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15lecture2a08

15lecture2a08 - Physics 2A Lecture 15 Feb 11 Vivek Sharma...

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Physics 2A Lecture 15: Feb 11 Vivek Sharma UCSD Physics
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Work is done when a force is applied on a body
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Work Done By a Constant Force SI Unit of Work: 1 Joule = (1N) (1m) = 1N.m When a constant force acts in same directi F displacement s, work done by forc on e F s s a W= G G When a constant force acts at angle to the displacem Fs ent work d W one =F.s ϕ G G G G Work Done is a Scalar Quantity
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Three Shades of Work WF . s | F | | s | c o s = ϕ = G G Relative direction between F & s Important G G
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Does Earth Do Work On The Moon ? Moon revolves around earth in nearly circular orbit, kept there by the attractive force exerted by earth. Does Earth do +,0,- work ? Gravitational force F on moon acts towards earth (centripetal force) inward along radius of moon's orbit Moon's displacement s always along a circle & F W=F.s G G G G G earth moon and artificial satellites can stay in orbit without expenditure of f . This is why , since no work is done against g W= ra ue y 0 l vit
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Doing Work (or Not) P No work is done on Groceries since F d G G Work done by hiker & earth on a 15.0 kg backpack carried up a height of 10.0m? yH B H H Draw freebody diag for Backpack & use 2nd law F =0 = F - m g F =147N Work done by hiker on backpa dc ck: W= F .d=F = os θ h h=1470 F=m g J G G G o H E Earth pulls on backpack: F = -mg Object displaced at angle =180 θ W dcos =Fdcos(180- )=-147 Work done by Earth o = n backpack: .= 0 F J Fd α αθ G G G
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Understanding Negative Work rd Artifact of 3 law: e.g. when hand catches ball, H & B have same displacement s G HonB BonH Hand exerts equal & opposite force F = -F in direction oppo. to Hand does . work on ball. negative s G G G () tot Total work done on a body is ALGABRAIC sum of work done by individual forces in displacing it. Which is to s W F ay W. s == G G BonH Ball exerts force F in direction o Ball does positive work on . f hd an . s G G
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Kinetic Energy & Work-Energy Theorem Energy is the ability of a body to do work Å Working definition
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15lecture2a08 - Physics 2A Lecture 15 Feb 11 Vivek Sharma...

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