10/3 Energy Intro

1
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!

2
Energy Introduction
The equation I never let you use
vi2 2a?x vf2
Multiply by the mass of the object, m
mvi2 2ma?x mvf2
Divide by 2
1/2mvi2 ma?x 1/2mvf2
Note that ma Fnet (2nd Law)
1/2mvi2 Fnet ?x 1/2mvf2 Defines Kinetic
Energy!
3
Kinetic Energy and Work
1/2mvi2 Fnet ?x 1/2mvf2 Defines Kinetic
Energy!
KEi Work KEf Shorthand for Work Energy
Relation
This is not a vector equation, its 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
4
Example 2nd law vs. Energy
vi 2m/s
2nd Law
m 3kg
Lets see. find acceleration
Find 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
3.0m
WE,B
Fnet mg
Wish for an easier way!!!
vf ?
5
Example 2nd law vs. Energy
Benefits Dont need highest point!(?y
3.0m) No acceleration calculation! No vectors!
Energy
vi 2m/s
Lets see Find KEi (1/2mvi2) Find Work (Fnet
?y) Add to Kei to get KEf Find vf from KEf
m 3kg
3.0m
WE,B
Fnet mg
KEi 1/2mvi2 6 Joules
Work mg?y 90 Joules
KEf 96J 1/2mvf2
vf 8.0m/s Oh Baby!
vf ?
6
Energy

• Always draw Buckets
• For Work the net force must be constant in
  Fnet?x
• ?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)