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/