Center of Gravity

Minnie E. Bounds                Dyett Middle School
                                551 E. 51st Street
                                Chicago, IL  60615
                                312-535-1825

Objective:

Predict and find the center of gravity of an object.

Materials Needed: 

2 boxes, string, lead sinker, dissecting needle, 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 oat meal 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 bread board 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.