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lec21 - Lecture Lecture 21 P112 Mar 7 2008 Agenda Agenda...

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Unformatted text preview: Lecture Lecture 21 P112 Mar 7, 2008 Agenda Agenda for today Momentum Impulse Inelastic Collisions Momentum Momentum Definition of linear momentum: N2: net external Force acting on a particle equals the time rate of the change of its linear momentum Physical meaning: How long (and in what direction) do you have to push to stop the object Impulse Impulse A constant force is acting on a particle during the time interval Δt. Impulse Definition: Impulse-Momentum Theorem: Problem Problem A tennis ball (m = 0.060 kg) strikes a racket with velocity 30 m/s (→) and rebounds with velocity 40 m/s (←) in the opposite direction. (a) What are the magnitude and direction of the ball's change in momentum? (b) If the ball is in contact with the racket for 6 ms (= 0.006 s), what is the average force on the ball due to the racket during impact? Side view h Top view B A d A ball of mass m moving with speed v rolls down a ramp: in case A it hits the ramp straight on, while in case B it hits at 45 degrees. In both cases the speed is the same. Q1: In each case, what is the magnitude of the final momentum? Direction? Q2: In which case is the magnitude of the change in momentum larger? What is the direction of the change in momentum? Q3: In which case is the accelerating force applied for a longer time? Q4: In which case is the displacement in the direction of the accelerating force greatest (during the acceleration)? Law Law of Conservation of Momentum The rate of change of the total momentum of a system equals the net external force acting on the system If the net external force acting on a system is zero then the total momentum of the system is conserved Elastic vs. Inelastic Collisions A collision is said to be elastic when kinetic energy as well as momentum is conserved before and after the collision. Kbefore = Kafter Carts colliding with a spring in between, billiard balls, etc. vi A collision is said to be inelastic when kinetic energy is not conserved before and after the collision, but momentum is conserved. Kbefore ≠ Kafter Car crashes, collisions where objects stick together, etc. When When can you use conservation of Momentum 1. No net external Forces 2. Time interval is short: § Impulse from external forces is negligable Ex: Ball hitting bat Neglect gravity during collision Summary Summary Impulse: Change in momentum Conservation of momentum ...
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