Master the basics - and everything after gets easier

What it is

Pressure is how concentrated a force is - the same push on a smaller area presses much harder.

Answer

Pressure is the force acting on a unit area, P = F/A. A sharp knife has a very small area at its edge, so for the same force the pressure it exerts is much greater, and it cuts easily. A blunt knife has a larger edge area, so the same force produces less pressure and cuts poorly.

What it is

Friction is the grip between two surfaces that resists sliding.

Answer

Friction is called a necessary evil because it is both useful and harmful. It is necessary as it lets us walk, hold objects, write and stop vehicles. It is an evil because it wears out machine parts and shoe soles and produces heat that wastes energy. So we increase it where needed (treads, brakes) and reduce it where it wastes energy (lubricants, ball bearings).

What it is

Sound is a vibration that travels by shaking the particles of a medium.

Answer

Sound is produced by vibrating objects; the vibrations push the particles of the surrounding medium, creating compressions and rarefactions that travel outward as a wave. Sound needs a material medium (solid, liquid or gas) because it moves by making particles vibrate. In a vacuum there are no particles, so sound cannot travel.

What it is

Electroplating uses electricity to coat one metal with a thin layer of another.

Answer

Electroplating is the process of depositing a layer of one metal over another using electricity (the chemical effect of electric current). When current passes through a solution of a metal salt, metal ions are deposited on the object connected to the negative terminal. It is used to coat iron with chromium or nickel to prevent rusting and to make cheap ornaments look like gold or silver.

What it is

Metals are shiny, bendable conductors of heat and electricity; non-metals are dull, brittle insulators.

Answer

Metals are generally lustrous (shiny), malleable and ductile, good conductors of heat and electricity, and sonorous. Non-metals are usually dull, brittle (break easily) and poor conductors of heat and electricity. For example, iron is a shiny conductor while sulphur is a dull, brittle insulator.

What it is

Burning needs three partners together: fuel, oxygen and enough heat.

Answer

For combustion, three things must be present together: a combustible substance (fuel), a supporter of combustion (usually oxygen from air) and heat to raise the fuel to its ignition temperature. If any one is removed, burning stops - which is why covering a fire with a blanket (cutting off oxygen) or water (cooling below the ignition temperature) puts it out.

What it is

Coal and petroleum are fuels formed from ancient buried life over millions of years.

Answer

Coal and petroleum are called fossil fuels because they were formed from the dead remains of prehistoric plants and animals buried under the earth for millions of years under high heat and pressure. They are exhaustible natural resources - they take millions of years to form but are being used up fast - so they must be conserved and used carefully.

What it is

Most plastics do not rot (they are non-biodegradable), so they pile up for centuries.

Answer

Most plastics are non-biodegradable, which means micro-organisms cannot break them down, so they stay in the environment for hundreds of years. Burning plastic releases harmful gases, and discarded plastic chokes drains, harms animals that swallow it and pollutes soil and water. The best approach is to reduce, reuse and recycle plastic.

What it is

Plant cells have a cell wall, a big vacuole and chloroplasts; animal cells have none of these.

Answer

A plant cell has a rigid cell wall outside its cell membrane and usually contains a large central vacuole and green chloroplasts for photosynthesis. An animal cell has no cell wall (only a cell membrane), has small or no vacuoles and lacks chloroplasts. This is why plants can make their own food while animals cannot.

What it is

Microbes are tiny living things that can help us (curd, bread, medicine) or harm us (disease).

Answer

Useful: yeast is used to make bread and in brewing (fermentation), and Lactobacillus bacteria turn milk into curd. Harmful: the bacterium that causes tuberculosis, and the Plasmodium protozoan that causes malaria. So micro-organisms can help or harm us depending on the type.

What it is

Manure is natural and releases nutrients slowly; fertiliser is a chemical that acts fast.

Answer

Manure is a natural substance made by decomposing animal dung and plant waste; it adds humus, improves soil texture and releases nutrients slowly. A fertiliser is a chemical made in factories that is rich in a specific nutrient (like nitrogen, phosphorus or potassium) and acts fast, but its overuse can harm the soil and water. Manure is eco-friendly; fertilisers give a quick nutrient boost.

What it is

At puberty, hormones bring body changes that mark a child becoming an adult.

