Make a stack of five or more pennies on a smooth tabletop.
Lay a dime on the table and flick it or slide it so that it hits the bottom of the stack. The bottom penny will pop out, but the others will stay in a stack.
What keeps the top pennies in a stack? Think of your own best answer before you read the explanation below.
How It Works
When the bottom penny shoots out, the upper pennies stay in a stack because any object that’s not moving tends to stay in place until an outside force pushes against it. This tendency is called inertia. When you flicked the dime into the stack, the force of the dime overcame the inertia of the bottom penny and pushed it out of place. But the inertia of the upper pennies kept them in place.