Re: Need soln explaination

[Chuck Britton <britton@odie.ncssm.edu>]

> I got a problem out of an older physics text (one without a solutions
> manual). My students and spent a while debating the method of attaining
> the solution. Can someone explain how they would solve it -using energy.
>
> PROBLEM
> Masses of 3 kg and 2 kg are hanging at opposite ends of a cord that
> passes over a frictionless pulley. The system is held stationary for a
> while, then released. What will be the speed of the masses when the 3 kg
> mass has descended 0.5 m below its starting point? (USE ENERGY
> RELATIONSHIPS TO SOLVE.)
>
> -tony

This sounds like a staight-forward KE & PE problem. I always teach a
'snapshot' approach to these.

Take snapshot#1 IMMEDIATELY as the system is released. (i.e. no motion yet)
Add up the various energies that are evident in this picture. (Ok, Leigh,
let's PRETEND that we can ascribe a localized mgh energy to an object that
is at a height h above our arbitrary reference level.)

Choose GPE = zero for the lower mass.

Take snapshot#2 when the final situation occurs. We can use the 'speed
lines' that ALWAYS show up on such snapshots OR we can calculate both KE's
by finding the NEW GPE's and ASSUMING that the

	total KE + GPE in #1 = the total KE + GPE in #2.

	(since this is the usual stretchless cord, we know that the two objects
	will always have the same speed and SPEED is what determines KE.)

This snapshot bookkeeping approach seems to work for many students.


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Chuck Britton  					Education is what is left when
britton@odie.ncssm.edu  			you have forgotten everything
North Carolina School of Science & Math		you learned in school.
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