Chapter 10: Movement and Forces

Chapter 10: Movement and

Forces10.1 The skeletal system provides movement and

protection10.2 The muscular system makes movement possible

10.3 Muscles exert forces10.4 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

• The distance from a fulcrum to a force is called the lever arm

• Each lever has a lever arm defined for the input force and another defined for the output force

• Contraction of the biceps exerts an input force on the lever of the forearm

• In the case of your forearm, the lever arm is very short, just a few centimeters from the elbow

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• 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

• 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

I-F-O

F-O-I

F-I-O

Classes of Levers

1stI-F-O

2ndF-O-I

3rdF-I-O

Examples of class 2 levers include:

* Wheelbarrow * Crowbar * Nut cracker

Examples of class 1 levers include:

* Teeter-totter * Oars on a boat * Catapult * Shoehorn * Scissors * Pair of pliers

Examples of class 3 levers include:

* Tweezers * Stapler * Mousetrap * Broom * Hockey stick

1stI-F-O 2nd

F-O-I

3rdF-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/simple-machines/frame_loader.htm

• http://www.msichicago.org/fileadmin/Activities/Games/simple_machines/