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• Hinge joints allow a person to move in almost any direction. • A hinge joint allows a person to move forward in one

direction and back.

• Muscles work together in coordinated groups to move bones. • True

• When a skeletal muscle relaxes and lengthens, it exerts a force that pulls bones.

• When a skeletal muscle contracts, it exerts a force that pulls bones.

Muscles provide forces that produce motion

• Force – a push or a pull• Force changes the motion of an object

• Forces exerted by your body comes from muscles• Contract muscles – pull bones movement• Pull harder? More force• Muscle fibers contract – more/larger fibers = more force

• Energy needed to apply force – from chemical energy (cellular respiration and fermentation)…from food

Forces act along the muscles

• Bicep curl – feel tendon (like a string) pulling the bone from the forearm to your shoulder

• The direction of the force from your biceps acts along this line parallel to the direction of the contraction

• Both size and direction of force are important• Forces from your muscles vary:• Start to move your arm – muscles stabilize• At 90o, most force

• When muscles pull in the direction in which the joints move most easily, their effective force is greatest

It’s physics! (and math!)

Muscle & using force effectively

• Pull the suitcase handle at an angle• Some applied force pulls object upwards• Some applied force pulls object forwards • you are not using all of the applied force to move

the object forward

• Pull the suitcase handle parallel to the direction to move• most of the applied force is used to move the suitcase

forward • (what about the rest of the force?)

More than one force affects your body

• Hold arms out• Downward force from…gravity! Called “weight”• Add a rock – even more “weight”• Forces acting on a book on the table?• Forces acting on you? • Forces can act in opposing directions• Balanced forces = no force no motion• Unbalanced forces = unequal forces motion

Gravity – downTable - up

Gravity – downFloor - up

Balanced forces = no force no motion

Unbalanced forces = unequal forces motion

Some body parts act like simple machines

• How to turn a small force into a larger one?• Can’t carry a large box up the stairs• How can you move the box?• Build a ramp (a type of inclined plane)

• Less force is needed because it is applied over a greater distance

• Work: the amount of energy that you transfer to an object when you move it over a distance• is a transfer of energy• you do work on an object

• If you exert a force on the object to move it, but it doesn’t move – no work was done

• If the object moves – work is done• Holding the box, or holding a rock in your hand, is not “work”

Simple machines (6)• (devices on which all other mechanical machines are based)• Help people do work (but don’t change the total amount of

work required)• pulley, lever, screw, wheel and axle, wedge, and inclined

plane• Household examples of simple machines?• using a pulley to raise a flag• using a hammer as a lever to pull out nails• screwing shelves into a wall• moving dirt with a wheelbarrow• using a knife acting as a wedge to slice an orange

(non-electronic)

Simple Machines

• Help people do work• Do NOT change the amount of work

required to move an object from location A to location B

• Allow us to apply less force over a longer distance to perform the same amount of work

W = F x d• tradeoff between force and distance• (ex: long ramp vs stairs)

Simple machines model how the body works

• Use them to model the way that limbs work• Not a perfect model, but a useful way to

describe and understand how complex systems interact

• Three simple machines help model movement in the body:• Pulleys• Inclined planes• Levers

Pulleys• Help change the direction of

force• Ex: fixed pulley at the top of a

flag pole (or mini-blinds)• You don’t change the distance the

flag moves or the force needed to move it – just the direction of the force

• force pulling the flag upward = your downward force

• allows you to take advantage of the downward pull of your weight to move a load upward

Pulleys in the body• Recall: muscles in the limbs are attached to two or more bones by tendons• Most of the skeletal muscles are attached to one end of a bone, stretch

across a joint, and are attached to another bone• When the muscle passes over a joint, the joint can act as a fixed pulley –

changes direction of force• Ex: shoulder• Muscles crisscross the shoulder joint, attaching to the bones at many points and

allowing many directions of motion• Ex: wrists and hands• Long ligaments and tendons allow the fingers to be moved by muscles far away in

the forearm • Wiggle the fingers of your hand – feel forearm

http://youtu.be/6RbDkz737oA

Inclined planes in the body

• A ramp, a simple machine that decreases the force required to move a heavy object• decreases the effective weight of a car

• Gliding joints - feet and wrists• have slightly slanted or inclined surfaces

where the bones meet• Nearly flat bone surfaces glide over each

other, allowing only limited motion

Levers in the body

• muscle pulling on a bone can be compared to a lever

• A lever is a solid bar, or rod, that moves around a fixed point, a fulcrum

• Ex: a crowbar• Body: each bone as a rod and each joint as

a fulcrum• (more in 10.4)

Bill Nye -Simple Machines

http://www.edheads.org/activities/simple-machines/frame_loader.htm

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