Year 13 Center of Mass Experiment:


Experiment part 1 (COM of stationary object):

Purpose:

  • The purpose of this experiment is to find the center of mass of a range of different shapes.


Equipment:

  • Paper / card (for students to cut out random shapes)
  • drawing pins (to connect through paper into a wall)
  • Wall which multiple pin holes is not a problem
  • string
  • bob

Method:

  • The center of mass is the point where all of the mass of the object is concentrated. When an object is supported at its center of mass there is no net torque acting on the body and it will remain in static equilibrium. An easy way to determine the location of the center of mass of a rigid pole is to support the pole horizontally on one finger from each hand. Gently slide your fingers together. When your fingers meet, you will be at the center of mass at which time you can easily hold up the pole with only one finger as long as it can withstand the entire weight of the pole. Try it with a bat or a broom. If the object is uniform, for example a meter stick, the center of mass will be at the exact geometric center; if the object is irregular in shape the center of mass will be closer to the heavier end.

COM.gif



  • Another method of finding the center of mass of an planar object is through the use of a plumb line. Suspend the mass from each vertex and trace the plumb line's location. Since the center of mass will fall below the suspension point (in order to reduce any torques from the object's weight) the center of mass will be at the intersection of all of the plumb lines.

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Full instructions can be found in the PowerPoint below.


  • Once you have determined the position of the center of mass, you should be able to suspend the object at that point. See COM PowerPoint by JHarris for a full introduction and instructions.
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Video instructions by Colorado University.

Another video to explain the above. No need to watch all of them:



Other examples:

  • There are also a heap of Physics toys and other examples demonstrating COM:
  • Balancing bird:
COM21.jpg
For video and more info click here
com22.jpg
  • Forks on match stick (which is lit on fire)


Experiment part 2 (COM of moving object):

When an object is thrown, although it may spin in somewhat a chaotic motion, the position of COM will follow a perfect parabolic motion.
  • Take your students outside and throw some asymmetric object. Ensure you have clearly marked the COM, so students can see the motion. I personally find this demo quite a big interruption, for something which is pretty hard to see and most students can visualize. The applets and videos I find a sufficient.
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Applets: Click to activate:

Motion_of_com_applet.jpg
Excellent Java applet for showing the motion of the centre of mass

Motion_of_com_applet2.jpg
Another good Java applet for showing the motion of COM


Experiment part 3 (COM of a set of objects):


  • This demo can be done with a set of books (hard cover) over the edge of a bench, Where each book is more over the edge of the table than the last.
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  • If you care for your books too much, I often set the following applet for homework:
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Click to go to applet




References / other comments:

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