Balance and Gravity

Stevenson, Mark James H. Bowen High School

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.
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