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Secrets of Lost Empires I—Obelisk
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Classroom Activity
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Objective
To investigate how levers can be used to lift objects.
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copy of "Lever Loads" student handout (PDF
or
HTML)
- two rulers, 30 cm (12 in.) plastic or lightweight wood
- pencil
- five weights (such as washers or coins)
- tape
- sheet of paper
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Let students investigate how levers can be used to lift objects
in this activity.
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Distribute materials and copies of the "Lever Loads" student
handout to pairs of students.
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In Part I students will discover that the amount of force
needed to lift a load changes depending on where the fulcrum is
placed.
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In Part II students repeat the investigation, this time
comparing the distance the load is lifted to the distance they
push down the other lever arm.
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After completing the activity, ask students to think about how
NOVA team members used levers to raise the two-ton obelisk.
Where did they place the fulcrum (the pivot point) in
relationship to the obelisk (load)? Why did the team need to
pack stone and earth underneath the lever to support it?
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Archimedes (287-212 B.C.E.) understood levers well enough to
claim, "Give me a lever long enough and a place to stand, and I
could lift the world." Discuss this quote with students. What
does Archimedes mean by a lever "long enough"? Where might he
place the fulcrum?
In Part I, students will notice that the closer the fulcrum
is to the load, the less force is required to lift the load. There
is a point where the fulcrum is so close to the load that it appears
no force is required at all. The weight of the lever itself (in this
case, the ruler) is actually providing the needed force. This is one
reason why crowbars are very heavy.
In Part II, students should note that the position of the
fulcrum also affects how far you push down and how far the load is
lifted up. Levers trade force (how hard you push) for distance (how
far you move the load). For example, when you apply a small force
over a long distance, you move the load a small distance.
Students should notice that when they use less force to lift the
load, the distance they push the lever arm down is larger than the
distance the load is lifted. When they use more force to lift the
load, the distance they push the lever arm is smaller than the
distance the load is lifted. When the fulcrum is placed in the
middle of the lever, the distance the load is lifted is about equal
to the distance that the other end of the lever is pushed down. In
this situation there is essentially no mechanical
advantage—the distance over which the force is applied equals
the distance the load is lifted.
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