Mechanics Flashcards
Kinematics
- Describes the motion
- Position, velocity, acceleration
Kinetics
- Describes how the motion is produced
- What forces create the motion
- Types of energy that motion utilizes
- The power required to initiate and guide the motion
Newton’s laws of motion describe the connection between
- Forces that act upon an object
- The manner in which the object moves
Newton’s First Law (the law of inertia)
- An object at rest stays at rest
- An object in motion stays in motion with the same speed and the same direction (unless acted upon by an unbalanced force)
- Predicts behavior of objects when forces are balanced
Two predictions of Newton’s First Law
- Resting objects will continue to rest
- Moving objects will continue to move
Forces applied in Newton’s First Law
- Horizontal and vertical forces are applied to the object
- These forces act upon the object
- For motion to occur, inertia must be overcome
Inertia
- Resistance
- An object has to change its state of motion
- Solely dependent upon the mass of an object
Newton’s Second Law
- The sum of the net forces acting upon an object equals its mass times acceleration
- Defines the behavior of objects when forces are not balanced
Newton’s Second Law is the relationship among
- Net force
- Mass
- Acceleration
- Net force is the sum of all forces acting upon an object
Force
- The change in momentum over time
- Occurs when forces are not balanced
Acceleration is dependent upon
- The net force acting upon the object
- The mass of the object
- F = m x a
Force definition
- Strength exerted upon an object
- Net force is the sum of all forces exerted on an object
Forces involved with net force calculation
- Gravity
- Resistance forces
Resistance forces
- Inertia
- Static Friction
- Dynamic Friction
- Compression, tensile, and torsional
- Air resistance
Acceleration
- Proportional to the magnitude of the net force
- Occurs in the same direction of the force applied
Acceleration is inversely proportional to
- Mass applied
Acceleration equation
- Equal to the final velocity minus the initial velocity divided by time
- (Vf - Vo)/(tf - to)
Acceleration relationships
- Greater mass means greater inertia
- More force is required to change their motion
- Increased force = increased acceleration
- Increased mass = decreased acceleration
Newton’s Third Law
- Whenever one body exerts a force on a second body, the second body exerts
an oppositely directed force of equal magnitude on the first body - “For every action there is an opposite and equal reaction”
Forces do not cancel each other because
- They act upon different systems
- When Body A exerts a force on Body B, then simultaneously, Body B exerts a force on Body A in the opposite direction
Momentum
- mass x velocity
Newton’s First Law (intertia) in walking
- Prior to heel contact, swinging leg has mass and forward momentum
- At heel strike, forward velocity and momentum go to zero
Newton’s Second Law at heel strike
- The change in momentum is great
- The time it takes to get to zero
- The force is very great as heel strike occurs quickly
As we walk faster
- Heel strike is shorter
- Force on the foot is greater (F = m x a, so more acceleration results in greater force)
Newton’s Third Law at heel contact
- The force generated by heel strike is applied to the ground
- The ground is pushing back of on the heel (ground reactive force)
Work
- Work is force x displacement
- W = F x d
- Measured in joules
Work results when
- A force causes displacement
- Mechanical Work
- Physiological Work
- No displacement means no work
Work applied to heel strike
- There is no displacement of the stance limb, therefore no mechanical work
- But there is physiological work
Power
- The rate at which work is done upon an object
- P = W/t
- Measured in watts
Watt
- Work = joules
- Time = seconds
- Joules/second = Watt
Efficiency
- A comparison of the energy output to the energy input
- Percent efficiency = (energy output/energy input) x 100%
- Symbol n
Torque
- Measure of the tendency of a force to rotate about an axis
- The product of a force around a turning point (axis) and the perpendicular distance to that axis
Torque relates to
- The axis of rotation that drives the rotation
- Measured in Newton-meters
Moment
- The measure of a force’s tendency to cause rotational acceleration about an axis
- It is calculated as force x distance at right angles
Energy
- The ability to perform work
- W = F x d
- Units are joules
Two primary forms of energy associated with movement
- Potential Energy
- Kinetic Energy
Potential energy
- Stored energy
- It is dependent upon gravity
- PE = m x g x h
- Note that g = acceleration of gravity (9.8 m/s^2)
Kinetic energy
- Energy possessed due to the motion of an object
- KE = (1/2)mv^2
Kinetic energy requirements
- An objection must be moving
- Its energy is dependent upon its mass and velocity
Law of conservation of energy (first law of thermodynamics)
- Energy is not created or destroyed
