NSEJS key terms

The tendency of an object to resist any change in its state of rest or uniform motion unless acted on by a net external force. It depends on mass, so a heavier body is harder to accelerate or stop.

The net force on a body equals the rate of change of its momentum, or mass times acceleration when mass is constant. It links force, mass, and acceleration quantitatively in one vector equation.

The product of an object's mass and its velocity, a vector pointing along the motion. In an isolated system the total momentum stays constant, which makes collisions and explosions solvable.

The product of a force and the time for which it acts; it equals the change in momentum produced. A longer contact time reduces the force needed for the same momentum change.

The net work done on a body by all forces equals the change in its kinetic energy. It converts force-and-distance information into speed information without tracking time step by step.

The energy a body possesses because of its motion, equal to half its mass times the square of its speed. Doubling speed quadruples kinetic energy, so stopping distance grows steeply with speed.

When only conservative forces act, the sum of kinetic and potential energy stays constant. Energy shifts between motion and position while the total remains fixed, simplifying pendulum and free-fall problems.

The net inward force that keeps a body moving along a circular path, directed toward the centre. It equals mass times speed squared divided by radius and changes direction, not speed.

The rate at which a rotating body sweeps out angle, measured in radians per second. It relates linear speed to radius through v equals omega times r for points on the rotating object.

The turning effect of a force, calculated as force times the perpendicular distance from the pivot to the line of action. It determines rotational acceleration and balance of levers.

The point at which the whole mass of a body may be treated as concentrated for analysing its overall motion. A freely falling or projectile body rotates about it while it follows a smooth path.

A body wholly or partly immersed in a fluid experiences an upward buoyant force equal to the weight of fluid it displaces. Floating occurs when this force balances the body's weight.

The force acting perpendicularly per unit area on a surface, measured in pascals. In a fluid at rest it increases with depth, which explains why dams are thicker near the base.

The mass of a substance per unit volume, usually expressed in kilograms per cubic metre or grams per cubic centimetre. Comparing an object's density with a fluid's predicts whether it floats.

The ratio of the speed of light in vacuum to its speed in a medium, always at least one. It governs how much a ray bends at a boundary and whether total internal reflection can occur.

The complete reflection of light back into a denser medium when it strikes a boundary beyond the critical angle. It carries light along optical fibres and makes diamonds sparkle.

The distance from the pole of a spherical mirror or the centre of a thin lens to its principal focus. It fixes image position through the lens or mirror formula and sets the power of the element.

For a metallic conductor at constant temperature, current is directly proportional to potential difference, giving voltage equals current times resistance. Devices that do not obey it are called non-ohmic.

The opposition a component offers to the flow of electric current, measured in ohms. For a uniform wire it rises with length and falls with cross-sectional area and with the material's conductivity.

The rate at which electrical energy is transferred by a component, equal to voltage times current. It can also be written as current squared times resistance for a pure resistor.

In a series arrangement the same current passes through each component and resistances add; in parallel the same voltage applies across each branch and conductances add. These rules allow reduction of networks.

The production of an electromotive force in a conductor when the magnetic flux through it changes. The induced effect opposes the change that caused it, as stated in Lenz's law.

The amount of substance containing as many elementary entities as there are atoms in twelve grams of carbon-12, about 6.022 times ten to the twenty-third. It bridges masses weighed in the lab and particle counts.

The mass of one mole of a substance in grams per mole, numerically equal to its formula mass in atomic mass units. It converts between mass and moles in every stoichiometric calculation.

The number of constituent particles in one mole, approximately 6.022 times ten to the twenty-third per mole. It lets chemists count atoms and molecules by weighing macroscopic samples.

The calculation of quantities of reactants and products using the mole ratios given by a balanced chemical equation. The limiting reactant sets the maximum yield of product.

The reactant that is completely consumed first and therefore determines the maximum amount of product formed. Others remain in excess and do not control the outcome of the reaction.

The simplest whole-number ratio of atoms of each element in a compound, such as CH2 for ethene. It is found from percentage composition and may differ from the actual molecular formula.

