Whack-a-Stack

Conduct a hard-hitting investigation into Newton's first law.

A simple stack of wood blocks demonstrates that an object at rest will remain at rest unless a force causes it to move.

Subjects:

Physics
- Mechanics

Keywords:

inertia

Newton's laws

force

Tools and Materials

Five or six wood blocks, each about 5 inches (13 centimeters) long, cut from a relatively smooth-surfaced two-by-four
Length of slender PVC pipe or a meter stick

Assembly

Stack the blocks neatly with the large sides parallel to the tabletop, and the long sides facing you.

To Do and Notice

Hold the meter stick, slender PVC pipe, or other suitable "whacker" along the tabletop, near the stack of blocks. Firmly and rapidly whack the meter stick back and forth just above the tabletop—with each whack, knock another block out from the bottom of the stack. Caution: your blocks will go flying, so make sure nobody is standing where they might get hit!

What happens when the bottom block is whacked out of the stack? The rest of the stack should essentially drop straight down onto the tabletop.

What’s Going On?

When the bottom block is knocked away by the "whacker" force, the rest of the stack resists moving horizontally along with it. This is due to a phenomenon called inertia—a resistance to change in motion. Inertia is the subject of Isaac Newton's first law of motion: An object remains at rest, or in uniform motion in a straight line, unless it's forced to change its state of motion by forces acting upon it.

Mass is a measure of inertia, as shown in Newton's second law: Force = mass x acceleration. In other words, the more massive something is, the harder you have to push or pull it to change its state of motion. The stack of blocks is fairly massive, so it has quite a bit of inertia.

You may notice, though, that the stack moves horizontally just a bit, especially as the stack gets smaller and therefore has less inertia. That's because there's a frictional force between the bottom block and the stack above it. If we could turn friction off, the stack wouldn't move sideways at all.

The stack doesn't stay in midair, of course, but falls to the tabletop. That's due to a different force, the force of gravity. (But you knew that, didn't you?)