Answer

Secondary sexual characters are the physical features that appear at puberty and distinguish males from females - such as growth of facial hair and deepening of the voice in boys, and development of breasts in girls. They are controlled by hormones released by the endocrine glands: testosterone from the testes in boys and oestrogen from the ovaries in girls.

What it is

Solving an equation means undoing the operations to leave the unknown by itself.

Answer

Subtract 5 from both sides: 3x = 20 - 5 = 15. Divide both sides by 3: x = 15/3 = 5. So x = 5. Check: 3(5) + 5 = 15 + 5 = 20, which is correct.

What it is

A square root asks: which number multiplied by itself gives this number?

Answer

Factorise: 1225 = 5 x 5 x 7 x 7 = 5^2 x 7^2. Pair the equal factors and take one from each pair: sqrt(1225) = 5 x 7 = 35. So the square root of 1225 is 35.

What it is

Standard form writes very large or very small numbers as a x 10^n.

Answer

Move the decimal point to just after the first non-zero digit to get 7.3. The point moved 6 places to the right, so we multiply by 10 raised to the power -6. Therefore 0.0000073 = 7.3 x 10^-6.

What it is

Profit percent measures your gain compared with the cost price, not the selling price.

Answer

Profit = selling price - cost price = 575 - 500 = 75 rupees. Profit percent = (profit / cost price) x 100 = (75 / 500) x 100 = 15 percent. So the shopkeeper makes a 15 percent profit.

What it is

Reflection is light bouncing off a surface following two fixed rules.

Answer

First law: the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. Second law: the angle of incidence is always equal to the angle of reflection (i = r).

What it is

Greenhouse gases act like a blanket that traps the Sun's heat around the Earth.

Answer

The greenhouse effect is the trapping of the Sun's heat near the Earth's surface by gases like carbon dioxide, methane and water vapour, which keeps the planet warm enough for life. It becomes a concern when human activities release extra greenhouse gases, trapping too much heat and causing global warming and climate change.

What it is

Animals either lay eggs (oviparous) or give birth to live young (viviparous).

Answer

Oviparous animals lay eggs that develop and hatch outside the mother's body, for example a hen or a frog. Viviparous animals give birth to live young that develop inside the mother's body, for example a human or a cow.

What it is

Algebraic identities are shortcuts that expand or factor expressions instantly.

Answer

Use the identity (a + b)^2 = a^2 + 2ab + b^2 with a = x and b = 5. So (x + 5)^2 = x^2 + 2(x)(5) + 5^2 = x^2 + 10x + 25.

What it is

Lightning is a huge electric spark between charged clouds or between a cloud and the ground.

Answer

During a thunderstorm, air currents make the top of a cloud positively charged and the bottom negatively charged. When the charge grows large enough it jumps as a giant spark - lightning - to another cloud or to the ground. A lightning conductor is a metal rod fixed at the top of a building and connected to the earth; it safely carries the lightning's charge into the ground and protects the building.

What it is

A good fuel gives lots of heat, is cheap, easy to store and burns cleanly.

Answer

A good fuel should have a high calorific value (release a large amount of heat per kilogram), a moderate ignition temperature, be readily available and cheap, be easy to store and transport, and burn without leaving much ash or releasing harmful smoke. For example, LPG is a good domestic fuel because it has a high calorific value and burns cleanly.

What it is

Biodiversity is the variety of all living things - plants, animals and microbes - in an area.

Answer

Biodiversity is the variety of living organisms - plants, animals and micro-organisms - found in a particular area. It should be conserved because every species has a role in keeping the ecosystem balanced, and the loss of one can affect many others. Biodiversity gives us food, medicines, and clean air and water, and losing it through deforestation and pollution can push species to extinction.

What it is

A trapezium's area is the average of its two parallel sides times the height.

Answer

Area of a trapezium = (1/2) x (sum of parallel sides) x height = (1/2) x (12 + 8) x 5 = (1/2) x 20 x 5 = 50 square cm.

What it is

Speed tells how fast something moves - the distance covered in unit time.

Answer

Speed = distance / time = 60 / 10 = 6 metres per second (m/s). If the same distance is covered in less time the speed is higher; if it takes more time the speed is lower.

What it is

Magnification tells how many times larger an object looks than its real size.

Answer

Magnification = image size / actual size = 60 / 6 = 10. The object looks 10 times larger than it really is. Magnification has no unit because it is a ratio of two lengths.

