Atwood machine: Unbalanced weights on a pulley accelerate slowly.

The Simple Atwood Machine. A light-weight pulley is arranged with string and weight-hangers of equal mass. A slotted weight is added to one of the hangers, and the system is put in motion. The distance is measured that the mass M2 moves in 1,2,3 and 4 seconds.Knowing the values of mass, distance, and time, the acceleration can be calculated. Note: can also be done with digital timing gates.
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This is a good demo to take to high school physics students because, after studying mechanics, they should be able to predict the behavior of the masses very exactly. We can lend you a metronome for timing and you could pose a challenge like "what masses should I use to make it drop exactly half a meter in 5 seconds" or some such. As always when you present a demo make sure you ask the participants questions before you demonstrate and after. Think about the concepts involved beforehand; having a friend ask you questions about it can be very revealing as to if you understand it well or not. Whenever you ask a question count to five (or at least three) in your head waiting for an answer before you stay anything else.