Momentum - Mechanics and Modern Physics Dr. Bernd Stelzer...

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Dr. Bernd Stelzer Mechanics and Modern Physics PHYS 120 SFU Fall 2010 Dr. Bernd Stelzer
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Announcements October 24, 2010 Bernd Stelzer PHYS120 2 Written Assignment 3 (PS3) is due on Monday, October 25 th , 9:30 AM Marks for Midterm1 are posted on WebCT Midterm 2, Friday, Nov 12 th , 2010
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Survey Compared to other exams I had this semester at SFU, I found the PHYS120 midterm 1 exam to be: A. Far too difficult B. A bit on the difficult side C. Just right D. A bit on the easy side E. Far too easy 10/24/10 Bernd Stelzer PHYS120 3
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Survey Out of 18points total, I expect to have scored in the range: A. 0 - 5 points B. 6 – 9 points C. 10 – 12 points D. 13 – 15 points E. 16 – 18 points 10/24/10 Bernd Stelzer PHYS120 4
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Midterm 1 - Results 10/24/10 Bernd Stelzer PHYS120 5 Average: 14.5 / 18
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Circular Orbits Could you throw a javelin around the world? October 24, 2010 Bernd Stelzer PHYS120 6 Ignoring air resistance +obstructions? What (tangential) velocity would be needed? F r = mg = m v t 2 r v t = rg v t = rg = 6370 10 3 m 9 . 8 m / s 2 = 7 . 9 10 3 m / s
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Circular Orbits October 24, 2010 Bernd Stelzer PHYS120 7 The space shuttle is “falling” around earth with the gravitational force causing the centripetal acceleration. Astronauts feel “weight less” because they are “satellites” themselves.
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Impulse, Momentum and Conservation Laws 10/24/10 Explosions and collisions obey some surprisingly simple laws Using these laws will make problem solving easier! The idea is to compare the situation before and after an interaction This introduces no new physics , just a new perspective . PHYS120 8 Bernd Stelzer
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Momentum and Impulse 10/24/10 • We can always use Newton’s 2 nd law to predict how an object will change motion (accelerate) under the influence of a net force . • In practice, however, this is not always easy when the net force changes in complicated way. E.g consider a collision between a baseball and a bat: PHYS120 9 Bernd Stelzer
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Example: Baseball 10/24/10 The forces on the bat and the ball during the collision are very complex ! The deformation of the bat and the baseball during the collision increases the time that the two objects are in contact. The harder the objects the shorter the contact time. E.g. the time of contact between a baseball and a bat is shorter than the contact time between a softball and a bat. PHYS120 10 Bernd Stelzer
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Molecular View of a Collision 10/24/10 PHYS120 11 Bernd Stelzer
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Force during Collision 10/24/10 The time dependence of the force during the collision may look like this: From this graph we see that the outcome of a collision depends on both, the magnitude of the force and the contact time ϶ t. The combined effect of these two variables is contained in a single physical quantity, called “impulse”. F PHYS120 12 Bernd Stelzer
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