12.1 - levers

lever

• a mechanism for doing work that consists of a rigid bar like structure, a fulcrum, and two forces

characteristics of a lever

• fulcrum (pivot point)

• resistance force (weight of arm/hand)

• effort force (muscular force) - ex. triceps

fulcrum (pivot point)

• the axis of rotation of a lever

effort force

• force applied to a lever, causing movement of the lever

• aka. muscle force

effort arm

• the perpendicular distance of the line of action of an effort force to the axis of rotation

resistance force

• a force that resists the effort force in a lever

resistance arm

• the perpendicular distance of the line of action of a resistance force to the axis of rotation

first class lever

• EFR

• fulcrum in middle

• ex.: head moving backwards

  • Effort force: back of head muscles

    • arm: perp. to spine (b/c force is down)

    • if you relax these muscles, head will rotate forward (wherre resistance force pulls it)

  • Fulcrum: atlantooccipital joint

  • Resistance force: weight of head

    • arm: perp. to spin (b/c force is down)

second class lever

• resistance in middle

• FRE

• ex.: man holding a wheel barrow

  • Fulcrum: wheel

  • resistance force: wheel barrow

    • Res. arm: just from fulcrum to res. force (barrow)

  • effort force: muscular force of body pulling it up (res. pulling it down)

    • effort arm: from fulcrum to effort force)

calf/heel raises second class lever example

• ERF/FRE

  • Effort: Gastrocnemius/tibialis - insert in heel

    • up

  • Resistance: down, bdyweight through tibia

  • Fulcrum: Heel

third class lever

• FER - most common

• majority of musculoskelteal systems are first class levers

• these favor speed and range of movement

thrid class example - elbow flexion

• fulcrum: back of elbow

• effort force: biceps up

• resistance force: lower arm down

what characteristic is the joint when talking about muscles

• the fulcrum

ex.: shoulder abducted - holding at side

• fulcrum: shoulder joint

• effort: deltoid: inserts on humerus - lateral to fulcrum

• resistance: weight of arm (around the elbow joint)

tunk fulcrums

• in between each vertebrae

• ex.: leg crunches

  • first class - because legs are the resistance and the abdomen is the effort

statistical analysiss

• a mechanical analysiss in which teh forces acting on the system are balanced - hence the system is not accelerating

• the system is either stationary or moving at a constant velocity

  • known as static equilibrium

2 conditions of static equilibrium

• sum of forces is 0

• sum of torque is 0

static equillibrium teeter totter

• sum of girls forces minus sum of boy’s

• if 0, then it will stay still

how to have boy pull the girl down

• shorten the girls resistance arm or lengthen the boy’s.

• takes more force when closer to the pivot point