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How are speed and acceleration calculated, and how do we read motion graphs?

Speed and average speed, acceleration, and interpreting distance-time and velocity-time graphs.

A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on motion, covering speed and average speed, acceleration, and how to read and interpret distance-time and velocity-time graphs.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Speed and average speed
  3. Acceleration
  4. Distance-time graphs
  5. Velocity-time graphs
  6. Speed, velocity and acceleration
  7. Typical everyday speeds
  8. Try this

What this dot point is asking

WJEC Double Award Unit 6 wants you to calculate speed and average speed and acceleration, and interpret distance-time and velocity-time graphs.

Speed and average speed

To use the equation, choose the form you need (distance=speed×time\text{distance} = \text{speed} \times \text{time}, or time=distancespeed\text{time} = \dfrac{\text{distance}}{\text{speed}}) and keep the units consistent.

Acceleration

For example, a car going from 0 to 20 m/s in 4 s has an acceleration of 204=5 m/s2\dfrac{20}{4} = 5\,\text{m/s}^2.

Distance-time graphs

On a distance-time graph:

  • the gradient (slope) is the speed - a steeper line means a faster speed,
  • a horizontal (flat) line means the object is stationary (not moving),
  • a straight sloping line means a constant speed.

Velocity-time graphs

On a velocity-time graph:

  • the gradient is the acceleration - a steeper line means a greater acceleration,
  • a horizontal line means constant velocity (no acceleration),
  • a line sloping down means deceleration,
  • the area under the line is the distance travelled.

Speed, velocity and acceleration

It is worth being clear about the difference between speed and velocity. Speed tells you how fast something is going (for example 20 m/s). Velocity is the speed in a stated direction (for example 20 m/s north). This matters because acceleration is a change in velocity, so an object can be accelerating even at a constant speed if its direction is changing (such as going round a bend). For straight-line motion at GCSE you can usually treat speed and velocity together, but knowing that velocity includes direction explains why turning is a form of acceleration.

Typical everyday speeds

Exam questions sometimes expect you to know rough everyday speeds, and to spot if an answer is sensible. A person walks at about 1.5 m/s, runs at about 3 m/s, and a car in a town travels at around 13 m/s (about 30 miles per hour). The speed of sound in air is about 340 m/s and the speed of light is about 300 million m/s. Knowing these helps you check whether a calculated speed is reasonable, for example noticing that a car speed of 1500 m/s must be wrong. Being able to judge whether an answer is sensible is a useful exam skill, and it can stop you losing marks for an answer that is the wrong size or in the wrong units.

Try this

Q1. State the equation for speed. [1 mark]

  • Cue. Speed = distance / time.

Q2. What does the area under a velocity-time graph represent? [1 mark]

  • Cue. The distance travelled.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC style3 marksA car travels 150 m in 10 s. Calculate its average speed.
Show worked answer →

A Unit 6 calculation. Use speed=distancetime\text{speed} = \dfrac{\text{distance}}{\text{time}} (1 mark). Substitute: 15010\dfrac{150}{10} (1 mark) =15 m/s= 15\,\text{m/s} (1 mark). Markers reward the equation, the substitution and the unit metres per second. A common error is to multiply instead of divide.

WJEC style4 marksDescribe what the gradient (slope) of a distance-time graph and a velocity-time graph each represent, and what a horizontal line means on each.
Show worked answer →

A Unit 6 graph question worth 4 marks. Reward: on a distance-time graph the gradient is the speed (1), and a horizontal line means the object is stationary (1); on a velocity-time graph the gradient is the acceleration (1), and a horizontal line means constant velocity (no acceleration) (1). Markers credit gradient = speed and a flat line = stationary for distance-time, and gradient = acceleration and a flat line = steady speed for velocity-time. A common error is to muddle the two graph types.

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