In this lecture you will learn about pressure(PSI), different types of force, and be introduced into a stress strain diagram.

 A force is some measurement of the amount of energy it takes to do something. ◦ Other ways to measure force are Newtons, kilograms, grams, stones…  An area is a measurement of a 2D space. ◦ Other ways to measure area are square Meters, square inches, square feet, square yards, square mile…

 Pounds per square inch  It measures pressure ◦ Pressure is force and area other measures of pressure. PSF, Newton per meter square…. Any combination where a force is over an area. Which would you rather get stepped on by?

 Force divided by area  Or lbs/inch^2  If you had a force of 10 pounds and an area of 5 inches square your PSI would be_______?

 If the weight was the same which would hurt more?  Converse = 48” square  High heel = 18” square  Just the heel =.75” square Assuming the force is the same (200 lbs) 200lbs/48” square = 4.16 psi 200lbs/18” square = 11.1 psi 200lbs/.75 square = psi

 Can you think of one example where varying the shape of something changes the pressure and how it is used.  You have 2 minutes to come up with an interesting example and share with your table.  I will call on each group to speak at the conclusion of that time.

 PSF is used in the building industry ◦ A classroom is required to withstand 40 PSF ◦ A hallway is required to withstand 100 PSF ◦ An attic is required to withstand 10 PSF  Pascal is used in the metric system and is a Newton per meter square

 Engineers and materials scientists need to know how much pressure a given type of material can take.  To do this they break a material sample.  The same sample can withstand different amounts of force depending on the type of force. Stress strain machine

 Group up at your tables and come up with a tool or process that uses one of the types of force I introduced and how it uses that force.  Be ready to share out the answer… anyone could be it….

 When materials have a force applied they change shape ◦ Sometimes it is a permanent shape change  If they permanently change shape without cracking it is called plastic deformation ◦ If the material springs back it is called elastic deformation

 Which type of steel would be best for making a sword and why?

 Engineers are very concerned about the straight potion of the stress strain curve.  The equation below show how to calculate this.  E is the Young's modulus (modulus of elasticity)  F is the force exerted on an object under tension;  A 0 is the original cross-sectional area through which the force is applied;  ΔL is the amount by which the length of the object changes;  L 0 is the original length of the object.

 Square materials E = W L^3 / 4 Δ b h^3 ◦ W=weight (or force) ◦ L= length of beam from support ◦ B= base ◦ H =height ◦ Δ = Amount of deflection  Round materials E = 4 W L^3 / 3 π Δ d^4 ◦ D= diameter of round material To determine young’s modulus from a deflection test you need these equations

1. Why do engineers building a bridge not pay much attention to how a material acts after the yield point? 2. What kind of force does rope withstand the best? 3. What kind of force do you use to break a paperclip with your hands?