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)