Acceleration
Acceleration describes a change in velocity over time. It occurs when something speeds up, slows down or changes direction.
Definition and videos ↗ 11 ▶STEP BY STEP KIDS TV
Concept glossary
Explore concepts from our films, read short explanations and find related videos.
Concepts: 50
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Additive colour mixing
Additive mixing combines light of different colours. A suitable combination of red, green and blue light produces white; mixing paint works differently.
Definition and videos ↗ 7 ▶Amber
Amber is ancient tree resin that became fossilized over time. It can preserve trapped insects or plant parts; it is not made from tree sap.
Definition and videos ↗ 9 ▶Angle vertex
An angle’s vertex is the common starting point of its two rays, which form its sides.
Definition and videos ↗ 10 ▶Angles
Two rays starting at the same point, called the vertex, form an angle. Its measure describes their opening; a full turn is 360°.
Definition and videos ↗ 17 ▶Archimedes’ principle
A liquid or gas pushes an immersed object upward with a force equal to the weight of the displaced fluid. This also applies to objects that sink.
Definition and videos ↗ 17 ▶Bacteria
Bacteria are single-celled organisms without a cell nucleus. Many are useful or harmless; only some cause disease.
Definition and videos ↗ 11 ▶Batteries and cells
An electrochemical cell turns energy from chemical reactions into electrical energy. A battery can contain one or more cells and power a closed circuit.
Definition and videos ↗ 23 ▶Blood circulation
In humans, the heart pumps blood through vessels to the lungs and the rest of the body. Blood carries oxygen, nutrients and waste products, among other substances.
Definition and videos ↗ 15 ▶Buoyancy
Buoyancy is the upward force a liquid or gas exerts on an immersed object, because pressure is greater below the object than above it.
Definition and videos ↗ 20 ▶Butterflies
Butterflies are insects whose wings are covered with tiny scales. Their life cycle includes an egg, caterpillar, pupa and adult.
Definition and videos ↗ 17 ▶Carbon footprint
A carbon footprint adds up greenhouse gas emissions associated with an activity, product or organization. Different gases are usually expressed as an equivalent amount of CO₂.
Definition and videos ↗ 8 ▶Chlorophyll
Chlorophyll is a green pigment that absorbs light used in photosynthesis. In plants, it is found in chloroplasts.
Definition and videos ↗ 1 ▶Density
Density tells us how much mass occupies a given volume: density = mass ÷ volume. A large object need not have a high density.
Definition and videos ↗ 14 ▶Electric circuits
A circuit connects electrical parts, such as a battery, wires and a lamp. In a simple battery circuit, current flows when the path is closed.
Definition and videos ↗ 12 ▶Electricity
Electric current is a directed flow of electric charge. Electrons carry it in metal wires, while ions can carry it in solutions.
Definition and videos ↗ 24 ▶Floating and sinking
For an object floating freely, buoyancy balances its weight. Under ordinary conditions, an object denser on average than the liquid sinks; trapped air counts in that average.
Definition and videos ↗ 23 ▶Freezing
Freezing changes a liquid into a solid while it releases heat. Pure water at ordinary atmospheric pressure freezes at about 0°C.
Definition and videos ↗ 11 ▶Greenhouse gases
Greenhouse gases absorb and emit heat radiation, slowing energy loss from Earth. They include carbon dioxide, methane and water vapour.
Definition and videos ↗ 8 ▶Hibernation
Hibernation greatly slows an animal’s body activity for a period: energy use, heart rate and usually body temperature fall. It is not ordinary sleep.
Definition and videos ↗ 13 ▶Internet
The Internet is a worldwide network of connected computer networks. Data travels between devices in packets; the Web is just one Internet service.
Definition and videos ↗ 19 ▶Lactic fermentation
Lactic fermentation converts pyruvate into lactate, allowing a cell to keep obtaining energy by breaking down glucose without using oxygen. Yogurt bacteria use this process.
Definition and videos ↗ 6 ▶Light
Visible light is the part of electromagnetic radiation our eyes detect. It can carry energy even through a vacuum.
Definition and videos ↗ 60 ▶Mass
Mass measures an object’s inertia, its resistance to a change of motion, and is measured in kilograms. Weight is the gravitational force on an object, not its mass.
Definition and videos ↗ 20 ▶Metamorphosis
Metamorphosis is a major change in body form as an animal develops. In butterflies, a caterpillar becomes an adult through a pupal stage.
