Speed — AQA GCSE Combined Science

In real journeys, speed changes from moment to moment. A person walking may slow at a crossing, speed up downhill, then stop; a car accelerates away from traffic lights, cruises, brakes and turns. Because speed is rarely constant, a single value usually describes an average over the whole journey rather than the speed at every instant. You calculate average speed using total distance divided by total time, which smooths out the changes. Instantaneous speed is the speed at one particular moment, such as a car speedometer reading. Distance–time graphs show these changes: a steeper line means higher speed, a horizontal line means stationary, and a curve means changing speed. Recognising that speed varies helps you interpret journey data and choose average speed when the motion is not steady.

Speed describes how far a person moves in a given time, but it is not a fixed personal constant. A child walking to school may average about 1 m/s, while a fit adult running may exceed 5 m/s; the same person slows on steep, muddy terrain or when carrying a heavy bag. Age affects strength, stamina and reaction time; fitness affects sustainable power output; terrain changes friction and gradient; and distance travelled matters because a short sprint can be faster than a long journey where fatigue builds. To compare journeys fairly, calculate speed using speed = distance ÷ time, keeping units consistent, for example 300 m ÷ 150 s = 2 m/s. Recognise that an average speed hides changes during the journey, so two people with equal averages may have moved very differently.

This statement introduces the typical speed values that support the earlier idea that walking, running and cycling speeds vary. In GCSE Combined Science you should recall approximate values and use them to judge whether a calculated answer is reasonable. A person walking typically moves at about 1.5 m/s, running at about 3 m/s, and cycling at about 6 m/s, though these are guides rather than exact constants. For example, a 1500 m walk at 1.5 m/s takes about 1000 s, while the same distance run at 3 m/s takes about 500 s. Use typical values to estimate times, check units and identify unrealistic answers, such as a walking speed of 30 m/s. Always state that actual values depend on age, terrain, fitness and distance travelled.

Walking is a familiar motion, and its typical speed is about 1.5 m/s. This means a walker covers roughly 1.5 metres each second. To use this value, recall that speed equals distance divided by time, so distance equals speed multiplied by time. For example, in 20 s a walker travels 1.5 m/s × 20 s = 30 m. The symbol ̴ means approximately, so 1.5 m/s is an estimate for an average person on flat ground; individual speeds vary with age, fitness and terrain. You should recognise this as a standard order-of-magnitude value for walking, compare it with running (about 3 m/s) and use it in calculations involving distance, time or speed. When measuring walking speed, time a known distance and divide distance by time, ensuring units are consistent.

Running is faster than walking, and a typical running speed is about 3 m/s. This means a runner covers roughly 3 metres each second. Using speed = distance ÷ time, you can calculate distance or time: for example, in 15 s a runner travels 3 m/s × 15 s = 45 m, and to run 60 m takes 60 m ÷ 3 m/s = 20 s. The symbol ̴ means approximately, so 3 m/s is an estimate for an average person jogging; sprinting speeds are higher and individual values vary. You should recognise this as a standard typical value, compare it with walking at about 1.5 m/s, and use it in calculations. When measuring running speed, time a measured distance and divide distance by time, keeping units consistent.

This fact states that a typical cycling speed is about 6 m/s. To use it, recognise that 6 m/s means 6 metres travelled each second, so in 10 s a cyclist covers about 60 m. Compare it with walking (about 1.5 m/s) and running (about 3 m/s): cycling is roughly twice running speed. Convert to km/h by multiplying by 3.6, giving about 22 km/h, a sensible everyday value. In calculations, treat 6 m/s as an approximate typical value, not an exact constant; a specific cyclist may be slower uphill or faster downhill. When estimating, choose 6 m/s for a bicycle unless the question gives a measured value, and keep the unit m/s with the number.

You need to recall approximate everyday speeds. A person walking is about 1.5 m/s, running about 3 m/s and cycling about 6 m/s. Transport examples include a car on a motorway at roughly 30 m/s, a train at roughly 50 m/s and a plane at roughly 250 m/s. These are typical values, not exact constants, so use them for estimates. For instance, a car at 30 m/s covers about 1800 m in one minute. To compare values, convert to the same unit: multiply m/s by 3.6 to get km/h, so 30 m/s is about 108 km/h. Learn the list and the unit m/s, and state that values are approximate.

Speed is the rate of change of distance with time, and it is not restricted to solid objects travelling along a track. Sound travels through a medium as a wave, and its speed depends on the medium and its temperature; in air at about 20 °C the speed is roughly 330 m/s, but in warmer air it is slightly greater and in water it is much greater. Wind speed also varies with time and place, from calm conditions to gales, and is measured with an anemometer. So when you calculate speed using speed = distance ÷ time, you must recognise that the value can change during a journey or a measurement. For example, timing thunder and lightning gives a rough distance because the sound speed is not fixed.

