Positioning the fulcrum in class one levers

Janis Gary Sullivan A. O. Sexton School
6020 S. Langley Ave.
Chicago, Ill. 60637
312 535-0640


1. To identify the parts of a class one lever
2. To discover and demonstrate the relationship between force and the
distance of the load from the fulcrum
3. To discover that work input = work output with the lever
4. To compute the mechanical advantage of a lever (ratio of the distances
through which the forces are exerted)

Materials Needed:

ruler, pencil, 30 pennies, wood block, wood plank (at least 4 feet),
bathroom scale, meter stick, various text books


Each group receives the above materials. The group then makes a lever by
placing the ruler across the pencil. The pencil (fulcrum) is first placed under
the 4 inch mark on the ruler. Ten pennies (load) are placed between the end of
the ruler and the one inch mark. Have each group add and record the number of
pennies needed on the opposite end of the ruler to lift the ten pennies (load).
The experiment is repeated with the ten pennies at one end and the pencil
(fulcrum) at the 6 inch and 8 inch marks under the ruler. The groups record
the number of pennies needed to lift the ten pennies (load) at the different
points of the ruler on the chalkboard chart. The mechanical advantage is
worked out using the ratio of the distance of the effort arm to the resistance
(load) arm.

Using the discovery of the relationship between the distance of the load to
the fulcrum, the groups will lift the teacher (if willing) or a classmate. A
wood block and a wooden plank are provided and the class one levers are set up.
The student and the textbooks are weighed on the scales and the data recorded.
The pupil stands on the load arm and one by one the books are placed on the
effort arm until the load arm is raised and the effort arm is touching the
floor. Positioning the fulcrum at different points under the plank, find the
ratio of the effort arm length to the load arm length to get the highest
mechanical advantage. This information is charted on the chalkboard to again
note that when the load is placed nearer the fulcrum the effort needed to raise
the load is less. Therefore, the longer the distance of the effort arm, the
smaller the force needed. The groups then compute work input and work output by
using the equation:

Fdresistence = fDeffort.

(where F="load", d=vertical distance "load" is lifted, f="effort",
D=vertical distance the "effort" is pulled down)
with the data from the chart on the chalkboard. The answer is given in foot-
pounds and will indicate that work input is equal to work output.
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