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/
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