OPENING QUESTIONS:
Take a look at my sketch on the board of a suspended object.
(The forces exerted on that object may, or may not be balanced in the x and y directions)
Do a Newton's First qualitative analysis on that situation (your FIRST thought is that you can't do that with the information presented... but I say au contraire mes amis...
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LEARNING OBJECTIVES:
- I will be able to use Newton's 1st and 3rd Laws to provide qualitative interpretations of force/object ineractions after today's class.
- I will be able to use Newton's 2nd Law and my knowledge of vectors to provide quantitative analysis of force/object interactions after today's class.
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WORDS O' THE DAY:
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Newtons ("a unit of force" with units of (kg)(m/sec2) (Newtons are vector quantities so the direction in which the force is exerted on an object is crucial)
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Normal Force ("Opposing g Force")
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Weight ("mass multipled by gravity") weight is NOT equal to mass, this takes some getting used to.
- Equilibrium ("Forces Balance!")
CALENDAR: Labs are due today
WORK O' THE DAY:
Now let's review how to do a force diagram:
- Force diagrams ALWAYS use arrows to represent forces
- the direction of the arrow indicates the DIRECTION of the force
- the numeric value associated with the arrow indicates the MAGNITUDE of the force
- It is often times beneficial to include simple shapes when constructing a force diagram.
- Always refer to one of my favorite engineering axioms:KEEP IT SIMPLE, STUPID! <K.I.S.S.> Which is to say, use simple shapes (squares, rectangles, stick figures etc...) to represent objects.
Let's do some practice (including some practice with Newton's Laws)
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HOMEWORK (est minimum time: 45 minutes - 1 hr):
- Review & finish problem 5.4
- (Newbies) Work through conceptual problem # 6 on page 137 -- be prepared to talk about that tomorrow.
(Sensei): Consider the following paradox:
A 400 kg horse exerts a 2000 N of force on a 100 kg cart. By Newton's 3rd Law, the cart pulls back with 2000 N of force.
Apparently those two forces are equal and opposite, and by Newton's 1st Law there shouldn't be any acceleration at all.
Clearly there is a problem here... be prepared to present your interpretation to the class on Friday or Monday. PLEASE DO NOT LOOK FOR A SOLUTION ON LINE (really!). I want you to exercise your qualitative skills here!
3. Step through solved problem 5.6
- Write down the initial conditions as presented (try not to look at the picture yet)
- Close the book
- Sketch and label a force diagram to describe the situation
- Do a qualitative anlysis using Newton's Laws
- Pay close attention to the normal force and force of gravity
- Solve the problem
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When you get stumped:
- Find out where you get stumped
- Figure out WHY you got stumped
- Work through the problem AGAIN witht book closed
Time management... I am fully aware that we had a monster lab due today. I'm also very aware that we had a homework set due today.
Please note again, that VERY AGGRESSIVE pace in this class. If you wait until the night before to do your homework then you run the risk of something unexpected comming along and then you are TOAST.
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Now let's do some qualitative work:
1) Nate, c'mon up
2) Sensei's huddle up, I want an OUTSTANDING presentation of the horse & buggy paradox
3) Newbies huddle up, I want an OUTSTANDING explanation of the weights/ropes problem I gave you.
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I want to thank the folks that came in on FRIDAY. Their questions helped prompt me to come up with a different way of analyzing hanging object problems.
Let's take a gander
