the picture shows a physics lab setup.
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?
When the index card is quickly pulled away from the glass in the horizontal direction, the disk falls straight down into the glass due to the principle of inertia and the lack of external forces acting on the disk.
Inertia is the resistance of an object to changes in its motion. According to Newton's first law of motion, an object at rest or in uniform motion tends to stay in that state unless acted upon by an external force.
In this case, before the index card is pulled away, the disk is at rest on top of the card. When the card is pulled horizontally, the disk tends to stay at rest due to its inertia. As a result, when the card suddenly moves away, there is no immediate external force acting on the disk to prevent it from falling straight down due to gravity.
The absence of any horizontal external force allows gravity to act vertically, causing the disk to fall straight down into the glass.