Kin 300 Exam 1 CSULB

Definition of a force

Push or a pull that accelerates an

object.

A force can start, stop, speed up,

slow down an object or change

direction of the object

Unit of a force

Newton

W = mg

1 N = 1 kg X 1 m/s/s

1 lb force = 4.448 N

Characteristics of a force

Point of application

Line of action

Sense of direction (angle of pull)

What is a vector?

A vector is a "type" of quantity.

A force is a vector quantity.

This means that a force has both:

Magnitude Direction

Vector Representation

Size of the force (length of the arrow)

Line of application (shaft of the

arrow)

Sense/Direction along the line of

application (arrowhead)

Point of application of the force (end

of arrow)

How do we classify forces?

Internal forces & External forces

Internal forces -

forces acting inside an object or system.

External forces

forces acting on

an object generally due to the

object's environment.

Internal forces

Muscles, tendons, ligaments all

create internal forces.

The magnitude and direction of

internal forces are important when

trying to determine the cause of

injury

External Forces

Non-contact forces - forces acting

at a distance from the object. Gravity Magnetic Electrical

The force of gravity acting on an

object is defined as its weight

9.81 m/s/s (32.2 ft/s/s) = g

(acceleration due to gravity)

Calculate Weight

The weight of an object (in N) is its

mass (in kg) times the acceleration

due to gravity (g) (-9.81 m/s/s) OR

W = mg

W = weight (in N)

m = mass (in kg)

g = acceleration due to gravity

(-9.81 m/s/s)

External Forces

Contact Forces - forces

acting directly between

objects. Friction ,Joint Reaction Forces ,Ground Reaction Forces

Ground Reaction Force

Normal Contact Force - line

of action is perpendicular to

surfaces in contact

Second Component is Friction

Action/reaction of object with

the ground.

Important when looking at

overall resulting forces acting

on an object

Ground Reaction Force Walking vs. Running

slide 5/14

Joint Reaction Forces

Action/reaction of

forces on

segments in the

body.

Not the actual

bone-on-bone

force it is an

estimate of all

forces

Friction

Friction is a force

that opposes the

motion of an

object.

Frictional force is a

product of the

normal force and

the coefficient of

friction.

Friction can be

static (limiting) or dynamic (sliding or kinetic)

Static (limiting)

friction occurs

when the objects are not moving

with respect to each other.

Dynamic (sliding or kinetic)

friction occurs when the objects are

moving with respect to each other.

Coefficient of Friction

Accounts for different effects that

materials have on friction

Ratio of friction force over normal

contact force

Static

Fs = μs X R

Dynamic (kinetic)

Fd= μk X R

Fs

static friction force

μs

= coefficient of static friction

Fd

= dynamic friction force

μd

coefficient of dynamic friction

R =

normal contact force

Weight of the object affect friction?

Yes, this changes the Normal force

which affects the frictional force.

Surface Area affect friction?

No, friction is not affected by surface area.

Different contacting materials affect friction?

Yes, the different materials will change

the coefficient of friction.

Friction affects everything we do!

Walking/running

Skating/hockey

Skiing

Grip materials

Throwing

Catching

Chapter 1 - PART II

Introduction to Forces :)

Resultant

result of vector addition of

two or more forces

Net

- vector addition of all external forces

acting on an object

Colinear -

- forces that have the same line

of action

Concurrent -

forces that do not act along

the same line, but through the same point

Colinear Force Addition

When forces act along the same line

and in the same direction, they can

be added using regular algebraic

addition

If not in the same direction, must

use positive/negative signs

-400N -150N

Concurrent Forces

Gymnast hanging on a bar:

20N by coach pushing on front 30N by coach pushing on back

550N upward vertical reaction force

exerted by the bar on gymnast's hands

500N downward force (mass = 50kg,

so W = 50(10))

Force system is

all forces acting on

the object.

Force composition

adding forces

together (determining a resultant)

Force resolution -

breaking a

resulting force into its component

forms (determining components)

Right Triangles

Pythagorean Theorem

Trigonometry

Pythagorean Theorem

Know the length of s of any two sides we can determine the length of the other side

Trigonometry

Know the length of one side of a right triangle and the measurement of one of the angles (other than right angle), we can determine the lengths of the other sides and other angle

Relationships exist among the lengths of the sides in a right triangle and the angles in a right triangle.

Trig Ratios

Sin Θ = opposite side/hypotenuse

Cos Θ = adjacent side/hypotenuse

Tan Θ = opposite side/adjacent side

Inverse Functions

Need to calculate angles when you

know sides lengths?

Θ = arcsin (opposite/hypotenuse)

Θ = arccos (adjacent/hypotenuse)

Θ = arctan (opposite/adjacent)

We know all the sides and need to compute

the angle. Let's calculate using the lengths

that were given to us.

