Investigating the Speed of Water Waves (WJEC GCSE Physics: Combined Science)

Aim of the Experiment

• In this experiment, the goal is to measure the mean speed of a water wave using the equation:

Variables

• Independent variable = Depth of water
• Dependent variable = Speed of the wave
• Control variables:
  • Height at which the tray is lifted to
  • Distance the wave travels while being timed

Equipment List

Equipment	Purpose
Rectangular tray	To contain the water
Ruler	To measure the length of the tray and depth of water
Stopwatch	To measure the time taken for the wave to travel
Large beaker	To transfer water to the tray

  

Equipment Set Up

Measure depth of water in the tray on the flat surface

Method

1. Measure the length of the tray and record this

2. On a flat surface, fill the tray with water to a depth of 0.5 cm

3. Lift one end of the tray up off the surface by a set amount (e.g. 5 cm)

4. Place it back down gently on a desk - this will produce a wave from one end of the tray

5. Once placed down, start the stopwatch and record the time taken for the wave to travel the length of the tray 3 times (the wave will reflect back from the end surface of the tray)

6. Repeat this process with a water depth of 1.0 cm, 1.5 cm, 2.0 cm, 2.5 cm and 3.0 cm

7. Repeat measurements for each depth 3 times

8. Use the recorded quantities to calculate the mean wave speed and plot a graph of this against the depth of water

Tipping the Water Tray

Ensure the tray is raised to the same height for each experiment and lower it back down gently to avoid spilling water and affecting the value of the depth (and to avoid a slipping hazard)

 

Example Table of Results for a Tray of Length 30 cm

Depth of Water / cm	Time taken to travel 3 lengths of a tray / s			Mean time / s	Mean speed / cm/s
	1	2	3
0.5	5.39	5.44	5.42	5.42	16.6
1.0	3.77	3.84	3.85	3.82	23.6
1.5	2.88	3.04	2.95	2.96	30.4
2.0	2.56	2.64	2.66	2.62	34.4
2.5	2.49	2.45	2.48	2.47	36.4
3.0	2.51	2.40	2.43	2.45	36.7

 

Analysis of results

• The mean speed is calculated by dividing 90 cm (3 times the length of the tray) by the mean time taken to travel 3 lengths of the tray
  • This is given in cm/s, not m/s, as the values are easier to handle
• Plotting a graph with depth of water on the x axis and mean speed on the y axis will show the relationship between the two

Example Graph of Depth Against Mean Speed of Wave

The relationship between wave speed and water depth is not linear, the line of best fit here is curved

• The relationship this graph shows is that wave speed in creases with depth of water, but increases at a decreasing rate

Evaluating the Experiment

Systematic Errors:

• Make sure the measurements on the ruler are taken at eye level to avoid parallax error
• Ensure one person moves the tray and one person operates the stopwatch to avoid any delay

Random Errors:

• Lift the tray to the same height each time
• Perform the experiment on the same flat surface each time
• Measure the depth of the water when the water is still
• Measure depth in multiple places - if it varies then the surface is not flat

Safety Considerations

• Ensure water is not spilled on the floor, this is a slipping hazard
• Lower the tray when creating the wave, do not drop it as this will spill water
• Immediately wipe up any spillages to avoid other students slipping