What it is

Simple interest is the extra money paid for borrowing, or earned for saving, a sum.

Answer

Simple interest = P x R x T / 100 = 2000 x 5 x 3 / 100 = 300. So the interest is 300 in the same currency as the principal. Increasing the principal, the rate or the time each increases the interest.

What it is

Pressure is the force acting on unit area of a surface.

Answer

Pressure = force / area = 100 / 2 = 50 pascal (Pa). The same force pressed onto a smaller area produces a much higher pressure, which is why a sharp edge cuts easily.

What it is

A compound microscope's total magnification is the eyepiece magnification times the objective magnification.

Answer

Total magnification = eyepiece x objective = 10 x 40 = 400. The image appears 400 times larger than the real object. Choosing a stronger objective or eyepiece raises the total magnification.

What it is

The area of a circle grows with the square of its radius.

Answer

Area = pi x r^2 = 3.14159 x 7^2 = 3.14159 x 49 = 153.94 square units. Because the radius is squared, doubling it makes the area four times as large.

What it is

The volume of a cuboid is the product of its length, breadth and height.

Answer

Volume = length x breadth x height = 3 x 4 x 5 = 60 cubic units. Increasing any one of the three dimensions increases the volume in proportion.

What it is

A percentage expresses a part out of a whole as a number per hundred.

Answer

Percentage = (part / whole) x 100 = (45 / 180) x 100 = 25%. A percentage rescales any fraction to 'out of 100', which makes different quantities easy to compare.

What it is

Distance travelled is the product of speed and the time for which the body moves.

Answer

Distance = speed x time = 10 x 30 = 300 metres (m). This is simply the speed formula rearranged: since speed = distance / time, distance = speed x time.

What it is

The area of a triangle is half the product of its base and height.

Answer

Area = 1/2 x base x height = 1/2 x 10 x 8 = 40 square units. A triangle covers exactly half of a rectangle with the same base and height, which is where the 1/2 comes from.

What it is

The area of a rectangle is its length multiplied by its breadth.

Answer

Area = length x breadth = 5 x 4 = 20 square units. If the length and breadth are equal, the rectangle is a square and the area becomes side x side.

What it is

A discount is a reduction on the marked price, found as a percentage of that price.

Answer

Discount = marked price x rate / 100 = 2000 x 25 / 100 = 500. So you save 500 and pay 2000 - 500 = 1500. A higher rate or a higher marked price gives a bigger saving.

What it is

The area of a square is the length of its side multiplied by itself.

Answer

Area = side x side = 6 x 6 = 36 square units. Because the side is squared, doubling it to 12 makes the area 144 - four times as large.

What it is

The volume of a cube is the length of its edge raised to the power three.

Answer

Volume = side x side x side = 4^3 = 64 cubic units. Since the edge is cubed, even a small increase in the side makes the volume grow quickly.

What it is

The circumference is the distance around a circle, equal to 2 x pi x radius.

Answer

Circumference = 2 x pi x r = 2 x 3.14159 x 7 = 43.98 units. Unlike the area, the circumference grows in direct proportion to the radius - double the radius and you double the distance around.

What it is

Compound interest is calculated on the principal plus the interest already earned, so it grows faster than simple interest.

Answer

Amount A = P(1 + R/100)^T = 1000 x (1 + 10/100)^2 = 1000 x 1.21 = 1210. Compound interest = A - P = 1210 - 1000 = 210. This is more than the simple interest (200) because the second year's interest is earned on 1100, not just 1000.

What it is

Profit percentage is the profit expressed as a percentage of the cost price.

Answer

Profit = SP - CP = 2500 - 2000 = 500. Profit % = (profit / CP) x 100 = (500 / 2000) x 100 = 25%. The percentage is always taken on the cost price; if the selling price is below the cost price the answer is negative, showing a loss.

What it is

The perimeter of a rectangle is twice the sum of its length and breadth.

Answer

Perimeter = 2 x (length + breadth) = 2 x (5 + 3) = 2 x 8 = 16 units. Because a rectangle has two lengths and two breadths, we add one of each and double the total.

What it is

The perimeter of a square is four times the length of one side.

Answer

Perimeter = 4 x side = 4 x 6 = 24 units. Since a square has four equal sides, we simply multiply one side by four.

What it is

The perimeter of a triangle is the sum of the lengths of its three sides.

