Types of Force

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…

The beginning…

Pounds per square inch It measures pressure

◦ Pressure is force and areaother measures of pressure.

PSF, Newton per meter square…. Any combination where a force is over an area.

What is PSI?

Which would you rather get stepped on by?

How do you calculate PSI (or other pressure measurements)

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

Here is an example

• Assuming the force is the same• (200 lbs)• 200lbs/48” square = 4.16 psi• 200lbs/18” square = 11.1 psi• 200lbs/.75 square = 266.6 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.

Discussion

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

Other types of measures..

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.

One way pressure is used by engineers?

Stress strain machine

Types of stress

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….

Discuss…

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

Deformation

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

Discuss…

Engineers are very concerned about the straight potion of the stress strain curve.

The equation below show how to calculate this.

Young’s modulus

E is the Young's modulus (modulus of elasticity) F is the force exerted on an object under tension; A0 is the original cross-sectional area through which the force

is applied; ΔL is the amount by which the length of the object changes; L0 is the original length of the object.

Young’s modulus

Square materialsE = 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 materialsE = 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