Monday, February 20, 2012

Projectile Motion/Gravity: Ruler and Penny

Originally posted on February 4, 2010

To demonstrate that gravity pulls objects to the ground at the same speed, regardless of the path they take…

Place a ruler flat on the demonstration table, with about 2 inches hanging over the edge of the table. Place a penny on the table next to the ruler (between the edge of the table and the ruler).

Instruct students to listen for the pennies hitting the ground. Quickly flick the ruler, so that you are pushing the penny off the table at the same time you are “pulling” the ruler out from under the penny sitting on it. The penny that was on the ruler will fall straight to the ground (due to inertia, 1st law of motion), while the penny that was on the table will travel along a path of projectile motion.

If executed properly, the pennies will hit the ground at the same time. You will no doubt have to repeat this demonstration several times; first, because students take that long to determine that they are in fact landing at the same time and second, because it’s a bit mind-boggling to students (and teachers).

Do as I say, not as I photograph...
1 - Don't perform this demonstration with little people sitting on the floor where pennies may land!
2 - Don't perform this demonstration in a carpeted area of the room, you won't hear the pennies land!

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