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IB DemystifiedMYP Sciences

Speed

How fast something moves is a comparison: how much distance it covers in each second. Once you can read that comparison from numbers, graphs and experiments, you can describe any journey.

Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D

Figure 1: distance–time graph of a journey in three stages Stage A: 0 to 20 metres from 0 to 4 seconds. Stage B: flat at 20 metres from 4 to 8 seconds. Stage C: 20 to 40 metres from 8 to 10 seconds. 0246810 time / s 010203040 distance / m ABC
Figure 1. One journey, three stages. The slope of each stage is its speed.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding speed
  5. Distance–time graphs
  6. Speed in the real world
  7. Worked examples
  8. Check your understanding
  9. Practice questions
  10. Investigation
  11. Criterion-linked questions
  12. Challenge questions
  13. Topic check
  14. Review your mistakes
  15. Your progress

Learning objectives

By the end of this topic you should be able to:

  • state what speed means and give its units
  • calculate speed from distance and time, and rearrange the equation to find distance or time
  • convert between km/h and m/s
  • calculate the average speed of a journey with several stages
  • interpret a distance–time graph and find speed from its gradient
  • plan a fair test and process experimental data about speed

Before you start

You will use these skills. If any feel shaky, review them first.

  • SI units for length (m) and time (s), and converting km to m
  • dividing decimals and rounding to a given number of decimal places
  • reading values from a line graph

Key vocabulary

Speed
The distance an object travels per unit of time.
Average speed
Total distance travelled divided by total time taken, including any stops.
Instantaneous speed
The speed at one particular moment, as shown on a speedometer.
Gradient
The steepness of a line: change in the vertical value divided by change in the horizontal value.
Stationary
Not moving; speed is zero.
m/s
Metres per second, the SI unit of speed. Also written m s−1.

Understanding speed

  1. What is it?

    Speed tells you how much distance is covered in each unit of time. A speed of 8 m/s means 8 metres are covered every second.

    speed = distance ÷ time

  2. Why does it happen?

    An object changes position when a force has set it moving and nothing has yet stopped it. Speed does not explain why it moves; it measures how quickly its position is changing. Forces, which you meet next, explain why speed changes.

  3. How do we know?

    We never measure speed directly with a ruler or clock alone. We measure two things, a distance and a time, and calculate their ratio. Light gates, radar guns and GPS all do this very quickly and very precisely.

  4. Why does it matter?

    Speed limits, athletics records, weather forecasts of storm movement and the timing of medicine through the bloodstream all depend on measuring speed accurately and in the right units.

  5. What does it connect to?

    Speed leads to velocity (speed in a direction), acceleration (how fast speed changes), and kinetic energy. In mathematics it connects to ratio, rates of change and the gradient of a straight line.

Distance–time graphs

Figure 1 at the top of this page shows one journey. Time is on the horizontal axis and distance from the start is on the vertical axis.

  • Stage A is a straight sloping line: the object covers equal distances in equal times, so it moves at a constant speed.
  • Stage B is flat: time passes but the distance does not change, so the object is stationary.
  • Stage C is steeper than stage A: more distance is covered each second, so it is faster.

The gradient of a distance–time graph equals the speed. For stage A: 20 m ÷ 4 s = 5 m/s.

Speed in the real world

Average-speed cameras time cars between two points a known distance apart. Sports scientists use light gates to measure sprint speeds to a hundredth of a second. Seismologists compare the arrival times of earthquake waves at different stations to work out how far away an earthquake happened.

Worked examples

Example 1: finding speed

A sprinter runs 100 m in 12.5 s. Calculate her average speed.

  1. Write the equation: speed = distance ÷ time.
  2. Substitute: speed = 100 m ÷ 12.5 s.
  3. Calculate and add the unit: speed = 8.0 m/s.

Example 2: finding time, with a unit change

A bus travels 6 km at an average speed of 10 m/s. How long does the journey take?

  1. Make the units match: 6 km = 6000 m.
  2. Rearrange: time = distance ÷ speed.
  3. Substitute: time = 6000 m ÷ 10 m/s = 600 s.
  4. Convert if helpful: 600 s = 10 minutes.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: ramp height and speed

Guided investigation · about 50 minutes · pairs

Research question
How does the height of a 1.00 m ramp affect the average speed of a toy car rolling down it?
Scientific background
A car at the top of a higher ramp has more gravitational potential energy. As it rolls down, more of this energy becomes kinetic energy, so it should reach a higher speed.
Hypothesis
If the ramp height increases, then the average speed of the car will increase, because more gravitational potential energy is transferred to kinetic energy.
Independent variable
Ramp height: 5, 10, 15, 20 and 25 cm.
Dependent variable
Time for the car to travel the 1.00 m ramp, used to calculate average speed.
Control variables
  • Same car every trial, because a different mass or wheels change friction.
  • Ramp length fixed at 1.00 m, marked with tape.
  • Car released from rest from the same start line, never pushed.
  • Same ramp surface throughout.
Apparatus
Toy car, 1.00 m board, stack of books or blocks, metre rule, stopwatch (or light gates), masking tape.
Method
  1. Set the ramp to 5 cm high, measured at the start line.
  2. Release the car from rest and start the stopwatch at the same moment.
  3. Stop timing when the car's front passes the end of the ramp.
  4. Repeat twice more, then repeat for each height.
  5. Calculate the mean time and average speed for each height.

Safety. Keep the floor clear where the car leaves the ramp so nobody trips. Stack blocks securely so the ramp cannot fall.

The results one student collected are used in the Criterion C questions below. Then evaluate: stopwatch reaction time is the largest source of error here. How would light gates improve the results?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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