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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 = vf2 Multiply by the mass of the object, m: mvi2 + 2max = mvf2 Divide by 2: 1/ 2 + max = 1/ mv 2 mv 2 i 2 f Note that ma = Fnet :(2nd Law) 1/ 2+F 1/ mv 2 Defines Kinetic Energy! mv x = 2 i net 2 f Kinetic Energy and “Work” 1/ 2+F 1/ mv 2 Defines Kinetic Energy! mv x = 2 i net 2 f KEi + “Work” = KEf Shorthand for “Work Energy Relation” This is not a vector equation, it’s a “Bucket” equation! “Work” KEi KEf Think of Energy as a “fluid” Think of “Work” as the “change in Energy” or the amount of fluid I pour into (or out of) a Bucket Example: 2nd law vs. Energy vi = 2m/s m = 3kg Find the final velocity for a ball moving up at 2m/s when it is 3m below 3.0m its starting point. 2nd Law Let’s see…. find acceleration find time to top find y to top WE,B =Fnet find total y = mg find vf Wish for an easier way!!! vf = ? Example: 2nd law vs. Energy Benefits: Don’t need highest point! (y = 3.0m) Energy vi = 2m/s m = 3kg No acceleration calculation! No vectors! 6J KEi “Work” 90J 3.0m 96J KEf vf = ? Let’s see… Find KEi (1/2mvi2) Find “Work” (Fnet y) Add to Kei to get KEf Find vf from KEf WE,B =Fnet = mg KEi = 1/2mvi2 = 6 Joules Work = mgy = 90 Joules KEf = 96J = 1/2mvf2 vf = 8.0m/s Oh Baby! 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)