Chapter 10: Movement
and Forces
10.1 The skeletal system provides movement and protection
10.2 The muscular system makes movement possible
10.3 Muscles exert forces
10.4 Bones and joints at as levers
Warm-up True or False
• The biceps exercise a pulling force on your arm.
• True
• More force is required to push objects up ramps.
• Ramps reduce the amount of force needed to move objects.
• A lever is made up of a rod and an inclined plane.
• A lever is made up of a rod and a pivot point.
The body uses levers
• Examples:
• Lift something close to your body:
• shoulder muscles apply less force than when you lift
something with your arm stretched out
• A small movement from your muscles moves the end of
your arm a large distance
What does your arm act like when you lift something with
it?
A lever!
Levers can change the
effects of a force
• Recall: A lever is a solid bar, or rod, that
moves around a fixed point, a fulcrum
• a bone as a rod
• a joint as the pivot point
• bend your arm at the elbow - you are
pivoting your forearm around the joint
in your elbow
Levers - fulcrum
• Fulcrum – a fixed point around which
the rod of a lever turns
• The lever’s “pivot point”
• Can be located anywhere along the rod
• If in middle – both ends of rod may move
• If at end – just one end may move
• Ex: stapler
• Use less force when pressing…
• When a force is farther from the
fulcrum, it can turn the lever more
effectively
Levers - fulcrum
• Elbow
• Weight location relative to
fulcrum:
• When a weight is at your wrist––
farther from the fulcrum––it is
harder to lift
• Stapler and hand weight: the force
is more effective when it is farther
from the fulcrum
Input Force and Output Force
• Input force: the force exerted on a machine
• Often called the “effort force”
• The effort you apply to use the machine produces the input
force
• Output force: the force the machine exerts on the object
• The weight is called the “load”
• Generally: output force = load (weight)
• Also called: the “resistance force” (because it resists, or acts
against, the load)
• The load causes the lever to pivot in one direction
• The output force acts to balance the load
Input Force and Output
Force
• The input force on a body
limb comes from muscles
pulling on bone
• Contraction of the biceps
exerts an input force on
the lever of the forearm
• The distance from a fulcrum
to a force is called the lever
arm
• In the case of your
forearm, the lever arm is
very short, just a few
centimeters from the
elbow
• Each lever has a lever arm
defined for the input force
and another defined for the
output force
Changing Force Size and
Movement Distance
• A lever can change
• a small input force into a large output force
• can also change the direction of a force
• The farther the input force is from the
fulcrum, the greater the output force will
be
• Trade off between distance and force
• Apply less force to lift the rock, but must
move the lever over a greater distance than
the rock actually moves
Changing Force Size and
Movement Distance
• Some levers, including many in the
body, change a large input force into a
small output force
• Trade off is between having a larger
force and having a larger range of
motion
• Like a rake! And your bicep!
Classes of Levers
I-F-O
• Depend on placement of input force,
output force, and fulcrum
• First-class levers: the fulcrum is between
the input and output forces
• Second-class levers: the output force is
between the fulcrum and the input force
F-I-O
• Third-class levers: the input force is
between the fulcrum and the output
force
• Third-class levers always decrease the
output force in favor of speed or distance
F-O-I
Classes of Levers
1st
I-F-O
2nd
F-O-I
3rd
F-I-O
Examples of class 1
levers include:
Examples of class 2 levers
include:
* Teeter-totter
* Oars on a boat
* Catapult
* Shoehorn
* Scissors
* Pair of pliers
* Wheelbarrow
* Crowbar
* Nut cracker
Examples of class 3 levers include:
* Tweezers
* Stapler
* Mousetrap
* Broom
* Hockey stick
1st
I-F-O
2nd
F-O-I
3rd
F-I-O
The body’s levers can be used
effectively
• Using a lever puts stress on both the lever and the
fulcrum––the bone and the joint
• Lifting can strain your muscles
• Take care of yourself!
• Bending over causes your back to become a lever, and
puts stress on your spine
• Lifting with your legs uses the leg bones as levers
• strong muscles in your thighs and calves provide the input
force
The body’s levers can be used
effectively
• Your hand is a lever
with the fulcrum at
the wrist
• Your forearm is a
lever with a fulcrum
at the elbow.
• Your upper arm is a
lever with a fulcrum
at the shoulder
• http://www.edheads.org/activities/simplemachines/frame_loader.htm
• http://www.msichicago.org/fileadmin/Activities/G
ames/simple_machines/