Forces Flashcards
Force
Effect that one body has on another
A push or a pull applied to an object
That required to change the state of motion of an object (i.e., that which causes acceleration)
What is force?
Push or pull
Causes deformation and/or motion
Vector quantity
Magnitude, direction & point of application
F = ma Units: Newton (N) Weight is similar to force, should be in N, not lbs or kg Weight = Mass * Gravity Gravity = -9.81 m/s2
Characteristics of Force
1. Force is a vector quantity Magnitude (“size”) 2. Direction (orientation) 3. A third, unique characteristic: Point of application Especially important relative to the determination of moments or torques 4. Must know all three characteristics 5. Other important characteristics: Line of action Angle of pull / orientation
Classifying Forces
Internal Forces
External Forces
Normal Force
Tangential Force
Internal Forces
Act within the object or system
Hold together when acted on by external forces
Muscle tension, ligament tension, bone compression
External Forces
- Act on an object as a result of interaction with environment
- Contact forces: forces resulting from objects coming into contact
- Non-contact forces: forces that occur even if objects are not in contact
Gravity, magnetic, electrical
Normal Force
Force acting perpendicular to surface of object
Tangential Force
Force acting parallel to surface of object
Contact Forces
Types: Ground reaction force (GRF) Joint reaction force (JRF) Friction Fluid resistance Inertial force Muscle force Elastic force
Non-Contact Forces
Non-contact forces: forces that occur even if objects are not in contact Types: Gravity Magnetic Electrical
What is a net force?
Single resultant force derived from the vector composition of all acting forces
Net force determines the net effect of all acting forces on a body
Vector Composition and Resolution (Types of Variables)
Scalars
Vectors
Scalars
A quantity that is defined by its size/magnitude
Examples:
Mass, energy, power, temperature, etc…
Vectors
A quantity that is defined by its size/magnitude and direction
Represented as arrows
Length = size/magnitude
Pointing = direction
Examples:
Force, moment, velocity, acceleration, etc.
Vector Composition
The process of determining a single force (vector) from two or more forces (vectors) by vector addition
Finding the resultant vector or net vector
Steps depend upon if vectors are colinear or concurrent
Colinear vectors = share the same line of action (parallel with each other)
Concurrent vectors = do NOT share the same line of action
Vector Addition: Colinear Forces
Forces that have the same line of action
Can be same or opposite direction
Tip to tail
Vector Subtraction (graphical)
Composition of vectors with opposite direction requires subtracting their magnitudes
Colinear vectors
Vector Addition: Concurrent Forces
Forces do not act on the same line, but they do act through the same point
Resolution of Vectors
- Replacing a single vector with two perpendicular vectors such that the vector composition of the two perpendicular vectors yields the original vector
- Resolve original vector into horizontal & vertical components
- Horizontal & vertical components should point in the same direction of the original vector - Methods
Graphical
Trigonometric
Trigonometry
Many problems in biomechanics involve the use of right triangles
Sum of 3 internal angles = 180º
Triangle Parts: 3 angles, 3 sides
Sides: hypotenuse, opposite, adjacent
Mechanical Behavior of Objects in Contact: Friction
Force acting at interface of surfaces in contact
Acts in a direction parallel to the area of contact
Opposes the motion or tendency to move
“Translational Friction”
Friction Force Depends On
(max static friction: Fm) depends on two things:
Normal reaction force (Rn)
Coefficient of friction (μ) (nature of surfaces)
Fm = μs ∙ Rn
Coefficient of Friction
Indicates relative ease of sliding between two surfaces in contact
Not describing a single surface
Factors
Roughness & hardness of surfaces
Type of molecular interaction of surfaces
Static & kinetic values differ
μs > μk
Kinetic Friction
Friction force generated between two surfaces in contact during motion
Remains constant
Fk < Fm
Fk = μk Rn
Is it easier to pull a desk than push it?
When you pull you usually have an upward component of force – so…
the normal force is decreased and therefore the friction force is decreased.
When you push you usually have a downward component of force – so…
the normal force is increased and therefore the friction force is increased.
