Lecture 4

Lecture 4 - PHYS 172 Modern Mechanics Fall 2007 CLICKER...

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1 Fall 2007 PHYS 172: Modern Mechanics Lecture 4 – Conservation of Momentum Read 2.7–2.14 CLICKER QUESTION #1 Reading Question (Sections 2.7 – 2.14) (This is a closed-book quiz, no consulting with neighbors, etc.) Which of the following are always conserved? A. The total velocity of a system B. The total momentum of a system C. The total momentum of a system + the total momentum of its surroundings D. The sum of the forces acting on a system E. The total momentum of a non-rotating system Important topics for today: • More on the momentum principle • Impulse • Reciprocity • More than one particle in a system • Conservation of momentum t F p p net i f ! + = ! ! ! KEY EQUATIONS t v r r ave i f ! + = ! ! ! position update formula momentum principle Impulse : For a short time interval, t F ! " ! Impulse So, the change in momentum of an object is equal to the net impulse applied to it. ! ! p = ! F net ! t = Impulse Applying the Momentum Principle (p. 46) 1. Choose a system (some portion of universe). Rest of universe is called surroundings. 2. List objects in surroundings that have significant influence on system; make appropriate diagrams 3. Choose initial and final times 4. Apply the momentum principle to the chosen system: Substitute known values into the terms of the momentum principle. Use position update if needed. 5. Check units and reasonableness of your answer t F p p net i f ! + = ! ! !
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2 A) p x B) p y C) p z D) p x y E) p x , p y z A ball is initially at rest on the ground, and you kick it so it has an initial velocity < 3,7,0> m/s. At this speed air resistance is negligible. Assume the usual coordinate system. Which components of the ball's momentum will change in the next half second? Which graph correctly shows p y for the ball during this 0.5 s? E D C B A The initial momentum of the ball was < 1.5, 3.5, 0 > kg*m/s. The final momentum after 0.5 s of the ball is < 1.5, 1.05, 0 > kg*m/s. Reciprocity : if you push on a spring, the spring pushes back on you (at the same time). These two forces are equal in magnitude, opposite in direction, and act on different objects . This reciprocity property is a feature of all electric and
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This note was uploaded on 04/21/2008 for the course PHYS 172 taught by Professor ? during the Fall '08 term at Purdue.

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Lecture 4 - PHYS 172 Modern Mechanics Fall 2007 CLICKER...

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