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The People of CIS

Mark Fairchild
Faculty

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Research Interests: Color Perception & Imaging: Color Appearance Modeling Color Perception Visual Systems Image Appearance Image Perception Color Measurement Image Quality Measurement High-Dynamic-Range Imaging Image and Video Rendering Science Literacy through Color Environmental Science

What's so great about the center of mass, anyway?

There are several reasons why it can be good to know the center of mass of an object:

if you want to hang it, or support it, the object will balance around the center of mass

if you push or pull the object along a line through the center of mass, it will translate -- move while remaining in its original orientation. If you push or pull along a line which doesn't pass through the center of mass, the object will rotate as well as translating.

extended objects may rotate or deform as they move under the influence of an external force, but their center of mass will follow a simple trajectory (exactly the same trajectory a little ball would follow)

To illustrate this last point, turn on your computer and start the Interactive Physics program. Go to the Student Shares -> University Physics -> Team Physics 311 -> Interactive Physics folder and choose the "CenterofMass1" file. You should see two objects, a small ball and a polygon similar to the one you measured earlier. When you Run the program, it will throw each object into the air and show you the resulting motion. If you use the controls in the lower-left corner of the screen, you can step forward one second at a time and watch things happen more slowly.

Run the simulation. Watch the path of the ball -- pretty simple, right? What is the path of the small dot marked on the polygon?

Now click on the little dot and drag it to the other end of the polygon. Run the simulation again. Is the path of the little dot the same as before? Did it become more or less complicated?

Can you find a location on the polygon for the little dot so that when you throw the polygon, the dot's path is as simple as that of the ball?

Suppose that we keep track of the center of mass of bunch of particles. Open the file "CenterofMass2" in Interactive Physics. You should see three objects (green ball, red square, blue banana) and two little markers.

Run the simulation. You should see all three objects fly across the screen. Complicated, isn't it? You can type Control-E to erase the tracks, or World -> Tracking -> Off to turn of the tracks entirely.

Go to the View menu and choose Show System Center of Mass. Run the simulation again, and watch the motion of the center of mass. Compare its motion to that of each individual object.

Drag the green ball so that it just to the right of the upper marker and run the simulation again. You should see a collision. What happens to the motion of the center of mass of the system?

Play with the program to cause more collisions and mayhem. Change the masses of the objects by selecting an object and then choosing Windows -> Properties. Keep watching the center of mass of the system -- what happens to it? What is its path as the objects fly about?

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First year doctoral student co-authors study on stable optical lift

The People of CIS

What's so great about the center of mass, anyway?

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Imaging Science Grad Student Responds to Disaster Needs

The People of CIS