18lecture2a08

18lecture2a08 - Physics 2A Lecture 18: Feb 20 Vivek Sharma...

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Physics 2A Lecture 18: Feb 20 Vivek Sharma UCSD Physics
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Momentum Definition of Forc ( : ) e dv Fm a m dt dm v dt == = G G G G Define new observable : Momentum p mv = G G dp F dt = G G Second Law Reloaded : Rapid change in momentum requires LARGE force but a more gradual change in p requires less force
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Impulse Of A Force If a net force acts over a time interval Impulse t J = t F Δ Δ constant GG 2 1 If a variable force acts over time interval t J = t t Fdt Δ 21 pp F tt F tp p =⇒ Δ= G G J ⇒= G G G ave = F J t = Δ G G G Change in momentum of body during a time Δ t equals the impulse of force that acts on body in that time interval
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Kinetic Energy, Momentum & Impulse A body of mass m, initially at rest, is acted upon by a force over a distance such that its final speed is F d . v G G 21 Impulse gain in momentum J pp F t = =− = Δ G G GG 2 2 Kinetic Energy gained = 2 E. 2 m p m v Fs == G G
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Quarterback Sack: You Pick ! Mike (m=50kg,v=8m/s) & Bubba (m=200kg,v=2m/s) coming to get you! By whom would, , the quarterback, prefer to be sacked y? b you 2 Both have same momentum (p=400kgm/s), will require impulse to be brought to rest (by you!). But Mike has times the kinetic energy of Bubba ! (K= same p /2) 4 m v Δ For a given force that you exert with your body, it takes same amount of time to is pushed back 4 mor stop either guy but e by faster guy ! (w your body ork done) × To avoid the instantaneous strain on body, Let Bubba get you !
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Principle of Conservation Of Momentum Consider two objects interacting with each other isolated 21 1on2 2on1 FF dp dp dt dt =− G G GG 2 2 2 1 11 () 0 cn s 0 ot dp dp d p p dt p dt dt p + += + =⇒ G G G G Sum of momenta of an isolated system of objects is constant, no matter what forces act between the objects making up the system If the vector sum of all external forces on a system is zero, then the total momentum of system is constant
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Quail Hunting With the Veep !
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18lecture2a08 - Physics 2A Lecture 18: Feb 20 Vivek Sharma...

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