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Inertia Demonstration

This demonstration can be used to fulfill Webelos Scientist activity badge requirement 3:

Read Newton’s first law of motion. Show in three different ways how inertia works.

It could also be used as part of a demonstration of the scientific method or a science experiment for the Science belt loop and pin from the Cub Scout Academics and Sports program. So it fits in well with a science theme.

Newton’s first law of motion

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Inertia Demonstration

Materials

  • Ten pennies
  • Jar or glass with a wide mouth
  • Piece of cardboard slightly larger than the mouth of the jar
  • A strip of paper approximately 1” by 11”
  • Plastic cup
  • water

Instructions

  1. Place one of the pennies in the center of the piece of cardboard.
  2. Place the cardboard with the penny on it over the mouth of the jar.
  3. Quickly slide the cardboard out from under the penny.
  4. Observe that the penny drops into the jar.
  5. Next, stack up nine of the pennies on a smooth flat surface, like a table.
  6. Put the last penny on the surface a few inches away from the stack.
  7. Flick the penny toward the stack of pennies with your finger.
  8. Observe that the penny strikes the bottom penny out of the stack, but the other pennies remain stacked up.
  9. Finally, fill the plastic cup half full with water.
  10. Place the cup of water on the strip of paper, about four inches from the end of the strip.
  11. Quickly pull the strip of paper out from under the cup of water.
  12. Observe that the cup of water remains where it was placed.

Explanation

In each case, an object did not move, even though the object beneath it was pulled or pushed away. This is inertia. An object tends to stay at rest (where it is) unless a sufficient force acts on it.

In the first case, the penny over the mouth of the jar fell into the jar. When the cardboard was pulled out from under it, instead of moving with the cardboard, it stayed in position until the force of gravity caused it to fall into the jar.

In the second case, the bottom penny on the stack was pushed out of the way by the force of the penny striking it. But the other pennies in the stack remained where they were.

In the third case, the cup of water remained in place even though it was sitting directly on top of the strip of paper which was pulled out from underneath it.

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