Quick answer: A wave is a repeating disturbance that carries energy from place to place without moving matter along with it. This grade 7 science lab on waves gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Do bigger waves travel faster? In this lab you time pulses along a stretched spring, compare small and large pulses, and then test whether v = f × λ predicts the wavelengths you see when you shake the spring steadily.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS4-1
Aim
To measure the speed of a pulse on a stretched spring, to find out whether the size of the pulse changes it, and to link frequency, wavelength and speed with v = f × λ.
Equipment
- A long spring toy or a loose skipping rope, about 5 m long
- Tape measure and masking tape for two marks
- Stopwatch or phone timer
- A helper holding the far end, or a fixed post
- A phone to film the wave, optional
Safety
Work on a clear, dry floor. Do not stretch the spring beyond its natural stretch, because it can snap back. Keep your face away from the free end and release it slowly.
Method
- Stretch the spring along the floor and mark two points exactly 4.0 m apart.
- Flick the hand end sideways once to make a small pulse of about 5 cm. Time the pulse between the two marks. Repeat three times.
- Repeat with a large pulse of about 20 cm, three times.
- Shake the end steadily and count the number of full shakes in 10 s. Estimate the wavelength by marking crest positions or from a film.
- Repeat with faster shaking and record the results.
Results
Sample measurements for the pulse over 4.0 m:
| Pulse size | Run 1 (s) | Run 2 (s) | Run 3 (s) |
|---|---|---|---|
| Small, 5 cm | 1.62 | 1.58 | 1.66 |
| Large, 20 cm | 1.60 | 1.64 | 1.60 |
Sample measurements for steady shaking (wave speed from the pulse test):
| Shaking | Shakes in 10 s | Measured wavelength (m) |
|---|---|---|
| Slow | 14 | 1.8 |
| Fast | 20 | 1.2 |
Analysis questions
Question 1 Mean times
Calculate the mean time for the small pulse and for the large pulse.Question 2 Pulse speeds
Calculate the speed of each pulse over the 4.0 m distance.Question 3 Effect of pulse size
State and explain your conclusion about the effect of pulse size on speed.Question 4 Frequencies
Calculate the frequency for slow and for fast shaking.Question 5 Predicting wavelengths
Use v = 2.5 m/s to calculate the wavelength predicted for each frequency and compare it with the measured values.Question 6 Another prediction
Predict the wavelength if the spring is shaken at 2.5 Hz, and explain your reasoning.Question 7 Timing error
Reaction time is about 0.2 s when you start and stop the stopwatch by eye. Calculate the percentage uncertainty in a 1.62 s time and name a way to reduce it.Question 8 Improving the lab
Suggest two improvements that make the speed more reliable.
