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Levers in the Human Body
lever (LEV uhr) a simple machine
that consists of a bar that pivots at
a fixed point called a fulcrum
mechanical advantage
(muh KAN i kuhl ad VANT ij) a
number that tells you how many
times a machine multiplies force
The action of a muscle pulling on a bone often works like a
type of simple machine called a lever. A lever is a rigid bar that
pivots at a fixed point known as a fulcrum. Any force applied
to the lever is called the effort force. A force that resists the
motion of the lever, such as the downward force exerted by a
weight on the bar, is called the load. Figure 3 shows the action
of three types of levers in the human body. In your body, the
rigid bar is a bone. The effort force is supplied by muscles.
And the fulcrum at which the bone pivots, is a joint.
Levers increase the amount of work that can be done by
the effort force applied to a load. This increase in work is
called mechanical advantage.
Figure 3
Levers in the Human Body
Load
Effort
Fulcrum
First-class lever
Third-class lever
Second-class lever
478
Chapter 15
Body Organization and Structure
Three Classes of Levers
As Figure 3 shows, there are three classes of levers. First-class
and second-class levers increase the amount of force applied to
a load. Third-class levers increase the speed of the motion. The
class of a lever is determined by the location of the fulcrum,
load, and effort force.
In a first-class lever, the fulcrum is between the effort force
and the load. First-class levers work like a car jack or seesaw.
To lift a load, a downward effort force must be applied to the
other end of the lever. There are few first-class levers in the
body. The joint in your neck acts as the fulcrum in a first-class
lever that lifts your head. The effort force is supplied by the
muscles at the back of your neck.
In a second-class lever, the load is between the fulcrum and
the effort force. Second-class levers allow you to use less effort
force than the force exerted by the load. Second-class levers
work like a shovel. There are also few second-class levers in
the body. The action of rising onto your toes represents the
action of a second-class lever.
The effort force in a third-class lever is between the fulcrum
and the load. Third-class levers increase the distance through
which the load is moved. Third-class levers work like a dolly
that is used to carry heavy boxes. Most movable joints in the
human body work like third-class levers. You use a third-class
lever to lift your textbook. Your elbow is the fulcrum, and the
biceps muscle in your upper arm provides the effort force.
Give an example of a lever in the body.
Mechanical Advantage
Force is measured using the
SI unit called a newton (N).
Use the equation below to
answer the questions that
follow.
mechanical advantage !
force applied to load "
effort force
If a second-class lever in
your body has a mechanical
advantage of 8. What is the
effort force applied to a load
of 40 N? Where is this lever
in your body?
7.6.i
Quick Lab
How Do Levers Work?
7.6.i
You can use simple materials to model how
levers in the human body work.
1. Draw a table like the one below.
Class
of lever
First
Second
Third
Sketch
of model
Human body
sketch
E
RIT
W
T
NO
OK
DO N BO
I
2. Use a meterstick, small wooden block,
and 100 g weight, to build a lever from
each class. Refer to Figure 3 as needed.
3. Sketch each of your models in your table.
4. Which part of the human body moves like
each class of lever? Sketch this body part in
the table that you created.
5. How does the location of the fulcrum
change the action of the lever?
6. What advantage does each class of lever
provide?
25 min
Section 3
The Muscular System
479