Investigating Momentum

·         

http://sites.google.com/site/winfailphysics/all-videos/roadrunner-human-cannonball

·         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 = u_l = 1m/s

Initial speed of RH glider = u_r = 0m/s

 

Final

Final speed of LH glider = v_l = 0m/s

Final speed of RH glider = v_r = 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 = u_l = 1m/s

Initial speed of RH glider = u_r = 0m/s

 

Final

Final speed of LH glider = v_l = 0.5m/s

Final speed of RH glider = v_r = 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 = u_l = 1m/s

Initial speed of RH glider = u_r = -1m/s

 

Final

Final speed of LH glider = v_l = 0m/s

Final speed of RH glider = v_r = 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 = u_l = 1m/s

Initial speed of RH glider = u_r = -1m/s

 

Final

Final speed of LH glider = v_l = 0m/s

Final speed of RH glider = v_r = 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