If we're using opposite and adjacent, we

need to use tangent. Since we know sides

and not angles, we need to use arctangent.

Θ = arctan (50N/10N)

Θ = arctan (5) Θ = 78.7º

(make sure set to degrees, not radians)

We know all the angles of a triangle add up

to 180º, so 180 = 90 + 78.7 + x

x = 11.3º

Force Composition

Addition

Graphical Technique

Pythagorean Theorem

Trigonometric Technique

Resolution of Forces

Not colinear and not acting in a

vertical or horizontal direction?

W = mg = 4(10) = 40N shot

We are told that the shotputter exerts 100N of

force upward at 60º above horizontal. We want to determine the horizontal and vertical components of that 100N

Resolution of Forces - Graphical &

Pythagorean

Knowing how long 100N line is, measure other two lines to

estimate their length. Double check using pythagorean

theorem

(87N)2 + (50N)2 = (100N)2

2500N2 + 7569 N2 = 10069 N2

vs 10000 N2

Resolution of Forces

Remember the 40N weight of the

shot is acting as a downward

force, so for the vertical

component of the force, the result

is:

-40N + 87N = 47N

(50N)2 + (47N)2 = C2

2500 N2 + 2209 N2 = C2

4709 N2 = C2 C = 68.6 N

We had the resultant force (100N)

and we worked backward to figure

out the components, graphically

Resolution of Forces -

Trigonometric Technique

We are told that the shotputter exerts 100N of

force upward at 60º above horizontal.

Cos 60 = adjacent/100N

Cos 60(100N) = adjacent

(0.500)100N = 50 N

sin 60 = opposite/100N

sin 60(100N) = opposite

(0.866)100N = 86.6 N

Resolution of Forces -

Trigonometric Technique

To calculate the net force, we can

now use the Pythagorean

theorem:

(46.6N)2 + (50N)2 = C2

2172N2 + 2500N2 = 4672N2 C = 68.4N

Now to calculate the direction (angle) between

the net force and the horizontal force: Θ = arctan (46.6N/50N) = arctan (0.932) = 43º

Static Equilibrium

The sum of all external forces

equals 0 because there is no

movement or constant velocity.

Σ F = 0 (net external force)

More Specifically: Σ Fx = 0 Σ Fy = 0

Ice Skater (page 40)

Free body diagram- simple

representation only of the object in

question along with all of the

external forces that act on it

W= mg = 50kg(-10m/s/s) = -500 N

ΣF = R + (-500N) = R-500N = 0

R = reaction force from the ice

R - 500N = 0

R = 500N

Child on Swing (page 43)

W = 20kg * (-10 m/s/s) = -200N

40N horizontal force

10N upward force

ΣFx = 0 (horizontal) and ΣFy = 0 (vertical)

ΣFx = Rx + 40N = 0, thus Rx = -40N

ΣFy = Ry + 10N + (-200N) = 0, thus Ry = (-10N) + 200N =

190N

40N to the left and 190N upward

Resultant Force = (40N)2 + (190N)2 = 37,700N2 = 194N =

C

Need its direction - what angle it makes with horizontal...

Child on Swing continued

Θ = arctan (190N/40N)

Θ = arctan (4.75) = 78º

Thus, the resultant force exerted by the

swing on the child is a force of 194N

acting backward and upward on the

child at an angle of 78º from

horizontal

INTRODUCTION:

What IS Biomechanics?

What is biomechanics?

Study of mechanical principles

(forces) as they are applied to living

organisms

Sport and Exercise Biomechanics

Orthopaedic Biomechanics

Cellular Biomechanics

Tissue Biomechanics

Applied Biomechanics

Why Study Biomechanics?

To better understand how

mechanical quantities, such as

gravity, friction, and other forces,

affect how we perform various

tasks.

Sport & Exercise Biomechanics

Focus of this class

Application of mechanical principles

as they relate to sport and exercise

activities

Goals of sport & exercise

biomechanist: Improve Performance Prevent Injury Improve Rehabilitation

How does biomechanics

IMPROVE PERFORMANCE?

Technique Improvement

Equipment Improvement

Training Improvement

Equipment Improvement

Tennis Materials of racquet

Cycling Bike materials Components

Running Shoes

Javelin Design and materials

Training Improvement

Specificity of training: Train for a specific activity Train muscle to do a specific activity

Understanding "needs" of activity: Strength needs Endurance needs Flexibility needs

How does biomechanics help to

REDUCE INJURY?

Technique Improvement

Equipment Improvement

Training Improvement

Kinesiology vs. Biomechanics

Biomechanics- application of

mechanical principles to living

organisms

Kinesiology - study of movement. This includes physiology, anatomy, psychology, motor control/learning,

AND mechanics

History of Biomechanics

Aristotle (380-320BC), Leonardo da Vinci (1400-1500s) •Biomechanics applications begin in late 1800s

•Posse and Skarstrom: first use of the term kinesiology in the United States in late 1800s

•1960s: conferences, organizations, graduate-level programs (first PhDs in the late 1960's)

•1970s: rapid expansion, sport medicine, dance kinesiology

(biomechanics)

•Late 20th century: continued expansion of university programs and organizations, switch from the term

Kinesiology to biomechanics to identify this subdiscipline

What makes a biomechanist?