Sound is a wave that travels through a medium by making particles vibrate. In air near room temperature, a typical value for its speed is 330 m/s, which is about one million times slower than light. This value is typical rather than exact because the speed of sound in air increases as the temperature rises and changes slightly with humidity. You can use 330 m/s in calculations, for example to estimate the distance to a lightning strike: if thunder is heard 3 s after the flash, the sound has travelled about 330 m/s × 3 s = 990 m. Remember that sound cannot travel through a vacuum, and that it travels faster in liquids and solids than in gases because the particles are closer together.

Speed describes how much distance an object covers each second, so measuring speed means measuring two quantities: distance and time. Choose a straight, clear section of the object's path and mark a start line and a finish line. Measure the distance between them with a metre rule or tape measure, in metres, and record the time the object takes to travel that distance using a stopwatch or light gate, in seconds. Repeat the timing several times and take a mean to reduce random error. Then divide distance by time: speed = distance ÷ time. For example, a trolley travelling 1.50 m in 2.5 s has a speed of 1.50 m ÷ 2.5 s = 0.60 m/s. Always give the unit, and check that the value is sensible for the object.

When an object moves at constant speed, it covers the same distance every second, so distance, speed and time are linked by a simple relationship. The equation is distance travelled = speed × time, often written s = v × t. If speed is in metres per second and time is in seconds, distance comes out in metres. For example, a cyclist moving at a constant 6 m/s for 20 s travels 6 m/s × 20 s = 120 m. Rearranged, speed = distance ÷ time and time = distance ÷ speed. Constant speed means the value does not change during the motion, so the same speed can be used throughout the calculation. Always convert units first: 3 minutes becomes 180 s, and 36 km/h becomes 10 m/s, before substituting into the equation.

This relationship links the distance an object travels to how fast it moves and how long it moves for. Speed is the rate of change of distance: a car travelling at 15 m/s for 8 s covers 15 × 8 = 120 m. Rearranged, speed = distance ÷ time and time = distance ÷ speed. The equation applies to motion along any path; if speed changes, it gives an average speed. Units must be consistent: distance in metres, time in seconds, speed in metres per second (m/s). Convert first, for example 3 minutes = 180 s, then substitute. In calculations, write the equation, substitute values with units, and state the unit in the answer.

This is the symbolic form of distance travelled = speed × time, where s is distance, v is speed and t is time. The symbols allow quick rearrangement: v = s ÷ t and t = s ÷ v. For example, a cyclist at 6 m/s for 25 s travels s = 6 × 25 = 150 m. The equation applies to steady motion in a straight line; for changing speed it gives average speed. Keep units consistent: s in metres, v in metres per second, t in seconds. When using the symbols, write the rearranged equation before substituting values, and give the final answer with its unit.

Distance, s, is the length of the path travelled by an object, measured in metres (m). It is a scalar quantity, so direction is not needed. For example, if a cyclist rides 150 m north then 50 m south, the total distance travelled is 200 m, even though the displacement is 100 m north. In speed calculations, s usually represents the distance moved during a time interval. You must be able to read distance from a description or a distance-time graph, convert units such as km to m by multiplying by 1000, and substitute s into v = s / t. Always include the unit m with your answer, and check that the value is reasonable for the context.

Speed, v, is the rate of change of distance with time, measured in metres per second (m/s). It is a scalar quantity, so direction is not specified. You calculate average speed using v = s / t, where s is distance in metres and t is time in seconds. For example, a runner who covers 400 m in 50 s has an average speed of 400 / 50 = 8 m/s. On a distance-time graph, speed is the gradient of the line. You must be able to convert units such as km/h to m/s, substitute correctly into the equation, and interpret the result. Always include the unit m/s and consider whether the value is realistic.

Time is the duration between two events and is measured in seconds (s), the SI base unit. In speed calculations, time is the denominator: average speed = distance ÷ time. You must read time from a stopwatch, timer or light gate and record it in seconds, converting minutes to seconds by multiplying by 60 and milliseconds to seconds by dividing by 1000. For example, a runner taking 2 minutes 30 seconds has t = 150 s. When timing oscillations, time several cycles and divide by the number of cycles to reduce reaction-time error. Always give the unit s with the value, and remember that a smaller time for the same distance means a greater speed.

Non-uniform motion means speed changes during a journey, so a single instantaneous speed does not describe the whole trip. Average speed is the total distance travelled divided by the total time taken: average speed = total distance ÷ total time. For example, a cyclist covers 300 m in 20 s, stops for 10 s, then covers 200 m in 15 s. Total distance = 500 m and total time = 45 s, so average speed = 500 ÷ 45 = 11.1 m/s (to 3 significant figures). Use the whole journey, not one stage, and do not average the separate speeds unless the times are equal. Give the unit m/s, or convert to km/h by multiplying by 3.6 if needed.