Friday, October 3, 2014

Q#2 review

From here on out, we will have a quiz every couple of chapters, so each will not cover too much material.

The chap 3-4 quiz will have:

2 2D problems
2 3D problems

Each problem will have

5 points for FBD's
15-25 points solving for M's and F's

What you need to know:

Forces at supports - don't memorize these, just think about what types of forces they can and cannot support!

2D examples:

3D examples:

Divide and Conquer!
Just take it one step at a time...

Steps for solving statics problems:
1. Draw a nice big FBD
2. List knowns
3. List unknowns
4. Figure out the geometry

Find and label Θ's, lengths, etc.

2 sides + one Θ? Use law of Cosines
1 side and 3 Θ's? use Sine law

Find unit vectors, split F's up into Fx + Fy + Fz

5. Apply equilibrium equ's:
Σ M = 0, ΣF = 0

Take your M about the point with the most unknowns to eliminate those unknowns from the M balance.
Solve Σ's with the fewest unknowns first.

6. Check your answer

Do the signs make sense?
Do the magnitudes make sense?

Steps for Drawing a good FBD:

1. Identify the object you want to isolate
2. Draw a sketch of the object isolated from it's surroundings
3. Draw vectors representing all of the external forces acting on the isolated object, and label them.
Understand reactions @ your supports and connections!
4. Choose how to orient your coordinate system.
5. Label all of your angles and distances.

2D Free Body Diagram Practice Problems:

3D Free Body Diagram Practice Problems:

2D Practice Problems:

Start thinking about bridge designs...
What are the most effective ways to support weight?

Which of the below configurations puts the least stress on A?

3D Practice Problems:

Monday, September 22, 2014

Sailboat Lab

Find the tipping point!

Looking down onto the boat:

Measure:

Weight of sail and boat (in grams)
Lengths of boat and sail (in cm)
Lengths to center of mass of boat and sail (where does it balance?)

Set up an excel file to calculate the angle between the sail and the boat that will make the boat tip over.

Example:
BL - boat length
al - axle length
S = Distance to sail
Fsj - force of sail pulling down
fbj = force of boat pulling down
BC = Distance to boat's center of mass
SC = Distance to Sail's center of mass
B° = Angle between boat and tipping axis
db = perpendicular distance between Fbj and tipping axis
ds = perpendicular distance between Fsj and tipping axis
S° = angle between sail and boat

Set up an excel file to calculate everything
(note, the numbers below are made up)