Description of Experiment

For my version of the coin drop trick, I used a $20, a jar full of dice, and a table. The jar sits on top of the $20 and is filled with dice. Without those dice, the jar isn't heavy enough and doesn't have sufficient inertia for the experiment to work. First I pull the $20 slowly from under the jar, and the jar slides along with it (fig.1). When I pull quickly on the $20 (fig.2), the jar stays in place and the $20 comes out.

Figure 1. Slowly accelerating the $20.

Figure 2. Quickly accelerating the $20.

Why it works/What I learned

Inertia is the resistance of an object to changing it's state of motion (source). This is Newton's first law of motion, that things that are moving tend to keep moving, or resist slowing down, and things that are not moving resist movement. My experiment demonstrates this by showing the cup of dice resist movement.

In figure 1, I pulled on the $20 slowly. The force of friction between the cup and the $20 acted to keep the cup on top of the dollar as it moved. While the cup resists this change in it's state of motion, the slow application of force is enough to over come this. The amount of resistance an object provides is related to the degree of change in motion which in this case is very small.

In figure 2, I pull very quickly on the $20. The forces of friction are the same, so we must accept that there is a similar force pulling the cup along with the $20. Since the cup does not move, there must necessarily be a force in the opposite direction resisting this motion. This is inertia. Because the change in motion from being at rest is very large, the cup resists with more force than before, which is more that the force of friction between the cup and the $20 can apply. Therefore the dollar slips out from under the cup and the cup remains at rest.

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I learned or was reminded how wordy it can be to explain a simple experiment. I understand what's happening, but it can be complicated to write it out in a way that is understandable. There are many interactions to consider that tracing them can take some time.

In completing the experiment I thought about the two different forces of friction, static and kinetic. The friction applied by something that is already sliding (kinetic) is less that when it is not sliding static). In my example once the $20 starts moving it is applying less force by friction overall, so it has less of a chance to pull the cup along with it. This is why the sharp "snap" to pull it out is helpful; quickly overcome the static force of friction so that the dollar can be removed.