Step-by-step tutorial
Fact
Newton’s first law of motion states that an object at rest will stay at rest unless acted on by an external force. In this activity, there are two external forces: the force of gravity, and the force you apply by whacking with a stick. Gravity affects all the boxes, pulling them down to stack on top of each other. When you swipe with your stick, though, your force affects only the bottom box that you hit. By Newton’s first law, the bottom box moves in the direction of your swipe, while the other boxes stay at rest. That is, until they fall straight down because of the force of gravity.
Step 1
Gather your materials.
Step 2
Stack 3 boxes on top of each other.
Step 3
Use the dowel to hit the bottom box out from under the top two boxes. Once the bottom box is pushed out, the top two boxes should remain stacked.
If this is too easy, continue adding boxes until you have stacked 5 or more boxes!
Tip
Try using a really quick sweeping motion, close to the table’s surface to hit the bottom box out!
Learn more
This activity is so surprising because you expect the stack of boxes to topple. And in fact, applying a force to the bottom of a stack of things usually does make it fall over. This is because of another force called friction, which tries to stop motion whenever objects slide over each other. Striking with a quick swipe overcomes the force of friction, while pushing slowly allows friction to keep the objects from sliding apart much, increasing the chance that the stack will tilt and fall. Try seeing what happens with slower and quicker swipes, or with materials that have different amounts of friction between them.