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Re: [Phys-l] Angular momentum problem



Even if you choose a reference frame in which the angular momentum of each block is zero, say, place the origin at the intersection of the "y"-line of the CM of 2 and the velocity vector through the CM of 1, they must rotate. Why? The initial Ang. Mom (AM) is zero, and the final will be zero because of the absence of external torques, like you said. Once both blocks are translating, Block 2 has angular momentum about the origin (remember, its CM will have a moment arm wrt the origin). Consequently, Block1 must rotate to get its CM off-axis so that it, too, can have some AM so that the total will remain zero wrt the chosen origin.

One could also say that the AM of the system-CM about the above origin is NOT zero, so the joined masses must rotate.

The AM must be calculated about a common point for both blocks.

Bill

Michael Meyer <mrmeyer@mtu.edu> 3/24/2009 8:30:37 pm >>>
Peter -

I think you'll see the issue if you ask yourself "the angular momentum AROUND WHAT?"

The angular momentum of the system around the system's center of mass is NOT zero before the collision, because the moving block's CM and the system's CM are not the same point.

Mike Meyer
Lecturer/Lab Coordinator
Mich. Tech Physics Dept.
mrmeyer@mtu.edu



----- Original Message -----
From: pschoch@nac.net
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Tuesday, March 24, 2009 8:13:32 PM GMT -05:00 Columbia
Subject: [Phys-l] Angular momentum problem

Hello all,

I have an especially inquisitive group this semester and they extended an
experiment in a new way which I am having trouble analyzing. I am sure it
has something to do with how I am defining the system, so that deltaL is
NOT equal to 0, but I just don't see it...

They had 2 rectangular blocks that would glide on an air table. Both
blocks have the same mass, length and width (to within our measurement
capability). They took Block 1 and Block 2 and oriented their long axes
with the y axis direction, a distance apart on the air table. They then
gave a light push to Block 1 in the +x direction toward Block 2 which was
still at rest (not easy on an air table, but they did quite well). The 2
blocks hit and stuck together at 1/4 of their length from the end; the
bottom of Block 1 1/4 from the end hit the top of Block 2 1/4 from its
end. They stuck because the students used velcro.

Now, the 2 blocks move off together in the +x direction, but they also
begin to spin/rotate cw.

I can explain the inelastic collision (I'm fairly sure). However, since I
view the 2 blocks as the system, I see no outside force and thus I can't
explain the non-conservation of angular momentum. I'm assuming I'm
defining the system wrong, so that there is a torque, but I don't see it.

Any help/nudge in the correct direction would be appreciated.

Thanks,
Peter Schoch
SCCC's lone Physics Prof.

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l