Inertia Trick

Physics:	Inertia, Laws of Motion
Grade Range:	Elementary School
Format:	Hands-on

This demonstration can be great fun, but it does require some practice before being presented. This works well for science nights and presentations, and kids can do it at home!

Materials

• Two Heavy Wide Mouth Glasses
• 5" by 8" Piece of Cardboard
• Two Small Cardboard Tubes
• Two Apples or Oranges

Safety Precautions

Please read the General Precautions section of the Demonstration Safety page before performing this demonstration.

Demonstration

1. Place the cups next to each other, then place the cardboard across the top. Stand the two cardboard tubes above the cups, and place the fruit on top of the tubes.
2. Ask students for ideas on what might happen when you pull out the piece of cardboard. Do they think the fruit will land in the cups? Can they explain why?
3. Pull the piece of cardboard out quickly and in a straight line, keeping it parallel to the cups. The fruit will land inside the cups!
4. Set up the demonstration again, and ask the student to try it. Can they do it correctly? If not, what's the secret?

Why This Works

This demonstration is showing Inertia: an object in motion stays in motion, until acted upon by an outside force. Also, an object at rest stays at rest until acted upon by an outside force. When you pull the piece of cardboard away, the tubes are pulled sideways. This is because they experience the sideways motion of the cardboard below them, and so they are put into motion as well. The fruit, instead of the cardboard pulling it sideways, will fall straight down into the cups! This is because the fruit is at rest while the cardboard is pulled out, and stays at rest until the cardboard tubes move out from underneath. Then, gravity does the rest and pulls the fruit down, and they land in the cups! This demonstration can be adapted to show how the mass of the moving pieces can affect the results of the demonstration. Smaller, less massive pieces of fruit will be affected more by the moving cardboard below than larger, more massive fruit. Also, the height of the cardboard tubes can affect whether the fruit falls into the cups or not.

Additional Information

• After mastering this demonstration, you can move on to the more dramatic form with a table, tablecloth, and dishware!
• This demonstration pairs well with the Uphill Roller demonstration.