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10/3 Energy Intro Text: Chapter 6 Energy HW 10/3 “2nd Law vs. Energy” due Monday 10/7 Tomorrow, Potential Energy and Projectile Motion Exam 2 Thursday, 10/17
5-7 Wit 116 6-8 Wit 114 (only if needed)
Use 5kg and 30N in “Block on a Ramp” but try it with all combinations of numbers!
Energy Introduction
The equation I never let you use:
vi2 + 2ax = vf
2
Multiply by the mass of the object, m:
mvi2 + 2max = mvf
2
Divide by 2:
1/2mvi2 + max = 1/2mvf
2
Note that ma = Fnet :(2nd Law)
1/2mvi2 + Fnet x = 1/2mvf
2 Defines Kinetic Energy!
Kinetic Energy and “Work”
1/2mvi2 + Fnet x = 1/2mvf
2 Defines Kinetic Energy!
KEi + “Work” = KEf Shorthand for “Work Energy Relation”
This is not a vector equation, it’s a “Bucket” equation!
Think of “Work” as the “change in Energy” or the amount of fluid I pour into (or out of) a Bucket
Think of Energy as a “fluid”
“Work”
KEi KEf
Example: 2nd law vs. Energy
2nd Law
3.0m
vi = 2m/s
vf = ?
Wish for an easier way!!!
m = 3kg
WE,B =Fnet = mg
Let’s see….
find accelerationFind the final velocity for a ball moving up at 2m/s when it is 3m below its starting point.
find time to top
find y to top
find total y
find vf
Example: 2nd law vs. Energy
Energy
3.0m
vf = ?
Let’s see…
Find KEi (1/2mvi2)
Find “Work” (Fnet y)
Add to Kei to get KEf
Find vf from KEf
KEi = 1/2mvi2 = 6 Joules
m = 3kg
Work = mgy = 90 JoulesKEf = 96J = 1/2mvf
2
vf = 8.0m/s Oh Baby!
WE,B =Fnet = mg
vi = 2m/s
“Work”
KEi KEf
6J
90J
96J
Benefits:
Don’t need highest point!(y = 3.0m)
No acceleration calculation!
No vectors!
Energy
Always draw “Buckets”
For “Work” the net force must be constant in Fnetx
x is the “Displacement,” as usual
Always consider Energy as an alternative to the 2nd law
Energy is a “Scalar” not a “Vector” (See Ch. 1)
For Friday, think about this example in the case where there is a horizontal component to the motion. (projectile motion)