Answer

Perimeter = a + b + c = 3 + 4 + 5 = 12 units. For any triangle, the perimeter is just the sum of its three side lengths.

What it is

The area of a parallelogram is its base multiplied by its perpendicular height.

Answer

Area = base x height = 10 x 6 = 60 square units. The height must be the perpendicular distance between the base and its opposite side, not the length of the slanting side.

What it is

The area of a rhombus is half the product of its two diagonals.

Answer

Area = 1/2 x d1 x d2 = 1/2 x 8 x 6 = 24 square units. This works because the diagonals of a rhombus bisect each other at right angles, splitting it into four right triangles.

What it is

Percentage change is the increase or decrease compared with the original value, expressed as a percentage.

Answer

Percentage change = (new - old) / old x 100 = (100 - 80) / 80 x 100 = 20 / 80 x 100 = 25%. It is always taken on the original (old) value; if the new value is smaller, the change is negative, showing a decrease.

What it is

The sum of the interior angles of a polygon depends only on its number of sides.

Answer

Sum of interior angles = (n - 2) x 180 = (5 - 2) x 180 = 3 x 180 = 540 degrees. A polygon can be split into (n - 2) triangles, each contributing 180 degrees.

What it is

Each exterior angle of a regular polygon is 360 degrees divided by the number of sides.

Answer

Each exterior angle = 360 / n = 360 / 6 = 60 degrees. The exterior angles of any polygon always add up to 360 degrees - one full turn - so for a regular polygon we just divide 360 by the number of sides.

What it is

The number of diagonals of a polygon is n times (n minus 3), divided by two.

Answer

Diagonals = n(n - 3) / 2 = 6 x (6 - 3) / 2 = 6 x 3 / 2 = 9. From each corner you can draw a diagonal to every other corner except itself and its two neighbours, and dividing by two avoids counting each diagonal twice.

What it is

Loss percentage is the loss expressed as a percentage of the cost price.

Answer

Loss = CP - SP = 2000 - 1600 = 400. Loss % = (loss / CP) x 100 = (400 / 2000) x 100 = 20%. Like profit, it is always taken on the cost price; if the selling price is above the cost price, this becomes a profit instead.

What it is

The amount to be repaid is the principal plus the simple interest earned on it.

Answer

Simple interest = P x R x T / 100 = 2000 x 5 x 3 / 100 = 300. Amount = principal + interest = 2000 + 300 = 2300. The amount is always the principal plus whatever interest has built up.

What it is

A plant cell is made of parts (organelles), each with its own job, working together to keep the cell alive.

Answer

The cell wall is a rigid outer layer that gives shape and support. Inside it, the cell membrane controls what enters and leaves. The cytoplasm is the jelly holding the organelles. The nucleus is the control centre with the DNA. The large vacuole stores water and keeps the cell firm, and the chloroplasts contain chlorophyll to carry out photosynthesis.

What it is

The selling price for a required profit is the cost price increased by that profit percentage.

Answer

SP = CP x (1 + profit% / 100) = 2000 x (1 + 25/100) = 2000 x 1.25 = 2500. The profit of 500 is 25% of the cost price, so the selling price is 2500.

What it is

The circumference of a circle is pi times its diameter.

Answer

Circumference = pi x diameter = 3.14159 x 7 = 21.99 units. Because the diameter is twice the radius, this is exactly the same as 2 x pi x r. In fact pi is defined as the ratio of the circumference to the diameter.

What it is

A percentage of a number is that fraction out of a hundred, multiplied by the number.

Answer

Result = (percent / 100) x number = (20 / 100) x 150 = 0.2 x 150 = 30. Finding a percentage of a number just means multiplying the number by that fraction out of a hundred.

What it is

Each interior angle of a regular polygon is the total angle sum divided by the number of sides.

Answer

Interior angle = (n - 2) x 180 / n = (6 - 2) x 180 / 6 = 720 / 6 = 120 degrees. We share the total interior angle sum, (n - 2) x 180, equally among the n corners.

What it is

Depreciation reduces an asset's value by a fixed percentage each year, like compound interest in reverse.

Answer

Value = P x (1 - r/100)^n = 50000 x (1 - 10/100)^2 = 50000 x 0.9^2 = 50000 x 0.81 = 40500. Each year the value becomes 90% of the previous year, so it falls faster at first.

