Which of the following shows the strongest force of attraction between particles?

The correct answer is D, Thallophyta. Algae belong to Thallophyta, the simplest division of the plant kingdom. Their body is a thallus, which means it is not divided into root, stem and leaf, and they have no vascular tissue. Most algae live in water, make their own food by photosynthesis and reproduce by spores; Spirogyra, Ulothrix, Chara and Volvox are common examples. Option A is wrong because angiosperms are flowering plants whose seeds lie enclosed in a fruit, the most advanced group of all. Option B is wrong because pteridophytes, such as ferns, are the first plants with proper vascular tissue, though they bear no seeds. Option C is wrong because bryophytes, such as mosses and Marchantia, are called the amphibians of the plant kingdom; they stand a step above thallophytes and need water for reproduction. Exam tip: the order of the plant kingdom is Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.

The correct answer is C, 500 W. The work done against gravity is mgh = 40 × 10 × 5 = 2000 joule, and power is work divided by time, so 2000 ÷ 4 = 500 watt.

Power is the rate of doing work, measured in watt, where one watt is one joule per second. In climbing stairs the useful work is the gain in potential energy, mgh, where only the vertical rise counts and the length of the staircase does not matter. If the same girl ran up in 2 seconds the work would be the same 2000 joule but the power would double to 1000 watt, which shows that power depends on how fast the work is done.

Option A is wrong because 100 W comes from dividing by the wrong quantity. Option B is wrong because 200 W would need 10 seconds. Option D is wrong because 2000 is the work in joule, not the power in watt.

Exam tip: work = mgh, power = work ÷ time, and one horsepower is about 746 watt.

The correct answer is C, Breaking a chalk stick. Force has to be applied to break the chalk because its particles hold one another together, and that pull is the force of attraction between particles.

Every substance is made of particles that attract one another, and the strength of that attraction decides how hard a thing is to break. The attraction is strongest in solids, less in liquids and least in gases, which is why an iron rod needs a hammer, water can be separated with a finger and air offers no resistance at all. The school activity of breaking chalk, then a pencil and then an iron nail shows the difference in this force from one solid to another.

Option A is wrong because compressing a sponge only squeezes out the air in its pores. Option B is wrong because dissolving sugar shows diffusion and the spaces between particles. Option D is wrong because heating ice shows a change of state, not the attraction itself.

Exam tip: attraction between particles is strongest in solids, weaker in liquids and weakest in gases.

The correct answer is C, zero. Constant velocity means no acceleration, and by Newton's second law force equals mass into acceleration, so the net force must be zero.

Newton's first law says a body continues in its state of rest or of uniform motion in a straight line unless an external force acts on it, which is why motion at a steady speed in a fixed direction needs no force to keep it going. Force is needed only to change the speed or the direction. A car moving at a steady 60 km per hour on a straight road is in the same state: the engine's push only balances friction and air resistance, so the forces cancel and the net force is zero. Such a body is said to be in equilibrium.

Options A, B and D are wrong because any net force in a direction would produce acceleration and so change the velocity, which contradicts the given condition of constant velocity.

Exam tip: constant velocity means zero acceleration and therefore zero net force, not zero force at all.

The correct answer is B, Platinum. Platinum is the least reactive of the four, so it is found in nature in the free or native state as the metal itself.

A metal occurs free only when it does not react with the air, water and other substances around it. In the reactivity series the metals at the bottom - gold, platinum and silver - are called noble metals and are found native; those higher up occur as oxides, sulphides or carbonates and must be extracted from their ores. This is why gold nuggets are found in river sand while aluminium has to be won from bauxite by electrolysis.

Option A is wrong because copper is mostly mined as copper pyrites and other ores, though small native deposits do exist. Option C is wrong because iron is always found as an oxide such as haematite or magnetite. Option D is wrong because potassium is so reactive that it burns on contact with water and is never found free.

Exam tip: gold, platinum and silver occur native; the more reactive a metal, the more firmly it stays locked in its ore.