Starter - Spot the mistakes!
· 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
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 Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 1m/s We can represent this graphically as Initial
Final
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 Final Final speed of LH glider = vl = 0.5m/s Final speed of RH glider = vr = 0.5m/s We can represent this graphically as Initial
Final
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 Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s We can represent this graphically as Initial
Final
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 Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s 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
Final
So something is conserved in the collision, but what is it? What does the area of the rectangles represent?! Time to label our axes!
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
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 Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 1m/s We can represent this graphically as Initial
Final
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 Final Final speed of LH glider = vl = 0.5m/s Final speed of RH glider = vr = 0.5m/s We can represent this graphically as Initial
Final
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 Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s We can represent this graphically as Initial
Final
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 Final Final speed of LH glider = vl = 0m/s Final speed of RH glider = vr = 0m/s 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
Final
So something is conserved in the collision, but what is it? What does the area of the rectangles represent?! Time to label our axes!
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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