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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 50kg 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. NonConservative 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) +(PEfPEi)
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.
 Spring '11
 UNKNOWN
 Energy, Force, Work

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