Forces Flashcards
Vectors and linear force
Always in a line
Pull NEVER push
Internal vs external
Internal – muscle contracting; always pulls in a straight line unless there’s a pulley redirecting the muscle like a ligament
Law of intertia
A body in motion (or at rest) will remain in that state unless acted on by another force.
Ways to stop the rolling: friction, gravity, outside force like a wall
We can use gravity to keep her in her chair by placing a slight wedge under her to tilt her slightly back into her chair.
Dycem is a non-slip material that can be placed under her to use friction to keep her from falling out the chair.
When increasing friction, sheer injuries can happen. Happened when forces due to friction happen in a parallel force.
Outside force can be a seatbelt. Could cause trauma. Can decrease the trauma by making the seatbelt wider to distribute force more evenly.
- could be seen as a type of restraint; a lap tray could also be seen as a restraint
- restraints can only be used when the patient is considered a danger to themselves or others
- If a patient can’t get themselves out of it, it is considered a restraint
Law of acceleration
It takes more force to move (accelerate) a bigger object than a smaller object.
If the same force is applied to a large and small object, the small object will move before the large object.
Law of action-reaction
When a force is applied to an object, there is no movement, an equal force must be acting in the opposite direction. (equilibrium)
A wrist with a flexion contraction will need a splint that has enough force to equalize the force of the contracture (stretch vs position)
If brace strength is greater it will do more harm than good. If it is too little or equal it will do nothing.
Normal forces
External forces that either push or pull joint surfaces together or apart
Compressive
Tensile
Compressive forces
push tissues together causing structures to widen and shorten
Tensile forces
Pulling apart, causing structures to narrow and lengthen
- Elasticity: the ability of a structure to stretch
and return to its position of rest without injury - Bones have very little elasticity (more as infants)
- Muscles have great elasticity
- Stress: external force causing increas
Shear (Tangential) forces
External forces that operate parallel to the surface
Often result in:
- Skin tearing
- Brain injury
Internal forces
Muscle contraction
- Muscles always produce straight line forces
- The number of fibers determines the force
- Tendons transmit the force to the skeletal segments
* They can redirect the force, creating a pulley effect
Resultant Force
- Muscles with fibers that go in different directions
Rotary forces
Doors, doorknobs, rolling wheels
- Motion occurs around a central point
Most skeletal movements are rotary motions that are created from linear movement
The brachialis contracts and creates a linear pull on the ulna to create a rotary movement of the forearm into flexion.
These rotary movements combine to give us the ability to move in linear paths.
How is Rotary force different from Linear movement?
- Rotary force occurs in a circular path around a center point, linear motion occurs along a linear path
- As objects rotate, they change orientation, linear movements remain in their original orientation
- Two points on a segment during linear movement, move at the same speed. Two points on a segment during rotational movement move at different speeds.
What factors affect movement in rotation?
The distance the force from the center of rotation and the greater the force the greater the effect and tendency to rotate
Tendency to rotate
The tendency of an object to rotate is related to the force and THE SPOT AT WHICH THE FORCE IS APPLIED
- Also called the mechanical advantage or leverage
- The force (or weight) and the distance from the pivot point determine tendency to rotate
* The further away from the pivot point, the greater the tendency to rotate
Torque
Torque is the effectiveness of a force in causing rotation, same as tendency to rotate
When we think about msucles, the point of application of force…
is the insertion of the muscle, not the muscle belly
The longer the tendon the greater the torque
Lever System
Because we move in a rotary manner, we have to consider both force and movement arm
- So we can’t use 5 kg of muscle contraction to counter act 5 kg of resistance.
- The amount of muscle force needed will depend on the muscle’s movement (force) arm (how far it is from the axis) and the movement arm of the resistance
- The arrangement of the two arms (force and resistance) can vary, there are three basic variations or levers
First class lever
Effort Axis Load
The mechanical advantage and the resistance are balanced on both sides of the axis. A 90# girl can move a 90# girl
- The force or effort arm and the resistance, weight or load arm are on opposite sides of the axis
- Examples: seesaw, steering wheel, revolving door
- Imagine if you had to steer your car with a wheel the size of a donut. Think about old cars-before power steering, what did the steering wheels look like?
- Equal advantage or balance.
Second class lever
Axis Load Effort
Favors the mechanical advantage of the force or effort (meaning the effect of the effort increases or is multiplied with this type of lever) A 90# girl can move a 200# boulder.
The force or effort and the weight, resistance or load are on the same side. The weight, resistance or load is between the axis (fulcrum) and the force or effort
The amount of force necessary to operate a second-class lever, decreases in proportion to the length of the handle (the longer the distance from axis and load to the effort the less effort it takes)
- The longer the handle the less effort it takes
- Examples: Bottle openers, wheelbarrow, wrench
Third class lever
Load Effort Axis
Favors the mechanical advantage of resistance or load, not the muscle. (a 90# girl can move a 50% girl, but send her flying)
Muscles must typically generate greater forces thatn the resistance they encounter.
The force or effort is between the axis(fulcrum) and the weight, resistance or load
Not as common in every day life
- Gain precision
- But most common in muscles
The catapult, bat, tweezers
Ineffective
Axis or fulcrum
what the motion happens around
Load or resistance
the weight of the extemity
Effort
the pull of the muscle
The Third Class Lever System, may appear to create a mechanical disadvantage, but:
It allows for more speed and degrees of movement
Muscles can increase in strength (they can’t increase excursion, which would be necessary if it worked like a 1st or 2nd class)
The only way an object (force) can be positioned at the end of the segment
- In a first class-very long arms
- 2nd class-hand would have to be in the middle of the forearm
Gravity
Perpendicular force
It is Consistent-everything falls down!
It takes tremendous power to break free from it
It effects every single movement on earth
It effects every thing the same
Gravity is CONSTANT! It doesn’t Change!
Humans are not constant, we move!
When you stand evenly on both feet, your weight is distributed between your feet, if you lift one foot off the ground, you will have to shift your body to center your weight over the one remaining foot or you will fall.
The wider the base of support the more stable and the easier it is to balance
The smaller the base of support the more effort it takes to maintain balance
Role of Core Muscles
How does gravity act as a stimulant?
In early life it stimulates reflexes that later lead to voluntary movement
Causes stretch which results in a resistive contraction leading to increase strength
Key Idea: Movement caused by gravity in one direction typically leads to muscle contraction in the opposite direction.
Gravity has subtle effects with MAJOR implications
Vestibular mechanism of the inner ear
Client who is unable to stand independently
Is it strength or fear?
Would vestibular tasks-Dancing (function) , help a client to improver her standing ability?
How do we find out center of gravity?
Much the same way,
IF we stood still
- About 6 inches above the pubic symphasis
- Level of the 2nd sacral vertebra
- 2 inches below the belly button
- Slightly lower in females due to hip width
Big Difference, we move and are not symmetrical
- As we move the center of gravity moves
- It typically moves toward the area of greatest mass
- As we bend forward, the COG moves interiorly. It can even move outside the body
- If you have really developed leg muscles, it may move down
Stability
The height of the center of gravity
The base of support
The wider the greater the stability
The projection of the Center of gravity
Harder to move heavy things
What determines stability? Why do some clients fall and some don’t?
The height of the center of gravity
- The higher the less stable
The base of support
- The wider the more stable
The center of gravity projection
- When the projection falls outside the base of support, we lose our balance
The weight
- The greater the mass the more force to move it
