Summary12 - Summary Lecture 12 Energy Work Force Energy is...

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Unformatted text preview: Summary Lecture 12 Energy Work Force Energy is the capability to do Work or To do some work you need to transform some energy. Definition of Work Work = Force x Displacement (Force component along path!) WF = F s cosθ cos Total work on object W = FNet s cosθ Energy and Work are measured in 1 Nm = 1 J [Joule] Work – Energy Theorem Total work done by the net force is equal to the change in kinetic energy to the change in kinetic energy. W = ΔKE Raising a Crate Bob raises a 50-kg crate by 2.0 m while raises 50 crate by 2.0 while increasing the crates velocity from 1.0 m/s to 3.0 m/s. How much work is done on the crate? How much work is done on the crate? W = Fnet h cosΘ (Fnet = FBob – mg) Work Energy Theorem h=2m W = ΔKE = ½ m vf2 – ½ mvi2 = 100 J FBob mg How much work does Bob do? FBob (Tension is non conservative) W = WC + WNC = Wgravity + WBob = -mgh + FBobh WBob = W + mgh = ΔKE + mgh = 1080 J In Physics 111 we study Mechanical Energies Kinetic Energy Kinetic Energy KE = m/2 v2 m/2 Potential Energy (Gravity) (Spring) ΔPE = mgh PE = k/2 x2 k/2 So for the crate example we can write WBob = ΔKE + mgh WBob = ΔKE + ΔPE Conservative Forces (e.g. Gravity) The work done moving an object is independent of path between the initial and final positions. No net work is done on an object moving around a closed path. Non-Conservative Forces (e.g. Friction, Push, Pull, the “Bob force” (Tension)…) The work done moving an objects depends on the path taken between the initial and final positions. Energy Conservation Replacing WBob with the more general WNC we find WNC = ΔKE + ΔPE In an isolated system, without friction, an isolated system, without friction, air resistance etc (i.e. WNC = 0) 0 = ΔKE + ΔPE 0 = (KEf – KEi) +(PEf-PEi) Or (KEi + PEi) = (KEf +PEf) Ei = Ef Energy is conserved is conserved ...
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This note was uploaded on 12/06/2011 for the course PHYSICS 111&112 taught by Professor Unknown during the Spring '11 term at Ohio State.

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Summary12 - Summary Lecture 12 Energy Work Force Energy is...

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