Measuring Speed — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions

Section 1 of 9Typical speeds

Moving thing	Typical speed (m/s)
Walking	about 1.5
Running	about 3
Cycling	about 6
Car	about 25
Passenger plane	about 250
Sound in air	about 330
Sound in seawater	about 1500

Section 2 of 9Why typical values vary

People

• People Walking, running and cycling speeds depend on age, fitness, terrain and the length of the journey.

Vehicles

• Vehicles Speeds of cars, trains and planes depend on their shape, design, cost and purpose.

Checking claims

• Checking claims A claim that people walk at 6 m/s is wrong; the walking value is nearer 1.5 m/s.

Section 3 of 9Measuring speed

1

Choose a known distance, for example 2 m for a falling ball or 100 m on a sports field.

2

Measure the distance using suitable equipment: a metre rule for short lengths, a tape measure or trundle wheel for long ones.

3

Time how long the object takes to travel that distance using a stopwatch.

4

Calculate the speed using speed = distance ÷ time.

5

Repeat several times and find the mean to reduce the effect of random errors.

Section 4 of 9Choosing equipment

Accurate timing and distance

Metre rule

• Metre rule Suitable for short distances, such as how far a tennis ball falls.

Trundle wheel

• Trundle wheel Best for measuring long distances like an athletics track.

Stopwatch

• Stopwatch Easy to use, but human reaction time adds uncertainty, especially for short times.

Light gates

• Light gates Sensors that start and stop a timer automatically when a beam is blocked, removing reaction-time error.

Section 5 of 9Sources of error when timing

Reaction time

• Reaction time A person takes roughly 0.2 s to react when starting and stopping a stopwatch.
  • Over a 1 s interval this is a large percentage error, but over a 20 s interval it matters much less.

Distance measurement

• Distance measurement Reading a tape measure at an angle (parallax) or not starting exactly at the zero mark gives errors in the distance.

Random and systematic

• Random and systematic Random errors change from one reading to the next and are reduced by repeating and taking a mean.
• A systematic error, such as a stopwatch that always starts late, shifts every result the same way and repeating does not remove it.

Section 6 of 9Worked example: speed of a cyclist

1

A cyclist rides a marked 100 m stretch of road three times, and the times recorded are 15.8 s, 16.2 s and 16.0 s.

2

Find the mean time: (15.8 + 16.2 + 16.0) ÷ 3 = 16.0 s.

3

Use speed = distance ÷ time = 100 ÷ 16.0.

4

The speed is 6.25 m/s, which is close to the typical cycling value of about 6 m/s, so the answer is sensible.

Section 7 of 9Planning a speed investigation

1Choose a question

• Choose a question For example, does a trolley travel faster down a steeper ramp?

2Control variables

• Control variables Change only the ramp height (independent variable), measure the speed (dependent variable) and keep the trolley mass, ramp surface and distance the same.

3Record and repeat

• Record and repeat Take at least three readings for each height, so that anomalous results can be spotted and a mean found.

4Conclude

• Conclude Compare the mean speeds, state the pattern found, and comment on how reliable the timing method was.

Section 8 of 9Key terms

Term	Meaning
Speed	The distance travelled per second, measured in metres per second (m/s).
Light gate	A sensor that records the time when an object breaks a light beam.
Trundle wheel	A wheel that clicks once per metre, used for measuring long distances.
Reaction time	The delay between an event and a person responding to it.
Mean	The average value, found by adding repeated readings and dividing by their number.

Section 9 of 9Exam technique

1

Quoting a walking speed of several m/s; learn the typical values in the table.

2

Using a metre rule to measure the length of a running track; pick equipment to suit the size of the distance.

3

Timing very short intervals by hand without considering reaction time.

4

Forgetting to give units, or leaving the time in minutes while the distance is in metres.