Lect05-f10.ConservationofMomentum.Ch3 (1)

# Lect05-f10.ConservationofMomentum.Ch3 (1) - PHYS 172 Modern...

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Fall 2010 PHYS 172: Modern Mechanics Lecture 5 – Predicting Motion, Conservation of Momentum Read 3.5 – 3.15

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Today •Predicting Motion •System of Many Objects •Conservation of Momentum •Electric Force – Large or Small? •Can we REALLY predict The Future?
3 Predicting motion of a planet Where will the planet be after one month? Use position update formula: If we assume that velocity is constant Does not work because the force is changing the velocity! The direction of force on planet changes with position. The momentum changes with position. In general, there is no algebraic equation to predict the motion of more than 2 interacting objects.

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Iterative Prediction of Motion Simple case: One Star (fixed in space) + One Planet 1. Calculate gravitational force: 3. Calculate v and update position 2. Update momentum Choose Δ t short enough ( F & v do not change much) 4. Repeat Critical parameter: Δ t
5 Iterative Prediction of Motion Constant Force (mg) + Momentum Principle = Projectile Motion (curved path) But, can also add air resistance = non-constant force

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Iterative prediction of motion Real case: many objects objects are free to move 1. Calculate net force on each mass: 3. Calculate v and update position of each mass 2. Update momentum of each mass Choose Δ t short enough ( F & v do not change much) 4. Repeat Δ t is a critical parameter!
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Lect05-f10.ConservationofMomentum.Ch3 (1) - PHYS 172 Modern...

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