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
