Lecture 12 - PHYSICS 220 Lecture 12 Momentum and Impulse...

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PHYSICS 220 Lecture 12 Momentum and Impulse Lecture 11 Purdue University, Physics 220 1

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Three conservation laws discussed in Physics 220 1. Energy 2. Momentum 3. Angular momentum These three quantities (there are some others) are strictly conserved in our universe, insofar as experimental verification allows Lecture 9 Purdue University, Physics 220 2
Momentum definition Lecture 11 Purdue University, Physics 220 3 p = m v • The magnitude of the momentum is linearly proportional to both mass and energy • Momentum is a vector quantity, so direction is extremely important and must always be considered

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Why do we need another conserved quantity? Lecture 11 Purdue University, Physics 220 4 Coach Hope conducts an off-season football camp. On a running play a 100 kg fullback runs with a velocity of 10 m/s and runs directly into a defensive back, who weighs 60 kg. The defensive back is a real beast in the weight room and even though he is perfectly still at the time of the collision the running back recoils off of the defensive back with a velocity of 10 m/s as a result of the collision. The collision doesn’t even budge the defensive back. The assistant coach took physics in high school and even though his intuition is that this couldn’t have possibly happened he remembers that energy must be conserved and he quickly calculates that there were 10 4 J both before and after the collision. So energy has been conserved. Even though this outcome seems unlikely the assistant coach reasons that there is no physical reason it can’t occur!
Momentum definition Lecture 11 Purdue University, Physics 220 5 p = m v • Evidently, a description of phenomena in our universe is incomplete if energy is our only conserved quantity • What is clear from our example is that direction plays an important role in the description of interactions

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How is momentum related to force? Lecture 11 Purdue University, Physics 220 6 p = m v d p dt = d dx m v ( ) = m d v dx d p dt = m a d p dt = F • A change is momentum indicates the presence of a force, or • A net force applied to an object produces a change in momentum
Newton’s third law - example Lecture 11 Purdue University, Physics 220 7 Fred and Jane are on skates facing each other. Jane then pushes Fred with force F.

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This note was uploaded on 05/03/2011 for the course PHYS 220 taught by Professor Chang during the Spring '09 term at Purdue University-West Lafayette.

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Lecture 12 - PHYSICS 220 Lecture 12 Momentum and Impulse...

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