4 (a) Figure 6 shows a large tank of water - Edexcel - GCSE Physics Combined Science - Question 4 - 2020 - Paper 1

To calculate the wavelength, we can use the formula:

$\text{Wavelength} = \frac{\text{Distance (L to M)}}{\text{Number of wavelengths}}$

Given that the distance between L and M is 32 m:

1. Number of wavelengths in the wave: 10 (as inferred from the wave crest count)
2. Hence, $\text{Wavelength} = \frac{32 m}{10} = 3.2 m$

To find the frequency, use the formula:

$\text{Frequency} = \frac{\text{Number of peaks}}{\text{Time}}$

Here, 12 peaks pass in 15 seconds:

1. Substitute into the formula: $\text{Frequency} = \frac{12}{15} = 0.8 Hz$

Draw arrows parallel to the direction of the wave propagation (from Q to S) to indicate that the rock at R moves in the same direction as the incoming wave.

Using the wave speed and frequency, we use the formula:

$\text{Wavelength} = \frac{\text{Wave Speed}}{\text{Frequency}}$

1. Given the wave speed = 7 km/s = 7000 m/s and frequency = 12 Hz:
2. Substitute into the equation: $\text{Wavelength} = \frac{7000 m/s}{12 Hz} = 583.33 m$ (approximately 580 m) or just 580 m when rounded.

This method is unsuitable because seismic waves, when they arrive, cannot be seen visually. Additionally, the technician counting peaks would need a different method to detect seismic waves, as the speed and nature of seismic waves may lead to difficulty in accurately counting without error. The technician may miscount if the waves arrive too quickly, making it easy to lose track of the count.