Starter - Spot the mistakes! 13 March 2012 16:35
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-human-canno...
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-spring-punch
· You know that these situations are wrong, but why are they wrong?! Guided discovery - Investigating Momentum 14 March 2012 07:20 When we collide two gliders on the air track, what happens? Situation 1: Elastic collision with a stationary glider Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = 0m/s [cid:image001.jpg@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 1m/s [cid:image002.jpg@01CD0767.BCDE8CE0] We can represent this graphically as Initial [cid:image003.png@01CD0767.BCDE8CE0] Final [cid:image004.png@01CD0767.BCDE8CE0] Conclusion
· It appears that the speed is "transferred" to the RH glider Situation 2: Inelastic collision with a stationary glider Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = 0m/s [cid:image005.jpg@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0.5m/s Final speed of RH glider = vr = 0.5m/s [cid:image006.jpg@01CD0767.BCDE8CE0] We can represent this graphically as Initial [cid:image003.png@01CD0767.BCDE8CE0] Final [cid:image007.png@01CD0767.BCDE8CE0] Conclusion
· Speed is conserved in the collision
· Total Initial speed = Total Final speed Situation 3: Head on collision Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = -1m/s [cid:image008.png@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s [cid:image009.png@01CD0767.BCDE8CE0] We can represent this graphically as Initial [cid:image010.png@01CD0767.BCDE8CE0] Final [cid:image011.png@01CD0767.BCDE8CE0] Conclusion
· Velocity is conserved in the collision
· Total Initial velocity = Total Final velocity Situation 4: Head on collision with different masses Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = -1m/s [cid:image012.png@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s [cid:image013.png@01CD0767.BCDE8CE0] Problem! Our previous conclusion that
o Velocity is conserved in the collision doesn't hold for this situation! Why do they move off to the left? Because the RH glider has twice the mass What could I change about the LH glider to make both gliders stop after the collision?
o Double the mass (obvious)
o Double the initial velocity We can represent this graphically as Initial [cid:image014.png@01CD0767.BCDE8CE0] Final [cid:image015.png@01CD0767.BCDE8CE0] So something is conserved in the collision, but what is it? What does the area of the rectangles represent?! Time to label our axes! [cid:image016.png@01CD0767.BCDE8CE0] Final Conclusion
· The area of the rectangles are mass x velocity
· Momentum = mass x velocity
· So momentum is conserved in collisions
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-human-canno...
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-spring-punch
· You know that these situations are wrong, but why are they wrong?! Guided discovery - Investigating Momentum 14 March 2012 07:20 When we collide two gliders on the air track, what happens? Situation 1: Elastic collision with a stationary glider Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = 0m/s [cid:image001.jpg@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 1m/s [cid:image002.jpg@01CD0767.BCDE8CE0] We can represent this graphically as Initial [cid:image003.png@01CD0767.BCDE8CE0] Final [cid:image004.png@01CD0767.BCDE8CE0] Conclusion
· It appears that the speed is "transferred" to the RH glider Situation 2: Inelastic collision with a stationary glider Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = 0m/s [cid:image005.jpg@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0.5m/s Final speed of RH glider = vr = 0.5m/s [cid:image006.jpg@01CD0767.BCDE8CE0] We can represent this graphically as Initial [cid:image003.png@01CD0767.BCDE8CE0] Final [cid:image007.png@01CD0767.BCDE8CE0] Conclusion
· Speed is conserved in the collision
· Total Initial speed = Total Final speed Situation 3: Head on collision Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = -1m/s [cid:image008.png@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s [cid:image009.png@01CD0767.BCDE8CE0] We can represent this graphically as Initial [cid:image010.png@01CD0767.BCDE8CE0] Final [cid:image011.png@01CD0767.BCDE8CE0] Conclusion
· Velocity is conserved in the collision
· Total Initial velocity = Total Final velocity Situation 4: Head on collision with different masses Initial Initial speed of LH glider = ul = 1m/s Initial speed of RH glider = ur = -1m/s [cid:image012.png@01CD0767.BCDE8CE0] Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s [cid:image013.png@01CD0767.BCDE8CE0] Problem! Our previous conclusion that
o Velocity is conserved in the collision doesn't hold for this situation! Why do they move off to the left? Because the RH glider has twice the mass What could I change about the LH glider to make both gliders stop after the collision?
o Double the mass (obvious)
o Double the initial velocity We can represent this graphically as Initial [cid:image014.png@01CD0767.BCDE8CE0] Final [cid:image015.png@01CD0767.BCDE8CE0] So something is conserved in the collision, but what is it? What does the area of the rectangles represent?! Time to label our axes! [cid:image016.png@01CD0767.BCDE8CE0] Final Conclusion
· The area of the rectangles are mass x velocity
· Momentum = mass x velocity
· So momentum is conserved in collisions
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