- Energy can only be converted into other forms
- In the human body it is converted to heat if not conserved
In trauma the degree of injury is related to
- The energy of the injuring element
- The interaction between that element and the victim
Linear versus angular terms
- Walking is rotational or angular
- All joints will rotate about their axis in all three cardinal planes
- Muscles generate the force to initiate and maintain movement
- These movements occur against resistance of some sort
Translation
- Forward linear movement
- Requires rotation for walking
Rotation
- Movement occurs through our joints and about the joint axes
- Movement around a fixed axis
- No forward movement
Walking combines
- Translation
- Rotation
Archimedes Law of the Lever assumption 1
- Equal weights at equal distances balance
- Equal weights at unequal distances do not balance
- The greater weight will tilt its end of the lever down
Archimedes Law of the Lever assumption 2
- When two weights balance if something is added to one, they no longer balance
- The side with the increased weight goes down
Archimedes Law of the Lever assumption 3
- If two weights balance and something is taken away from one, they no longer balance
- The unchanged side goes down
First class levers
- Skull
- Ankle joint
The skull as a first class lever
- Head nods forward and backwards
- Head nods side to side
The ankle joint as a first class lever
- Open kinetic chain
- The lever is suspended (no axial load)
- Axis is the ankle joint
Ankle joint lever components in open kinetic chain
- Plantarfelxion = effort of tendo Achilles posterior to ankle joint
- Dorsiflexion = effort of anterior muscle group
- Foot accepts the load
Second class lever
- The load exists between the fulcrum and the effort
- Force is applied in an opposite direction from load
Equal load and force on second class lever
- MA = 1
If effort provides greater force than load on a second class lever
- MA: Load force x distance ÷ effort force x distance = <1
- Object will move towards the direction of effort
If the load force is greater than effort on a second class lever
- MA: Load force x distance ÷ effort force x distance = >1
- Object will move away from the direction of effort
The ankle joint as a second class lever in closed kinetic chain motion
- Weight bearing (there is an axial load)
- Fulcrum (axis): ankle joint
- Effort: tendo Achilles
- Load: the body and any weight it carries
Third class lever
- The effort is between the fulcrum and the load
In a third class lever, the object will move in the direction of the load if
- MA > 1
- Load force x distance from fulcrum ÷ effort force x distance = >1
In a third class lever, the object will move in the direction of effort if
- MA < 1
- Load force x distance from fulcrum ÷ effort force x distance from fulcrum
Third class lever components example
- Fulcrum (joint axis): elbow
- Effort: contraction of the biceps brachii
- Load: what is held within the hand
Extension of knee joint
- Class 3 lever is operating
- Effort is between the axis (fulcrum) and the load (lower leg)
First and second class lever efficiency
- Generally efficient
- The closer the load to the fulcrum, the more efficient
Third class lever efficiency
- Least efficient
- Make up the majority of body levers
Benefit of efficiency
- Get the maximum work from the least amount of effort
Mechanical advantage
- Measures the amplitude of force
- If is a ratio between the load and the force applied by the effort
Movement in the direction of effort requires:
- Force of the load x distance from fulcrum must be less than force of muscular effort x distance from fulcrum
- MA < 1
Movement in the direction of load requires
- Force of the load x distance from the fulcrum must be greater than force of muscular effort x distance from fulcrum
- MA > 1
Equilibrium
- No movement
- MA = 1
If load and effort are equal forces (regarding mechanical advantage)
- Effort distance twice as far from fulcrum compared to load
- Effort has “twice” the leverage or mechanical advantage
Mechanical advantage (MA) equation
- MA = (Force B/Force A)
- MA = (Load/Effort)
If the effort and load forces are equal (regarding mechanical advantage)
- Load is three times the distance from the fulcrum
- Load is three times greater than effort
- Load has the mechanical advantage
Pulley system
- Change the effective direction of the applied force
- Transmit force
- Gain mechanical advantage by changing the angle of pull for the muscle
The knee as a pulley system (extension) is an interaction between
- Patella
- Quadriceps
- Patellar tendon
Role of the gracilis muscle in the knee as a pulley system
- Course along condyles
- Increase the angle of pull which minimizes force required
The ankle as a pulley system
- Peroneus longus changes direction
Role of the peroneus longus in the ankle as a pulley system
- Posterior to fibular malleolus: assists with plantarflexion
- Cuboidal notch: everts foot and plantarflexes first ray