Part engineer

Part physicist

Part anatomist

Part physiologist

Part neuroscientist/motor control

Part computer geek

Part electronics whiz

Job Titles for Biomechanists

Researcher

Clinical biomechanist

Performance enhancement specialist

Ergonomist or human factors engineer (industrial task analysis

specialist)

Forensic biomechanist

University professor

Certified orthotist or prosthetist

Earn additional degrees in allied health fields (PT, ATC, OT, PA, MD, etc)

What is mechanics?

Analysis of forces applied to an

object and the result of these

forces.

Science that analyzes the effects of

forces on objects.

Key Mechanics Definitions

:)

Statics

the object undergoes zero

acceleration.

Dynamics

the object undergoes some

acceleration.

Kinematics

description of the object's

motion: position, velocity, acceleration.

Kinetics -

forces that cause the motion:

gravity, friction, applied force.

Putting it all together

pg 6 of 9

Types of problems

Qualitative vs Quantitative

Qualitative

Non numeric method of describing movement/performance

Quantitative

Numeric method of describing movement/performance

Our "Toys"

Equipment is highly specialized and VERY

expensive!

Gait analysis

EMG

Plantar Pressure

Concept/Terminology Review

1) Define biomechanics

2) What is the difference between

biomechanics and kinesiology?

3) What are the general goals of a sport and

exercise biomechanist?

4) Define statics and dynamics.

5) What is an example of a kinematic

variable?

6) What is an example of a kinetic variable?

7) Define mass

8) Define weight

9) Define inertia

Introduction to Linear Kinematics

Types of motion, position, & velocity :)

What do you need to describe

motion?

Defined Space: Tells you where object starts Tells you where object stops

Defined Time: Tells you how long object was moving

Kinematic Quantities

As long as we have a defined space

and time, we can determine all the

kinematic quantities that we need to

describe motion.

Position - Distance & Displacement

Speed & Velocity

Acceleration

Recall the following model:

pg 2/19 not necessary

Linear (Translational) motion

occurs when all the points on the object move

the same distance, in the same

direction, over the same amount of

time. Rectilinear Curvilinear

Rectilinear

when all the points move

in a straight line (skiing straight down slope)

Curvilinear

when all the points move

along a curved path (hang-gliding straight, while arcing downwards)

Angular motion (rotational)

occurs when all the points move through the

same angle, in the same direction

over the same amount of time. (laying down on stomach doing hamstring curl)

General Motion

Most complex human movements fit

into this category.

It is a combination of linear and

angular motion.

Angular motion of the joints results in

linear motion of the rest of the body or

an object.

(moving forward in wheelchair, doing sideways 360 on snowboard in straight direction)

How do you define position?

Position is an object's location in

space.

In this class, we will constrain our

movement space to be 2 dimensional.

Since we are in 2D, we need to have

an X coordinate and a Y coordinate to

describe the object's position in

space.

Distance

Distance is the measure of the length of the

true path that the object traveled.

Units for distance are meters.

Distance is a scalar quantity, it has only

magnitude.

Example: runner runs 3 blocks north, 2

blocks east, and 2 blocks south. What is

the distance that the runner traveled? (7 blocks)

Displacement

Displacement is the straight line from

the initial (start) position to the final

(end) position.

The units are meters.

Displacement is a vector having

magnitude and direction.

What would be the displacement for

the runner? (use pythag. theorem. 5 blocks)

Speed vs.

velocity

Speed

Distance over

time

Scalar quantity

Units: m/s, km/hr

Average speed

Instantaneous speed

Velocity

Displacement

over time

Vector quantity

Units: m/s

Average velocity

Instantaneous

velocity

Velocity Graph

velocity (m/s) vs displacement (m)

Check your Understanding

A swimmer completes a 200m swim in

an Olympic pool in 110 seconds.

Which quantity would you use to

determine "how fast" she was

swimming? (speed)

Going back to example:

If it took our runner 5 ½ minutes to run

the 7 blk. distance and each block is

400 meters. The displacement was

2.2 blocks. What was the runner's

average speed and average velocity?

Average Speed = distance/time 7 blks X 400 m/blk / 5.5 min. X 60s/min 2800meters/330 s 8.48 m/s

Average velocity = displacement/time 2.2 blks X 400 m/blk /330s 2.67 m/s at 26° N of due E

Acceleration

Change in velocity divided by time

Average acceleration and

instantaneous acceleration.

Direction of acceleration is important.