Forces

Question 1

Force

Answer 1

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)

Question 2

What is force?

Answer 2

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

Question 3

Characteristics of Force

Answer 3

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

Question 4

Classifying Forces

Answer 4

Internal Forces
External Forces
Normal Force
Tangential Force

Question 5

Internal Forces

Answer 5

Act within the object or system
Hold together when acted on by external forces
Muscle tension, ligament tension, bone compression

Question 6

External Forces

Answer 6

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

Question 7

Normal Force

Answer 7

Force acting perpendicular to surface of object

Question 8

Tangential Force

Answer 8

Force acting parallel to surface of object

Question 9

Contact Forces

Answer 9

Types:
Ground reaction force (GRF)
Joint reaction force (JRF)
Friction
Fluid resistance
Inertial force
Muscle force
Elastic force

Question 10

Non-Contact Forces

Answer 10

Non-contact forces:  forces that occur even if objects are not in contact
Types: 
Gravity 
Magnetic
Electrical

Question 11

What is a net force?

Answer 11

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

Question 12

Vector Composition and Resolution (Types of Variables)

Answer 12

Scalars

Vectors

Question 13

Scalars

Answer 13

A quantity that is defined by its size/magnitude

Examples:
Mass, energy, power, temperature, etc…

Question 14

Vectors

Answer 14

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.

Question 15

Vector Composition

Answer 15

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

Question 16

Vector Addition: Colinear Forces

Answer 16

Forces that have the same line of action
Can be same or opposite direction
Tip to tail

Question 17

Vector Subtraction (graphical)

Answer 17

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

Question 18

Vector Addition: Concurrent Forces

Answer 18

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

Question 19

Resolution of Vectors

Answer 19

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

Question 20

Trigonometry

Answer 20

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

Question 21

Mechanical Behavior of Objects in Contact: Friction

Answer 21

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”

Question 22

Friction Force Depends On

Answer 22

(max static friction: Fm) depends on two things:
Normal reaction force (Rn)
Coefficient of friction (μ) (nature of surfaces)
Fm = μs ∙ Rn

Question 23

Coefficient of Friction

Answer 23

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

Question 24

Kinetic Friction

Answer 24

Friction force generated between two surfaces in contact during motion

Remains constant
Fk < Fm
Fk = μk Rn

Question 25

Is it easier to pull a desk than push it?

Answer 25

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.