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?
When the index card is quickly pulled away from the glass in a horizontal direction, the disk falls straight down into the glass due to the principle of inertia. The disk, which was initially at rest on the index card, tends to stay at rest due to inertia.
However, when the index card is pulled away, there is no longer any support or friction to prevent the disk from falling. As a result, the disk falls straight down due to the force of gravity acting on it.
The horizontal motion of the index card does not affect the vertical motion of the disk because these motions are independent of each other. The disk falls vertically because gravity pulls it downwards, regardless of the horizontal motion of the index card.