Inertia: we’re full of it, the lot of us. Any time I don’t want to get off the couch, it’s not laziness, it’s inertia. (I swear!) If you want to get all technical about it, according to Sir Isaac “Fig” Newton’s Law of Inertia, an object at rest tends to stay at rest until acted upon by another force. Sounds about right to me—I will stay at rest on the sofa until my beverage runs out, forcing me to get up and refill it.
Unfortunately, as I’ve been told many times, lounging on the ol’ davenport doesn’t count as “teaching.” So, how can one effectively demonstrate the concept of inertia to a classroom full of students (or whoever cares to listen)?
Spheres & Cylinders: What is This, Geometry?
This Demo Science science demo is so easy, even an untrained monkey could do it. All you need is a tall plastic drinking cup (that’s your cylinder, more or less), a small sphere of some type (like ball or a marble), a table, and an untrained monkey. If you don’t have access to a monkey, you can skip that part and just do the experiment yourself.
Put your sphere, whatever it is, in the drinking cup (can’t call it a glass, since it’s made of plastic). Lay the cup on its side, gently, so as not to disturb the ball and cause it to roll out and ruin the demonstration ere it has even begun.
With the open end facing forward, slide the cup quickly across the surface of the table. Then, stop suddenly. The ball will come rolling out at high speeds and continue on its trek. But everything else stopped—how’d the sphere do that?
The Friendly City of Inert, Iowa
This part is a bit harder to explain than the demo was to execute, so stay on your toes here… Because the ball itself was not being acted upon by a force—it was the cup that was being moved—the ball’s inertia held it in place inside the cup. The inertia of the sphere canceled out the force of the cup’s movement. But, when the cup suddenly stopped, the forward motion of the ball—which was moving with the cup, even though it wasn’t actually moving from its position inside the cup—overcame this inertia and off it went.
Does that make sense? I hope so, because I can’t figure out how to describe it better. Go science!