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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 • 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 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? Gravity – down Table - up Gravity – down Floor - up • Forces can act in opposing directions • Balanced forces = no force no motion • Unbalanced forces = unequal forces motion 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) (non-electronic) • (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 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=Fxd • 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/simplemachines/frame_loader.htm http://www.msichicago.org/fileadmin/Activ ities/Games/simple_machines/
