Additional Info

• ID Code: C3-05
• Purpose: Dramatically demonstrate inertia
• Description:
  A large-tip felt pen is balanced on a 12" embroidery hoop, which in turn is balanced on a wide-mouth bottle. Yanking the hoop out from under the pen (by striking inside the leading side horizontally) allows the pen to fall straight downward into the bottle. Note that this does take a bit of practice; try it out before class.

  Engagement Suggestion:

  Ask your students: • Why does it matter if the hoop moves up or down while you are moving it?
  • Does it make a difference if you grab the hoop from the outside or the inside?
  

  Background:

  Newton’s First Law of Motion states that an object’s velocity is constant unless there is a net force acting on it. What this means is that if an object is not moving (at rest), it will not start moving until there is a force pushing or pulling on it. If an object is moving at a constant speed and direction, it will keep going with that same speed and direction unless a force pushes or pulls on it to change that. When the pen is sitting on top of the hoop, the force of gravity is pulling it down, but the normal force of the hoop is exactly equal to the gravitational force and holds it up. If another force suddenly affects the pen (such as if you walk up and tap on its side, or jiggle the hoop up and down), that force could cause it to move, and probably fall.

  But if the hoop is snatched sideways quickly and smoothly, it does not give any force to the pen. Now the only force acting on the pen is gravity, and the pen falls straight down into the bottle.

• Availability: Available
• Loc codes: C3