What it is

The cost price can be recovered from the selling price and the profit percentage by dividing.

Answer

CP = SP / (1 + profit% / 100) = 2500 / (1 + 25/100) = 2500 / 1.25 = 2000. Because the selling price is 125% of the cost price, we divide by 1.25 to get back to 100%.

What it is

When a boat travels downstream, the stream's speed adds to the boat's own speed.

Answer

Downstream speed = boat speed + stream speed = 10 + 4 = 14 km/h. Going with the current, the stream pushes the boat along, so the two speeds add up.

What it is

When a boat travels upstream, the stream's speed is subtracted from the boat's own speed.

Answer

Upstream speed = boat speed - stream speed = 10 - 4 = 6 km/h. Going against the current, the stream slows the boat, so we subtract the stream speed.

What it is

For equal distances at two different speeds, the average speed is the harmonic mean of the two speeds.

Answer

For equal distances, average speed = 2ab / (a + b) = 2 x 40 x 60 / (40 + 60) = 4800 / 100 = 48 km/h. It is less than the simple average of 50 because more time is spent at the slower speed.

What it is

Sales tax is a percentage of the price added on top of it.

Answer

Tax = price x rate / 100 = 1000 x 12 / 100 = 120. The final amount paid is the price plus this tax, so 1000 + 120 = 1120.

What it is

The side of a square is the square root of its area.

Answer

Side = sqrt(area) = sqrt(144) = 12 units, because 12 x 12 = 144. Finding the side of a square from its area is exactly what taking a square root means.

What it is

The rate of interest is found by rearranging the simple interest formula.

Answer

Rate = SI x 100 / (P x T) = 300 x 100 / (2000 x 3) = 30000 / 6000 = 5% per year. This is SI = P x R x T / 100 rearranged to make R the subject.

What it is

The time is found by rearranging the simple interest formula.

Answer

Time = SI x 100 / (P x R) = 300 x 100 / (2000 x 5) = 30000 / 10000 = 3 years. This is SI = P x R x T / 100 rearranged to make T the subject.

What it is

The principal is found by rearranging the simple interest formula.

Answer

Principal = SI x 100 / (R x T) = 300 x 100 / (5 x 3) = 30000 / 15 = 2000. This is SI = P x R x T / 100 rearranged to make P the subject.

What it is

The fourth proportional completes a proportion a : b = c : d.

Answer

Since a : b = c : d means a x d = b x c, we get d = (b x c) / a = (3 x 4) / 2 = 12 / 2 = 6. So 2 : 3 = 4 : 6.

What it is

The marked price is recovered from the selling price and the discount percentage.

Answer

MP = SP / (1 - discount% / 100) = 1500 / (1 - 25/100) = 1500 / 0.75 = 2000. Because the selling price is 75% of the marked price, we divide by 0.75 to get back to the full price.

What it is

To pass a pole, a train travels a distance equal to its own length, so its speed is length divided by time.

Answer

Speed = length / time = 200 / 10 = 20 m/s. When a train crosses a pole (a point), the distance it covers is just its own length, so the speed is the length divided by the time.

What it is

The number of workers and the days to finish a job are inversely proportional.

Answer

Since workers x days is constant, days2 = (workers1 x days1) / workers2 = (10 x 12) / 15 = 120 / 15 = 8 days. More workers share the same total work, so it finishes in fewer days.

What it is

To divide a total in a given ratio, each share is the total times that part over the sum of the parts.

Answer

First share = total x a / (a + b) = 500 x 2 / (2 + 3) = 500 x 2 / 5 = 200. The second share is 500 x 3 / 5 = 300, and the two shares add back to 500.

What it is

The unit rate is the cost of a single item, found by dividing the total cost by the quantity.

Answer

Unit rate = total cost / quantity = 240 / 12 = 20 per item. This is the unitary method: find the value of one unit, then you can scale up to any number.

What it is

A force is a push or a pull on an object that arises from an interaction between two objects.

Answer

A force is a push or a pull acting on an object due to its interaction with another object. A force can make a stationary object move, stop a moving object, change its speed, change its direction of motion, and change the shape or size of an object.

What it is

When forces on an object cancel out they are balanced; when they do not, the net (unbalanced) force changes the object's motion.

