving. Option C is wrong because high specific heat explains why water warms up slowly, not why it dissolves things. Option D is wrong because being an oxide of hydrogen does not by itself make a liquid a good solvent. Exam tip: polar water dissolves polar and ionic substances, because like dissolves like.

The correct answer is C, 10 million. The erg is the unit of work in the CGS system, being one dyne of force acting through one centimetre, and since one newton is one lakh dyne and one metre is a hundred centimetre, one joule works out to ten million erg, that is 10 raised to the power seven.

Option A, ten thousand, and option B, one lakh, are the partial products a candidate gets by converting only the force or only the distance, which is exactly the slip the setter expects. Option D, a hundred million, overshoots by a factor of ten. The safe method is to convert both quantities and multiply: one lakh dyne into a hundred centimetre. Keep this beside the other conversions of the chapter, one calorie as about 4.2 joule and one kilowatt hour as 3.6 million joule.

The correct answer is B, Na₂CO₃·10H₂O. Washing soda is sodium carbonate decahydrate: each unit of sodium carbonate holds ten molecules of water of crystallisation. It is made from common salt in steps: baking soda is heated to get anhydrous sodium carbonate, called soda ash, which is then recrystallised from water to give washing soda crystals. It is a basic salt used for cleaning, in the glass, soap and paper industries, and to remove the permanent hardness of water. Paper setters like to put these related compounds side by side as options, so learn them together. A is wrong because NaHCO₃ is baking soda (sodium hydrogencarbonate), used in cooking and in antacids. C is wrong because Na₂CO₃ without water is soda ash, not the crystals called washing soda. D is wrong because NaCl is common salt, the starting material for all of these. Exam tip: NaCl → NaHCO₃ (baking soda) → Na₂CO₃ (soda ash) → Na₂CO₃·10H₂O (washing soda).

The correct answer is C, kinetic energy. At the top the object holds only potential energy; as it falls, that store is steadily converted into kinetic energy, and just as it touches the ground almost all of it is kinetic. Potential energy is mgh and kinetic energy is half of mv squared, and in free fall under gravity the sum of the two stays constant, which is the law of conservation of mechanical energy. At the instant of touching the ground the height is zero, so the potential energy is zero, while the speed is greatest, so the kinetic energy is at its maximum. A is wrong because heat appears only after the impact, from friction and deformation, not at the instant of touching. B is wrong because chemical energy is stored in bonds and plays no part in a falling body. D is wrong because potential energy has already dropped to zero at ground level. Exam tip: in a falling body potential energy decreases, kinetic energy increases, and the total mechanical energy stays the same.

The correct answer is C, Zinc. Galvanisation is the coating of iron or steel with a thin layer of zinc so that the iron underneath does not rust. Zinc sits above iron in the reactivity series, so even when the coating is scratched, zinc corrodes first and keeps protecting the iron; this is called sacrificial protection. The zinc layer is usually put on by dipping the article in molten zinc (hot-dip galvanising) or by electroplating. Galvanised sheets are used for roofs, water pipes, buckets and electricity poles. A, Gallium, is a rare, low-melting metal and is never used for rust protection. B, Aluminum, forms its own protective oxide layer, but coating iron with aluminium is not called galvanisation. D, Silver, is too costly and is less reactive than iron, so it would not protect it sacrificially. Exam tip: galvanisation means a zinc coating on iron; a tin coating on iron is called tinning.