lecture9 - Physics 231 Lecture 9 Impulses: forces that last...

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Physics 231 Lecture 9 Impulses: forces that last a short time •M o m e n t u m : •I m pulse-Momentum theorem: () i f m m v v v p t F = Δ = Δ = Δ v p m = Main points of this lect. Momentum conservation: Totally inelastic collisions t o t , f1 , f2 , , i 2 , it o t , i ≡+ = +≡ pp p p p p  ( ) 1 2 2 1 2 vmm m m =+ + vv
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Bicycle challenge rules Contestants must warm up by riding for 20 s at P > 200 W. Then each contestant is judged by the peak or maximum power generated anytime in the next 10 s. Can I have two volunteers? Should the instructor participate? –a ) y e s –b ) n o “Carmichael (Armstrong’s coach) told me that a decent pro cyclist would have put out at (an average power)least four hundred watts (during a 4 hour period), and that the stragglers at the end of the peloton (known as the gruppetto) (which don’t feel as much drag force from air resistance) would clock in at perhaps three hundred and fifty. Armstrong—in top Tour shape—would have come close to five hundred.” (from New Yorker article)
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Impulse: forces that last a very short time There are many processes in which forces last a very short time and are difficult to mathematically describe. Examples are: – Kicking, striking batting, dribbling a ball. – Various types of explosions, firearms, etc. The typical time dependence of such impulse forces is described below: 0.01s
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Momentum The linear momentum of a particle of mass m is: The change in velocity is related to the change in momentum, i.e. impulse: v p m = It is related to the average impulsive force: ( ) i f i f m v v p p p = = Δ ( ) () m t t m t a m t i f i f Δ = = Δ Δ = Δ = Δ p v v v v F m t i f Δ + = F v v
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Conceptual question about momentum Consider two carts, of masses m and 2 m , at rest on an air track. If you push first one cart for 3 s and then the other for the same length of time, exerting equal force on each, the momentum of e light cart is the light cart is – a) four times –b ) tw i c e – c) equal to – d) one-half – e) one-quarter the momentum of the heavy cart.
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This note was uploaded on 05/03/2011 for the course PHYS 231 taught by Professor Nagy during the Fall '11 term at Michigan State University.

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lecture9 - Physics 231 Lecture 9 Impulses: forces that last...

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