Answer

Balanced forces are equal in size and act in opposite directions, so their net force is zero and the object's state of motion does not change - for example, a book resting on a table. Unbalanced forces do not cancel out, so there is a net force that changes the object's speed or direction - for example, when one team in a tug of war pulls harder, the rope moves towards them.

What it is

Contact forces act only on touching (muscular, friction); non-contact forces act from a distance (magnetic, electrostatic, gravitational).

Answer

Contact forces act only when two objects are in physical contact, such as muscular force and frictional force. Non-contact forces act even without contact, over a distance, such as magnetic force, electrostatic force and gravitational force.

What it is

Pressure is the force acting per unit area on a surface.

Answer

Pressure = force / area = 100 / 2 = 50 N/m^2 (pascal). If the area is halved to 1 m^2 with the same force, pressure = 100 / 1 = 100 Pa, so halving the area doubles the pressure.

What it is

Liquids exert pressure on the container walls and base, and this pressure increases with depth.

Answer

A liquid exerts pressure in all directions on the walls and base of its container. The pressure increases as the depth increases, and at the same depth it is the same in all directions. This is why the walls of a dam are made much thicker at the bottom than at the top.

What it is

The weight of the air column above us presses on every surface; this is atmospheric pressure.

Answer

Atmospheric pressure is the pressure exerted by the weight of the column of air above a surface. It is very large, but we are not crushed because the pressure of the blood and fluids inside our body is equal to the atmospheric pressure and balances it from inside.

What it is

Muscular force comes from the action of muscles; gravitational force is the pull of the Earth on every object.

Answer

Gravitational force is the force with which the Earth pulls every object towards its centre; it gives an object its weight. The force used to lift a bucket is muscular force, a contact force produced by our muscles.

What it is

Friction is the force that opposes the relative motion between two surfaces in contact.

Answer

Friction is the force that opposes the relative motion between two surfaces that are in contact. It always acts in the direction opposite to the motion (or attempted motion) of the object. Friction is caused by the interlocking of tiny irregularities on the two surfaces.

What it is

Friction depends on how rough the surfaces are and on how hard they are pressed together.

Answer

Friction depends on the nature (roughness or smoothness) of the two surfaces in contact - rougher surfaces produce more friction - and on how hard the surfaces are pressed together, that is, the force pressing them, so a heavier object experiences more friction.

What it is

Static friction acts before motion starts, sliding friction during sliding, and rolling friction when a body rolls; rolling is the smallest.

Answer

The three types are static friction (acts when the object is at rest and about to move), sliding friction (acts when one surface slides over another) and rolling friction (acts when a body rolls over a surface). In decreasing order they are static friction > sliding friction > rolling friction, so rolling friction is the smallest.

What it is

Friction is useful (walking, gripping, braking) but also wastes energy and wears out parts.

Answer

Advantages: friction lets us walk without slipping and lets vehicles stop when brakes are applied; it also helps us write and hold objects. Disadvantages: friction wears out the soles of shoes, tyres and machine parts, and it produces heat that wastes useful energy.

What it is

Friction is reduced using lubricants, ball bearings, streamlined shapes and polished surfaces.

Answer

Friction can be reduced by applying lubricants such as oil and grease, which form a thin layer between surfaces; by using ball bearings that convert sliding friction into much smaller rolling friction; and by polishing surfaces or giving vehicles a streamlined shape to reduce the friction of air and water.

What it is

Sound is produced by vibrating objects and travels as a disturbance through a medium.

Answer

Sound is produced when an object vibrates, that is, moves rapidly to and fro. For example, a drum makes sound when its stretched membrane vibrates, and in humans sound is produced by the vibration of the vocal cords in the voice box (larynx). When the vibration stops, the sound stops.

What it is

Sound needs a material medium (solid, liquid or gas) to travel and cannot pass through a vacuum.

Answer

No, sound cannot travel through a vacuum because it needs a material medium (solid, liquid or gas) whose particles vibrate and pass on the disturbance. In a vacuum there are no particles to carry the vibrations. This is shown by the bell-jar experiment, where the sound of a ringing bell fades as the air is pumped out.

What it is

Amplitude is the maximum displacement of a vibration, frequency is the number of oscillations per second, and time period is the time for one oscillation.

Answer

Amplitude is the maximum distance a vibrating object moves from its central rest position. Frequency is the number of complete oscillations made in one second, measured in hertz (Hz). Time period is the time taken to complete one oscillation. Frequency and time period are reciprocals of each other.