Definition and videos ↗ 13 ▶Microorganisms
Microorganisms are organisms usually too small to see without a microscope, such as bacteria and yeasts. They are not all single-celled or disease-causing.
Definition and videos ↗ 20 ▶Net force
Net force is the vector sum of all forces on an object, including their directions. Two equal forces in opposite directions can balance each other.
Definition and videos ↗ 10 ▶Newton’s laws
Newton’s laws connect forces and motion. His second law, for constant mass, is F = m × a, where F means the net force from all the forces acting.
Definition and videos ↗ 10 ▶Oxygen
Oxygen is a chemical element with the symbol O. In air it mainly occurs as O₂ gas, which our cells use in cellular respiration.
Definition and videos ↗ 28 ▶Peristalsis
Peristalsis is a sequence of muscle contractions that moves material along the digestive tract. It also pushes swallowed food through the oesophagus.
Definition and videos ↗ 1 ▶Photosynthesis
Plants use light energy, water and carbon dioxide to make sugars, releasing oxygen. They also carry out cellular respiration, as well as photosynthesis.
Definition and videos ↗ 5 ▶Pigments
Pigments absorb selected parts of light, helping give materials their colour. Plant pigments include green chlorophyll and yellow or orange carotenoids.
Definition and videos ↗ 10 ▶Pixels
A pixel is one element in the grid of a digital image and stores colour information. Many pixels together form a photo or video frame.
Definition and videos ↗ 7 ▶Pollination
In flowering plants, pollination transfers pollen from an anther to a stigma. It can enable fertilization and seed formation, but is not fertilization itself.
Definition and videos ↗ 30 ▶Pressure
Pressure is perpendicular force per unit area: pressure = force ÷ area. The same force acting on a smaller area produces greater pressure.
Definition and videos ↗ 39 ▶Ray in geometry
A ray has one starting point and extends forever in one direction. Two rays with a common start can form an angle.
Definition and videos ↗ 10 ▶RGB colour model
RGB describes a colour using red, green and blue light components. Screens combine these components to display many different colours.
Definition and videos ↗ 7 ▶Sequence of video frames
A video displays successive still images called frames. Showing them rapidly in the right order lets us perceive motion.
Definition and videos ↗ 7 ▶Signal decoding
Decoding reads data stored according to a particular code. During video playback, a decoder turns encoded data into images or sound that can be played.
Definition and videos ↗ 7 ▶Skeleton
The human skeleton includes bones and cartilage: it supports the body, protects organs and works with muscles for movement. Other animals can have different skeleton types.
Definition and videos ↗ 10 ▶Smoke detectors
A smoke detector senses smoke particles and can trigger an alarm. Optical detectors use light, while ionization detectors sense changes in charge flow; a smoke detector does not replace a carbon monoxide detector.
Definition and videos ↗ 9 ▶Sorting and recycling
Sorting separates waste by type, while recycling processes suitable materials into new products. Collection rules vary by place; not every waste item can be recycled.
Definition and videos ↗ 9 ▶Sound
Sound is a travelling vibration in a material, such as air, water or metal. It needs a medium, so it does not travel through a vacuum.
Definition and videos ↗ 17 ▶States of water
Water can be solid ice, liquid water or gaseous water vapour. Its state depends on temperature and pressure; water vapour itself is invisible.
Definition and videos ↗ 18 ▶Subtraction
Subtraction finds what remains or the difference between numbers. For example, 7 − 3 = 4 because 4 + 3 = 7.
Definition and videos ↗ 10 ▶Surface tension
Attraction between molecules makes a liquid surface tend to shrink its area. This effect helps form droplets; the surface is not an actual skin.
Definition and videos ↗ 21 ▶Temperature
Temperature describes how hot or cold something is and can be measured with a thermometer. It is not the total energy stored in an object.
Definition and videos ↗ 1 ▶Units of measurement
A measurement unit is an agreed reference for describing a quantity. In SI, length uses metres, mass uses kilograms and time uses seconds.
Definition and videos ↗ 10 ▶Water treatment
Water treatment removes selected pollutants using methods such as settling, filtration and biological processes. Treated wastewater is not automatically safe drinking water.
Definition and videos ↗ 20 ▶Wetlands
Wetlands are places where water covers or saturates the ground all year or for part of the year. That wetness shapes the soil, plants and animal life.
Definition and videos ↗ 10 ▶