Flashcards in Forces Deck (25)

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1

## 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)

2

## 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

3

## Characteristics of Force

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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

4

## Classifying Forces

###
Internal Forces

External Forces

Normal Force

Tangential Force

5

## Internal Forces

###
Act within the object or system

Hold together when acted on by external forces

Muscle tension, ligament tension, bone compression

6

## External Forces

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1. Act on an object as a result of interaction with environment

2. Contact forces: forces resulting from objects coming into contact

3. Non-contact forces: forces that occur even if objects are not in contact

Gravity, magnetic, electrical

7

## Normal Force

### Force acting perpendicular to surface of object

8

## Tangential Force

### Force acting parallel to surface of object

9

## Contact Forces

###
Types:

Ground reaction force (GRF)

Joint reaction force (JRF)

Friction

Fluid resistance

Inertial force

Muscle force

Elastic force

10

## Non-Contact Forces

###
Non-contact forces: forces that occur even if objects are not in contact

Types:

Gravity

Magnetic

Electrical

11

## 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

12

## Vector Composition and Resolution (Types of Variables)

###
Scalars

Vectors

13

## Scalars

###
A quantity that is defined by its size/magnitude

Examples:

Mass, energy, power, temperature, etc…

14

## 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.

15

## 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

16

## Vector Addition: Colinear Forces

###
Forces that have the same line of action

Can be same or opposite direction

Tip to tail

17

## Vector Subtraction (graphical)

###
Composition of vectors with opposite direction requires subtracting their magnitudes

Colinear vectors

18

## Vector Addition: Concurrent Forces

### Forces do not act on the same line, but they do act through the same point

19

## Resolution of Vectors

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1. 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

2. Methods

Graphical

Trigonometric

20

## 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

21

## 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”

22

## 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

23

## 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

24

## Kinetic Friction

###
Friction force generated between two surfaces in contact during motion

Remains constant

Fk < Fm

Fk = μk Rn

25