Saturday, 9 March 2019

homework and exercises - Elastic collision of point particle and rod




A 1 meter long rod on the ice with mass m2=1 kg is perpendicularly hit on one end by a point particle with mass m1=0.1 kg. The collision is elastic and the point particle is bounced back in the same direction. After the collision the rod's frequency is ν=2 Hz. What was the initial velocity of the point particle?



My attempt:


Since the collision is elastic, the kinetic energy of the system is the same before and after the collision: 0.5m1v21=0.5J2ω22+0.5m2v22+0.5m1v23

Where v3 is the velocity of the point particle after the collision.


Now, in the case of a rod: J=112L2m

And, we also know: ω2=2πν
And there are also no external forces, therefor the momentum of the system is the same before and after the collision: m1v1=m1v3+m2v2
Here v1 is the quantity we're looking for, v3 is the point particle's velocity after the collision and v2 is the velocity of the rod's center of mass. It follows: v2=m1v1m1v3m2
From this it follows: 0.5m1v21=124L2m24π2ν2+0.5m2|m1v1m1v3m2|2+m1v23
This is 1 equation with 2 unknowns, and this is where I get stuck. Any help is appreciated.




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