Forces Flashcards Preview

KNES 361 Exam 1 > Forces > Flashcards

Flashcards in Forces Deck (25)
Loading flashcards...
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

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

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

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

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.