When the index card is quickly pulled away from the glass in a horizontal direction, why does the disk fall straight down into the glass?
kinetic friction
equilibrium
static friction
inertia
1 answer
This phenomenon is due to inertia. The disk or coin that was placed on the index card while it was on top of the glass is initially at rest, so it has no horizontal velocity. When the index card is quickly pulled away in a horizontal direction, the disk remains in place due to static friction between the card and the disk. However, when the card moves out from under the disk, there is nothing to keep it from falling due to gravity. Because the disk has no horizontal motion, it falls straight down into the glass.