MOAS activity – balancing a stick

Balancing a mass on your finger

Mass: Demonstrating how the distribution of the mass of an object determines the position of its center of gravity, its angular momentum, and your ability to balance it.

Sunshine State Standards: SC.C.1.2 SC.C.2.2 SC.H.1.2 SC.H.2.2

Materials:

  • 1/2 inch wooden dowel, approximately 3 feet long,
  • Lump of plastic modeling clay

To Know:

  • Inertia – the tendency for a mass or object that is at rest to remain at rest or if it is in motion for it to continue in motion.. Think of pushing someone in a wagon or merry go round. It is hard to get them started (inertia at rest) and then once they are moving it is hard to stop them (inertia in motion).
  • Rotational inertia – a mass (object) that is fixed to a point about which it must move (rotate) which is not in a straight line. Think of a door that moves around its hinges or the wheel of a bike that rotates around the hub of the wheel.
  • Axis of rotation – the point about which the mass must move such as the hinge point or hub as mentioned. If you stand a pencil on its point and then let it fall over the axis of rotation is the point where it touches the table. The mass of the pencil must rotate about that point as it falls over.

To Do and Notice:

  • Place the lump of clay about the size of your fist eight inches from the end of the dowel.
  • First balance the stick on the tip of your finger by placing your finger under the end that is closest to the clay.
  • Now turn the stick over and balance it with the clay on the top.
    Is it easier to balance the stick with the clay on the bottom or with the clay on the top?
  • You will notice that the stick is easier to balance when the clay is near the top.

What’s Happening?

  • The dowel rotates more slowly when the clay is at the top, allowing you more time to adjust and maintain balance. When the clay is at the bottom, the stick has less rotational inertia and tips more quickly.
  • Think of your hand as the axis of rotation. The farther away the clay is located from your hand the greater the rotational inertia and the slower the stick will turn.
  • An object with a large mass is said to have a great deal of inertia.
  • It is hard to change the rotational motion of an object with a large rotational inertia.

The following experiment will allow you to feel the change in inertia.

  • Take the end of the dowel closest to the clay in your hand
  • Hold the dowel vertically, and rapidly move the dowel back and forth with same motion you would use to cast a fishing line.
  • Now turn the dowel upside down and hold it at the end that is farthest from the clay. Repeat the casting motion.
    Is it more difficult to move the stick in a casting motion with the clay at the top or with the clay at the bottom?
    Notice it is more difficult to move the dowel rapidly with the clay at the top.
  • The mass of the stick has not changed, but the distribution of the mass of the stick with respect to your hand has changed.
  • The rotational inertia depends on the distribution of the mass of the stick.

Extensions
Look around your classroom at school and find things to balance. On paper describe how each items reacts.

Developed by the GE Volunteers in partnership with the Museum of Arts and Sciences (MOAS) – Daytona Beach, FL