## How can you lift a marble off a table without touching it with your hands?

### Listing Details

Concepts
gravity; forces
Time needed
• 2 hours
Age
12-14
Setting
Home
Materials Needed
• Paper or Styrofoam cup
• Small marble
• Utility knife or sharp scissors
• Table
Doing the activity!
1. Place the marble on a table.
2. Provide students with a Styrofoam cup with the entire bottom removed.
3. Ensure that the hole at the bottom of the cup is completely round and fairly smooth.
4. Challenge students to lift the marble off the table top using the Styrofoam cup.
What's happening?

The marble can be raised in the following manner: with the marble resting on the table, place the bottom of the cup over the marble so that the marble touches the side of the cup. Rotate the cup to make the marble spin. Rotate the cup as fast as you can and eventually the marble will rise up inside the cup. The marble can then be removed from the table by lifting the cup up off the table.

In order for the marble to come up off the table, it must overcome gravity, which has a constant force. Once the marble starts to spin, it is held against the walls of the cup by centripetal force. The marble wants to move in a straight line, but the rouded edges of the cup are forcing it to move in a circle. Centripetal force is the force that pushes or pulls an object inward and forces it on a circular path. If the centripetal force exceeds the force of gravity, the marble will lift off the table. Once moving, the marble will continue to travel along the inside of the cup because of its inertia. That is, the tendency to stay in motion.

Why does it matter?
Centripetal forces are at work in many everyday interactions. When a car banks on a turn, this is due to centripetal forces. A rollercoaster is forced through a loop by the tracks applying centripetal forces on the coaster cars. Earth\'s gravity exerts centripetal force on communication satellites, to keep them in orbit and prevent them from flying off into space.

Design by Ms.O & Co., Development by Intermix.ca