Balance and Gravity

Stevenson, Mark                          James H. Bowen High School 
                                         933-7200
                           

Objectives:

Students will learn the basic concepts of balance and how to determine 
the location of the center of gravity using a model of the leaning 
tower of pisa and various other items. 

Apparatus Needed:

plexiglass (5cm x 20cm)                triple-beam balance 
string (35cm)                          beaker (250ml)
overhead pens                          salt (2g) 
lead weight (100g)                     pencil

Recommended Strategy

     Have students examine the triple-beam balance and weigh several 
items. Ask them to notice what are the results of their measurements 
when the balance is in equilibrium.  Raise the question; Where is the 
center of gravity found using this apparatus? 
     
     Make a model of the leaning tower of Pisa from the plexiglass. The 
model should be slightly larger at the base. Explain that the center of 
gravity is an imaginary point at which we can consider the entire 
weight of an object to be concentrated.  Place one hole in each of the 
top corners of the model. Suspend the model from one hole at a time 
while allowing the string and weight to come to rest below the base.  
Draw a line from the point of suspension to the base.  Repeat this 
procedure for the opposite side. The X where the lines intersect marks 
the center of gravity. Place another hole at the point determined as 
the center of gravity. If you now suspend your weight-on-a-string so 
that it hangs freely from the center-of-gravity point and lean the 
tower against a wall while its base stands on the edge of a book, you 
can find out just how far over it can go without falling.  As long as 
the line remains within the base line of the tower, it will not fall. 
The instant the line passes outside the base, it falls. Explain to the 
students that the reason the leaning tower of Pisa has not yet fallen 
is because its center of gravity still lies within the base. 
     The lower the center of gravity, the farther over an object can 
lean. Thus, by concentrating the weight of buildings, cars and boats as 
low as possible, designers make them more stable. 
     Ask the students to bring to class at least one item which 
demonstrates balance or the center of gravity for class discussion.