This is related to the following picture:

, which is explained in this posting. So for the physics behind it, read on…

# Physics of the dropping coin

Formulas to use (see http://en.wikipedia.org/wiki/Acceleration#Uniform_acceleration ):

- Acceleration is the gravitational force, g = 9.81 m / (s*s)
- Speed (v) at a specific time (t): v = g * t
- Traveled (s) distance at a specific time: s = ½ g * t * t
- Time required to travel a specific distance = sqrt (2 * s / g )

# Some calculations for our setup

Our travel distance for the coin drop was about 0.5 meters between the “drop point) and the surface of the water/milk. Using the formulas from above, we get

- Time for drop = sqrt (2 * 0.5 m / (9.81 m / (s*s))) = about 0.32 seconds = 320 ms
- Speed at impact with water = 9.81 * 0.32 = 3.13 m/s
- Distance travelled in 1 ms at impact speed = 3.13 m / 1000 = 0.3 cm = 3 mm

# Theory and Practise

Jan L. A. van de Snepscheut is attributed to saying

*“In theory, there is no difference between theory and practice. But, in practice, there is.”*

Whoever said this, consider this here as there are various other factors at play:

- A relay needs a few milliseconds to ‘fire’
- The magnet takes a while to drive the aluminum plate
- The formulas above apply only in a vacuum

So prepare for a rather different delay you’ll need to use in your setup as compared to the one you calculated. We had to use 430 ms rather the 320 ms from above.

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