Katy and Hannah’s Physics Web Page

 

Center of Gravity- The point at the center of an object’s weight distribution, where the force of gravity can be considered to act.  (CG)

 

*      What it is- The CG of an object is the point located at the object’s average position of weight.  On a baseball, for example, this point is at the center.  Objects that are not symmetrical such as a baseball bat have a center of gravity towards the heavier end. 

*  CG is often called the center of mass. 

 

*        How you find it- Suspend any object at a single point and the CG of the object will hang directly below or at the point of suspension.  To locate the object’s CG construct a vertical line beneath the point of suspension.  The CG lies somewhere along this line.  Locate the CG by suspending the object from some other point and drawing a second vertical line.  The CG is where the two lines intersect. You should be able to balance the object at this point on the tip of your finger if you found the CG correctly. 
 
*        Try It Yourself! An experiment…
                               Objective:
 
Predict and find the center of gravity of an object.
 
Materials Needed: 
 
2 boxes, string, lead sinker, dissecting needle, and metric ruler
 
Strategy:
 
A) Use a pair of boxes of the same size for each team of students.  If different 
   weights are secured in different places in the boxes, all the teams will find 
   different centers of gravity. 
 
B) Weight half the boxes in advance.  Secure the weights with glue.  Blocks of 
   wood or metal make excellent weights. 
 
C) Examine an unweighted box. 
 
D) Predict where the center of gravity is. 
 
E) Attach a lead sinker to a 30-cm long piece of string.  Tie the string to a 
   dissecting needle.  Stick the needle into the broad surface of the box at any 
   point near the edge of the box and mark this point. 
 
F) Let the box and the weighted line hang freely from the needle.  With a 
   pencil, mark the position of the weighted line at the edge of the box. 
 
G) Remove the dissecting needle.  Use a ruler to draw a line between the two 
   points that you marked. 
 
H) Repeat this procedure for three more points on the surface of the box. 
   Remember to mark each point near the edge of the box.  The point at which the
   four lines cross shows the center of gravity of the box. 
 
I) Examine a weighted box. 
 
J) Predict where the center of gravity is. 
 
K) Repeat steps E through H with the weighted box. 
 
L) Extend your index finger and try to balance each box on your finger. 
 
M) Place each box, with the marked side up, on a flat surface.  Spin each box. 
 
Results and Conclusions:
 
1.  How do your predictions compare with the actual location of the center of 
    gravity of each box?
 
2.  How is the center of gravity related to the way each box balances?
 
3.  How is the center of gravity related to the way each box spins?
 
Summary:
 
1.  Answers will vary.
 
2.  The box balances at the center of gravity.
 
3.  The box spins with the center of gravity as its axis.
 
Culminating Activity:
 
1.  Clean out an empty soup can or an oatmeal box and fully remove both the top 
    and the bottom.  Now you have a cylinder.
 
2.  Tape a quarter or some other small weight inside it.
 
3.  Prop up one end of a breadboard or some other flat board.
 
4.  Place your cylinder at the low end of the board with the quarter in the two 
    o'clock position.  When you release the cylinder, it will roll up the hill.
 
Summary:
 
   Because its center of gravity is very near the position of the quarter.  The 
center of gravity will go down, causing the cylinder to roll up the hill.

 

 

 

 

 

 


*          Real Life Applications- The first application of CG is in toppling.  If the CG of an object is above the area of support, the object will remain upright.  All car manufacturers use the center of gravity in tilt tests to test the

stability of their cars. 

              When buildings are constructed, especially tall narrow structures, the CG is very important.  The Seattle Space Needle, for example, has its CG below ground level.  Because of this, the building cannot fall over intact. 

            People also have a CG.  The CG of a woman is slightly lower than a man because women tend to be proportionally larger in the pelvis and smaller in the shoulders.  The CG is typically two to three centimeters below your naval and midway between your front and back.  CG is important to help us stand and for athletes, such as high jumpers. 

                                                                                                       

 

*      Conclusion- If the CG extends beyond the support base, toppling occurs. 

 

 

*      CG Links-         www.phschool.com

                                www.whyfiles.org

                                www.hummer.com

        http://www.pbs.org/wgbh/nova/teachers/programs/2403_sle1ston.html

                                http://www.phy.ntnu.edu.tw/java/block/block.html

                                http://jersey.uoregon.edu/vlab/

                                http://www.phy.ntnu.edu.tw/java/indexPopup.html

 

*  About the geniuses behind this- Katy and Hannah are stellar Physics students at FHS. They chose to do this web page because it was required. They choose the topic of CG because this is a very elementary, yet important concept in Physics. If one does not understand CG, they will have trouble in construction of experiments and mechanics. For questions or comments contact Mr. Young at dyoung@fayar.net.