class5- statics,lever and equibrium

7/30/2019 Class5- Statics,Lever and Equibrium

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Class 5 Statics, Levers, andBalance


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2/15/2007 Class 5 2

Statics

The study of rigid bodies that are inequilibrium

Equilibrium = a state of rest or balance dueto the equal action of opposing forces dictionary.com

Why study things that dont move in a classabout how things move?

A static problem is a snapshot of a dynamic one Most of the time we can isolate a statics problem

that represents the worst case scenario in adynamic system


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2/15/2007 Class 5 3

Force as a vector

Whats a vector? Scalar: speed, mass, length, time

Vector: velocity, weight

When describing a force, think of Point of application (simplify)

Magnitude

Direction

Gravity is a force F=ma

Difference between mass and weight Quick Trigonometry lesson

SOHCAHTOA


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2/15/2007 Class 5 4

Free Body Diagram

A FBD is a simplified graphical depiction of all the

forces and moments happening on a body Applied, normal, friction, weight, gravity, spring, etc.

Examples: Book on a table (Newtons 3rd law)

Hanging from two ropes

Weight on a high crane

3 steps1. Isolate the body from all supports

2. Identify all external forces acting on the body

3. Sketch the body, showing all forces


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2/15/2007 Class 5 5

Torque, Moment

We already learned that Torque(moment)=Force*distance

Distance (d) stands for the PERPENDICULAR

distance between the force line of actionand the point of rotation, sometimes calledthe lever arm or moment arm

For something to be in Static Equilibrium,the sum of forces in any direction and thesum of moments about any point must = 0


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2/15/2007 Class 5 6

Solving a staticsproblem

Draw a FBD Choose a reference frame (x

horizontal, y vertical)

Choose a convenient point to calculatemoments around

Sum forces and moments in alldirections until unknowns are found


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2/15/2007 Class 5 7

Levers

Last week we learned about how gear ratiosgive you a mechanical advantage

Mechanical advantage, or the difference

between the work you get out and the workyou put in, can also be achieved with levers

A lever is a rigid object used with a pivot

point or fulcrum to multiply the mechanicalforce on an object

Three classes of levers


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2/15/2007 Class 5 8

1st Class Levers

Pivot is betweeninput and outputforces

Ex: seesaw,crowbar,shoehorn, oars,

can opener Double lever:pliers, scissors


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2/15/2007 Class 5 9

2nd Class Levers

The output islocated betweenthe input and

the fulcrum Ex:

wheelbarrow,

stapler


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2/15/2007 Class 5 10

3rd Class Lever

The input force ishigher than theoutput force

Input force moves

over shorterdistance thanoutput load

Ex: humanmandible, bicep,tweezers


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2/15/2007 Class 5 11

Ex: Biomechanics

Sum the momentsabout the elbowjoint

link


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Anecdotal evidence is the enemyof good design

- modified JP quote

Assignment:Find something in real life that experiences forces when used in

context (i.e. wine bottle opener, your toothbrush, a hanging streetlight, etc.) and draw its FBD. Try to guess/measure numbers thatmake sense to solve for an unknown force or moment in yoursystem (using your thumb as the lever point, how hard does thebrush press against your teeth? What is the moment at the base

of the street light post?). Post your sketch on your student pagefor discussion in class. Note: I realize this is a first and dontexpect perfect answers. The object is to get you thinking aboutthe things around you in terms of forces, moments, angles, etc.and be able to simplify them so you can learn how to apply thispractical skill to your art.

class5- statics,lever and equibrium

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