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Balancing Pencil Challenge

In this exploration, students are given washers and wire to help stabilize a pencil on their finger. They use the principles learned in previous activities in order to solve the challenge.

In order for any object to be balanced and stable, a vertical line drawn from the centre of gravity should fall within the base of support.

It is much easier to balance an object with a broad base of support and a low centre of gravity. The broad base and low centre of gravity means that slight wobbles of the centre of gravity don't knock the object off its base.

In this activity, the base of the object is the tip of the pencil. Since we cannot broaden the base, we must somehow move its centre of gravity in order for it to be perfectly vertical above its tip. The pencil's natural centre of gravity is at its midpoint. By hanging weights below the pencil, we increase its stability by lowering its centre of gravity. Instead of being perched above the base of support, most of the mass is hanging below your finger.

It is also easy to line up the pencil's centre of gravity with its base by tweaking the position of the wires. You can adjust the washers to make the pencil lean to one side or stand up straight.

Solution:

  1. Wrap each end of the wire around a washer.
  2. Wrap the middle section of the wire around the middle of the pencil.
  3. Spread the weights out a little and try balancing the pencil on the tip of your finger. Adjust the weights as needed to get the pencil to balance.

Practical examples of these principles include tight rope walkers and rocking toys. Tight rope walkers use a heavy, downward-curving pole to lower their centre of gravity in order to stay balanced on the rope (base). A rocking toy's centre of gravity is at the bottom. When the toy is rocked, it returns to its most stable position, with the centre of gravity in line with its base.

Objectives

  • Find the centre of gravity of an object.

  • List the conditions required for an object to be stable.

Materials

  • A sharpened pencil

  • 2 washers

  • 30 cm of floral wire (or other thin, malleable wire)

Key Questions

  • Why does moving the washers affect the balance?
  • Where is the centre of gravity of your contraption?

What To Do

  1. Brainstorm ways in which we can increase the stability of an object.
  2. Write out the conditions necessary to increase the stability of an object (found in the activity intro) on the board.

Challenge:

  1. Try balancing the pencil on the tip of your finger (this should be difficult).

     
  2. By using the two washers and some wire with your pencil, you will have no problem balancing it on your finger.
  3. Use what you know about balance and stability to solve this challenge. You have 5 minutes. Feel free to brainstorm solutions with your classmates.

Extensions

  • Try to balance the pencil on other parts of your body (e.g. nose) or on other surfaces.
  • Cut out a cardboard animal and tape it to the wired pencil. Find the new centre of gravity to make it balance.
  • How do the results of this activity explain why a tightrope walker uses a heavy pole for balance?
  • How do the results of this activity explain how a rocking toy works?

Other Resources

Science World Resources | Full Unit |  Balance