Forces and Mechanics

Force and movement is used
in our muscles, bones, and
joint systems—many that act
as _____.
Newton’s _____ law of motion
• A body will stay at rest or in a
state of uniform motion unless
some force alters this behavior
• Friction due to gravity is what
stops the wheelchair/piano
• In a car accident, nothing stops us
from flying ahead accept airbag,
seat belt, or windshield
Newton’s ______ law of motion
• Acceleration of an object is
proportional to the force applied
to it
• Force = mass times acceleration
• F=m x a
• a= acceleration > how fast speed
increases or decreases
• Ex: to move the piano (high mass)
you have to push with a higher
force relative to the wheelchair to
get it to move (overcome inertia)
Newton’s ______ law of motion
• For every action, there is an
equal and opposite reaction
• Forces are balanced in
objects as rest (or constant
Work done = ?
force applied x
distance moved
A concept that an object’s entire
weight (mass) can be represented
as a SINGLE POINT in space [for
physics analyses]
Center of Gravity
Medical Gravity:

IV drip bags suspended
above the patient

Gravity provides the force to “push” solutions into veins
(saline drips, blood transfusion, etc.), height related to drip
rate of fluid
Medical Gravity:

Fluid drainage

Must ensure that collection bag is below point of drainage
from patient’s body as gravity pulls liquid “down”
Medical Gravity:

Raising a limb to reduce
venous bleeding

The heart has to pump against gravity, so the flow of blood
is reduced upon elevation
Medical Gravity:

Improving blood
circulation with a
fainting patient

Laying a person flat and raising their legs encourages blood
flow to the patient’s brain, to keep oxygen flowing to this
critical tissue/organ

• The force that opposes the sliding of
two surfaces, one over the other
• Due to surface imperfections
between the surfaces
• The reason we do not slip and fall
when we walk

True or False: Dynamic friction is higher than static
False: Static friction is higher than dynamic
– It is harder to start an object in
motion than to continue its motion
Give a few examples of how to reduce friction:
• Use of lubricant oils, ball
bearings, wheels, etc. are
excellent ways to reduce
friction when moving
objects past/over one
• Highly polished surfaces
have significantly lower
friction as well
Give some examples of how friction is reduced within/on the body
• Mucus
– In lungs, heart, and intestines
• Saliva
– Digesting foods
• Synovial fluid reduces cartilage bone friction
• KY jelly etc. for insertion of instruments and
catheters in the body
• Use of skin lotions/oils during massage
_____ _____ are thought to result from sustained pressure
on the skin causing restricted capillary blood flow beneath
the skin > cell death > tissue ulceration
– Keep the patient mobile to vary the point of contact with the bed
– Use varied types of cushions, mattresses, and bedding
– Goal is to relieve the prolonged pressure on the same points of
Pressure Sores
A _____ is a simple mechanical
device that enables us to apply a
force at one point and see an effect
at another
The _____ is the pivot point of a lever
Lever Force Equation for movement
The movement force is equal to
the applied force times the
distance from the pivot point
_____- is formally the weight
of the object desiring to be
moved (as a force)—also
called resistance sometimes
_____- is formally the force
needed to be applied to the
lever to make the load
Which class of lever is this:

The load and effort
on opposite sides of
the fulcrum

First class lever; a see-saw; the semispinalas capitis and the splenius capitis use the occipital condyles as a fulcrum to raise the from to of the head when they contract.
What class of lever is this:

The effort is outside
the load on the same
side of the fulcrum

Second-class; wheelbarrow; the quadriceps muscles of the anterior thigh elevate the knee when contracted. The hip joint is the fulcrum.
What class of lever is this:

The effort is inside
the load on the same
side of the fulcrum

Third class; using a paddle, such as when paddling in a boat; when flexing the elbow, the biceps brachii muscle exerts an effort on the radius. the fulcrum is the elbow joint.
Consider a human head sitting on top of the
spine (previous slide). If the head weighs about
10lbs. and its COM is situated about 4” in front of
the center of the spine, how much force would
the muscles situated 1” behind the spine need to
exert to keep the head from moving?
– 4in. x 10lbs. = 40 in•lbs. = 1in. x (muscle force)
– Muscle force = 40 in•lbs./ 1in. = 40lbs.
Consider a human leg and foot (slide 17). If the
leg/body weight exerted on the foot is about 40kg and
its COM is situated about 10cm in back of the ball of
the foot, how much force would the leg muscles
situated 14cm behind ball of the foot need to exert to
start to lift the foot off of the floor?
– 40kg x 10cm = 400 kg •cm = 14cm x (muscle force)
– Muscle force = 400 kg •cm / 14cm = 28.6kg
Note: this is lower than the original force /weight due it
being further from the fulcrum (the ball of the foot)
_____ are used to change the
direction of forces that we
apply to help us do work
Pulleys help by adding _____ _____ in their use.
Mechanical advantage
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