Loss of electrons by a species, accompanied by an increase in oxidation number. In terms of oxygen and hydrogen it means gain of oxygen or loss of hydrogen from a substance.

Gain of electrons by a species, accompanied by a decrease in oxidation number. It always occurs together with oxidation, since electrons released by one species are taken by another.

A formal charge assigned to an atom assuming bonding electrons go to the more electronegative partner. Tracking these numbers reveals which atoms are oxidised or reduced in a reaction.

A logarithmic measure of hydrogen ion concentration in a solution, defined as the negative logarithm to base ten of that concentration. Values below seven indicate acidity, above seven alkalinity.

An acid donates a proton while a base accepts one, according to the Bronsted-Lowry description. They react together in neutralisation to give a salt and water.

A solution that resists large changes in pH when small amounts of acid or base are added. It usually contains a weak acid with its conjugate base in comparable amounts.

The recurring pattern of chemical and physical properties of elements as atomic number increases across a period. Trends in size, ionisation energy, and metallic character follow from electron configuration.

The minimum energy needed to remove one electron from a gaseous atom or ion in its ground state. It generally increases across a period and decreases down a group as size grows.

A measure of how strongly an atom in a bond attracts the shared pair of electrons. Large differences between bonded atoms produce polar bonds and often ionic character.

A chemical bond formed when two atoms share one or more pairs of electrons, usually between nonmetals. The shared pair is attracted to both nuclei, holding the atoms together.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving different properties. Structural isomers differ in connectivity while stereoisomers differ in spatial arrangement.

A family of organic compounds with the same functional group and general formula, each member differing from the next by one CH2 unit. Members show gradually changing physical properties and similar chemistry.

The movement of water molecules across a partially permeable membrane from a region of higher water potential to one of lower water potential. It is passive and central to water uptake by plant roots.

The net movement of particles from a region of higher concentration to one of lower concentration due to random motion. It needs no membrane and is faster at higher temperature and shorter distances.

Movement of ions or molecules across a membrane against their concentration gradient, using energy from ATP and carrier proteins. It allows cells to accumulate substances in higher concentration than outside.

The shrinking of a plant cell's protoplast away from the cell wall when the cell loses water by osmosis in a hypertonic solution. It shows that the wall is rigid while the membrane is not.

The outward pressure of cell contents against the plant cell wall when the cell is full of water. It keeps non-woody plants upright and drives cell expansion during growth.

The process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen inside chloroplasts. It stores energy in chemical bonds and releases oxygen as a by-product.

The green pigment in chloroplasts that absorbs mainly red and blue light and reflects green. Absorbed light energy excites electrons that drive the light-dependent reactions of photosynthesis.

The breakdown of organic molecules such as glucose to release energy stored as ATP, with carbon dioxide and water as waste products. It occurs partly in the cytoplasm and partly in mitochondria.

A biological catalyst, usually a protein, that speeds up a reaction by lowering its activation energy. Each enzyme has an active site whose shape makes it specific to particular substrates.

The loss of an enzyme's or protein's three-dimensional shape and function caused by heat, extreme pH, or chemicals. The primary sequence stays intact, but the active site can no longer bind substrate.

Cell division that produces two genetically identical daughter cells with the same chromosome number as the parent. It underlies growth, repair, and asexual reproduction in multicellular organisms.

A two-stage reduction division that produces four haploid gametes from one diploid cell. Crossing over and independent assortment during meiosis create genetic variation among offspring.

A thread-like structure of DNA wrapped around histone proteins that carries genes in a linear sequence. Human body cells contain forty-six, arranged as twenty-three pairs.

The pattern of inheritance described by Mendel, in which alleles segregate and assort independently during gamete formation. Dominant alleles mask recessive ones in heterozygotes, giving predictable ratios.

The genotype is the pair of alleles an organism carries for a gene; the phenotype is the observable trait those alleles produce. The same phenotype can arise from different genotypes.

The process by which individuals with heritable traits better suited to their environment survive and reproduce more successfully. Over generations, favourable alleles become more common in the population.

The position an organism occupies in a food chain, such as producer, primary consumer, or secondary consumer. Energy transfer between levels is inefficient, so food chains rarely exceed four or five links.