When you quickly hit or flick the coin at the bottom of a stack (for example with a ruler), the bottom coin shoots out, while the rest of the coins stay almost in place and then drop straight down into a neat stack.

Why does this happen?

This is a simple demonstration of inertia , which is part of Newton’s First Law of Motion.

  • The bottom coin experiences a horizontal force from the ruler, so it accelerates sideways and shoots out.
  • The coins above it do not feel much horizontal force at that instant, so they “want” to stay at rest.
  • Gravity then pulls the remaining stack straight down, so they fall nearly vertically and often land where the bottom coin used to be.

In short:

  • The bottom coin moves because you directly applied a force to it.
  • The other coins stay (momentarily) where they are because of inertia, then fall due to gravity.

Mini example story

Imagine a tablecloth trick:

  • You pull the cloth quickly from under plates and glasses.
  • The dishes barely move because they resist changes in motion.

The stack-of-coins trick is the same idea on a smaller scale: hit fast and straight, and only the bottom coin is yanked out while the rest largely “ignore” the motion for a brief moment.

Key points (easy recap)

  • Bottom coin: gets hit → moves out quickly.
  • Upper coins: barely any sideways force → stay almost in place.
  • Then they fall straight down due to gravity.
  • Underlying reason: inertia (objects resist changes to their state of motion).