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# Physics of the dropping coin (or strawberry)

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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Categorised in: DIY, InEnglisch, Photography, Photography-Experimental