What it is

Loudness depends on the amplitude of a sound, while pitch depends on its frequency.

Answer

The loudness of a sound depends on the amplitude of the vibration - a larger amplitude produces a louder sound. The pitch (shrillness) of a sound depends on its frequency - a higher frequency produces a higher pitch, like a whistle, while a lower frequency produces a lower pitch, like a drum.

What it is

Humans hear sounds with frequencies from about 20 Hz to 20000 Hz.

Answer

The audible range for a normal human ear is from about 20 Hz to 20000 Hz. Sounds with frequencies below 20 Hz are called infrasound and those above 20000 Hz are called ultrasound; both are inaudible to humans, though animals such as dogs and bats can hear ultrasound.

What it is

Unpleasant, unwanted, loud sound is noise; too much of it in the environment is noise pollution.

Answer

The presence of excessive or unwanted loud sound in the environment is called noise pollution. Its harmful effects include loss of hearing, and stress-related problems such as lack of sleep, anxiety and high blood pressure. It can be reduced by planting trees along roads, which absorb sound, and by using silencers in vehicles and machines.

What it is

Liquids that conduct electricity (like salt or acid solutions) do so through the movement of ions.

Answer

No, not all liquids conduct electricity. Liquids such as solutions of common salt, acids and bases conduct electricity because they contain freely moving ions that carry the current. Liquids such as distilled water, oil and alcohol are poor conductors because they have very few free ions.

What it is

Electroplating is depositing a layer of one metal on another using electricity.

Answer

Electroplating is the process of depositing a thin layer of a desired metal over another material using electric current. It is an application of the chemical effect of electric current. It is used to coat iron taps and car parts with shiny chromium to prevent rusting and improve appearance, and to coat iron cans with tin for storing food.

What it is

Rubbing certain objects transfers charge; there are two kinds of charge - positive and negative.

Answer

When two objects are rubbed together, such as a plastic comb through dry hair, charge is transferred from one to the other, so both become electrically charged. There are two kinds of charge, positive and negative. Like charges (both positive or both negative) repel each other, while unlike charges (one positive and one negative) attract each other.

What it is

Lightning is a huge electric discharge between charged clouds or between a cloud and the ground; a lightning conductor safely carries it to earth.

Answer

Lightning is caused by the movement of accumulated electric charges - an electric discharge - between clouds or between a charged cloud and the earth. A lightning conductor is a metal rod fixed on the top of a tall building and connected to a metal plate buried in the ground. It provides an easy conducting path so that the lightning's charge passes safely into the earth without damaging the building.

What it is

When light reflects, the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.

Answer

First law: the angle of incidence is equal to the angle of reflection, where both angles are measured from the normal (the line perpendicular to the mirror at the point of incidence). Second law: the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

What it is

Reflection from a smooth surface is regular and forms images; reflection from a rough surface is diffused and does not.

Answer

In regular reflection, light falling on a smooth surface such as a plane mirror is reflected in one direction, so parallel incident rays stay parallel and a clear image is formed. In diffused (irregular) reflection, light falling on a rough surface such as paper or a wall is reflected in many different directions, so no clear image is formed. In both cases the laws of reflection are still obeyed at each point.

What it is

Speed tells how fast an object moves; it is the distance travelled per unit time.

Answer

Average speed = total distance / total time = 240 / 4 = 60 km/h. To convert to m/s, multiply by 1000/3600 (that is, divide by 3.6): 60 / 3.6 = 16.67 m/s approximately.

What it is

In a physical change no new substance forms and it is usually reversible; in a chemical change a new substance forms and it is usually not reversible.

Answer

A physical change is one in which no new substance is formed and only properties such as shape, size or state change; it is usually reversible, for example the melting of ice into water. A chemical change is one in which one or more new substances with new properties are formed; it is usually irreversible and often involves energy change, for example the burning of paper into ash and gases.

What it is

Chemical changes show signs such as change in colour, evolution of gas, formation of a precipitate, and change in temperature.

Answer

A chemical change may be recognised by a change in colour (iron rusting to reddish-brown), the evolution of a gas (bubbles when an acid meets a metal), the formation of a precipitate (an insoluble solid), and a change in temperature or the giving out of heat and light (burning). A change in smell may also occur.