Additional Info

• ID Code: C3-12
• Purpose: Dramatically demonstrate inertia
• Description:
  A pencil is accelerated to almost the speed of sound by blasting it through a four-foot tube using a carbon dioxide fire extinguisher. The pencil will readily impale itself through a piece of 3/8" plywood. With a little bit of luck the pencil point will be virtually intact, although sometimes you need to re-sharpen it after the demonstration.

  CAUTION: Be sure that the hose fitting is securely attached to the tube and that the plastic shield is in place before firing. The shield should be latched in place, with no debris blocking its edge from meeting the baseplate

  Engagement Suggestions

  • • Before using, encourage your students to predict what will happen to the pencil.
  • • For advanced students, discuss the energy involved in the problem and where the kinetic energy of the pencil went after the collision.

  Background

  This demonstration can be presented in multiple ways. It has been offered classically as an illustration of the principle of inertia – the pencil is in motion at a high velocity, and continues in motion despite the intervening wood until arrested by a greater force. Alternatively, consider the high velocity and high momentum of the pencil. The abrupt deceleration at the plywood means a high impulse. The pointed pencil has a very small cross-sectional area, resulting in force applied over a small area leading to a high momentary pressure.

  Linked below is a slow-motion video of the collision, shot at 600 frames per second. A fun class activity could be to use the video to measure the motion of the pencil and estimate its momentum and kinetic energy, based on what you see in the video and by measuring typical lengths and masses for wooden pencils.

  • Availability: Available
  • Loc codes: FS1
  • Videos-1: a slow motion video of the collision, shot at 600 frames per second
  • Videos-2: Demonstration video on YouTube