Investigating the Speed of Water Waves

Section 1 of 7Aim and variables

Aim

• Aim To find the mean speed of water waves in a tray and see how it changes with the depth of the water.
• Mean speed = distance ÷ mean time.

Variables

• Variables Independent variable: the depth of the water.
• Dependent variablethe speed of the wave.

Control variables

• Control variables The height to which the end of the tray is lifted.
• The distance the wave travels while it is timed, and the same tray each time.

Section 2 of 7Equipment and its purpose

Equipment	Purpose
Rectangular tray	Holds the water and guides the wave
Ruler	Measures the length of the tray and the depth of the water
Stopwatch	Measures the time for the wave to travel
Large beaker	Transfers water into the tray

Section 3 of 7Method

1

Measure the length of the tray with the ruler and record it.

2

Place the tray on a flat surface and fill it with water to a depth of 0.5 cm, checking the depth with the ruler.

3

Lift one end of the tray by a fixed amount, for example 5 cm, then put it down gently on the desk to start a wave.

4

Start the stopwatch when the tray touches down and time the wave as it travels the length of the tray and back, recording the time for three lengths.

5

Repeat the timing three times at this depth.

6

Repeat the whole procedure for depths of 1.0, 1.5, 2.0, 2.5 and 3.0 cm.

7

Calculate the mean time for each depth, then the mean speed, and plot a graph of speed against depth.

Section 4 of 7Processing the results

Analysis

Total distance

• Total distance For a 30 cm tray, three lengths is 90 cm, so distance = 3 × 30 = 90 cm.

Mean speed

• Mean speed Speed = 90 cm ÷ mean time. For example, a mean time of 5.42 s gives 90 ÷ 5.42 = 16.6 cm/s.

Trend

• Trend Speed increases as the water gets deeper, but the graph flattens, so deep water gives smaller extra gains.

Repeats

• Repeats Taking three readings and finding a mean reduces the effect of random errors in stopwatch timing.

Section 5 of 7Safety and accuracy

Safety

• Safety Lower the tray gently to avoid spilling water, because spills change the depth and create a slipping hazard.
• Mop up any water straight away and keep electrical equipment away.

Accuracy

• Accuracy Lift the tray to the same height every time.
• Read the depth at eye level, and start and stop the stopwatch on the same part of the wave.

Section 6 of 7Key terms

Term	Meaning
Mean	The average found by adding the readings and dividing by how many there are.
Independent variable	The quantity the investigator changes.
Dependent variable	The quantity that is measured to see the effect of the change.
Control variable	A quantity kept the same so the test is fair.
Wave speed	The distance a wave travels each second, in m/s or cm/s.

Section 7 of 7Exam technique

1

Using the length of the tray instead of the total distance of three lengths when calculating speed.

2

Using one reading instead of a mean: repeats improve reliability.

3

Changing the lifting height between trials, which makes the test unfair.

4

Forgetting unitscm and s give cm/s unless converted.

5

Writing that deeper water makes the wave travel slower: the speed increases with depth.