1. Class Notes 02 November Pick up HW Today: Turn in HW
All: Can’t use W = Fdcos for a spring!
#2: Nearly identical to class problem
#5: State Work = Area under the F-vs-x
Turn in HW
Test due tomorrow (graph & derivation)
Today:
Conservative and Nonconserv. Forces
General Work-Energy Principle

2. Bluto spots a 20. 0-kg, 4. 00-m-long ladder lying on the ground
Bluto spots a 20.0-kg, 4.00-m-long ladder lying on the ground. He stands it upright (). How much work did he do?
Rank
Responses 1
784 2 3
111 4 5 6
Other
Correct answer is 392 Joules. Must use half the height, identifying where the vertical height the “center of mass” moves.
Values: 0.053, {
Value Matches: 0

3. 22 Spyders at Fontana Dam!

4. Conservative vs. Non-conservative Forces
A force acting on a point object is conservative provided the total work done on the object as it is moved around any closed path is zero.
Non conservative forces dissipate mechanical energy.

5. Conservative Forces A conservative force is path independent.
Non conservative forces will dissipate energy.
Examples:
Gravity
Spring
Friction
Cons. PEg = mgh
Cons.  PEsp = ½ kx2

6. General Work-Energy Theorem
What if both conservative forces and non-conservative force act?
Conservative  Potential Energy
Non-conservative  Work done
Compare work done by the force and work done on the object
Next ask the question: What if there were no non-conservative forces acting? From above derive E1 = E2.
WNC = DKE + DPE

7. Conservation of Mechanical Energy
E1 = E2
…when no work is done by nonconservative forces

8. Conservation of Mechanical Energy
If no non-conservative forces act on an object or system then mechanical energy is constant, that is, conserved.
Or, if the net work done by non- conservative forces is not zero…
Another word for non-conservative is “dissipative”.
E1 = E2
E1 + WNC = E2

9. Example
A small toy uses a spring to launch an object. When compressed 5.00 mm and fired, the 11.1-g Dexter rises 25.0 cm in the air. Determine the spring constant of the toy.
This is the Dexter/Katheryn(?) toy. Values measured by me.
k = N/m

10. Example
A 1.00-kg ball is thrown from a m-tall platform with a speed of 15.0 m/s at an angle of 40.0°. What is the speed of the ball just before it hits the ground?

11. Homework Read chapter 6, sec. 8-10 Test rework due tomorrow
No HW due Monday!!!
But I may post worksheet 28
Use conservation of mechanical energy and modified (E1 + WNC = E2)
Go to volleyball match!