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Re: PSE Example 8.6 via Reduce

• To: mathgroup at smc.vnet.net
• Subject: [mg112881] Re: PSE Example 8.6 via Reduce
• From: Daniel Lichtblau <danl at wolfram.com>
• Date: Tue, 5 Oct 2010 05:31:34 -0400 (EDT)

Eduardo Cavazos wrote:
> On Sep 26, 1:44 am, Eduardo  Cavazos <wayo.cava... at gmail.com> wrote:
> 
>> Here's example 8.6 from PSE [1]:
>>
>>    http://i.imgur.com/HJLqs.png
>>
>> Here I'll be working with the "exercise" portion of the example; i.e.
>> the case where there is friction between point A and point B.
>>
>> Use Reduce on the set of equations which describe the system between
>> point A and point B:
>>
>>    http://i.imgur.com/PLWza.png
>>
>> The result shows that vB is about 12.9.
>>
>> Now use Reduce on the set of equations which describe the system
>> between point B and point C. Also, plug in our value of 'vB' manually:
>>
>>    http://i.imgur.com/rujUz.png
>>
>> We get the correct answer for dBC; i.e. about 40.3.
>>
>> Above I solved the problem in "piecemeal" fashion; first I used Reduce
>> on the system from A to B and used the result from that to work with
>> the system from B to C, also using Reduce.
>>
>> I don't have alot of experience with Reduce, but it seems like, in
>> theory, I should be able to throw the whole set of equations at Reduce
>> in one step, instead of in two steps. However, when I try to do so,
>> Mathematica seems to have trouble with it (at least on this system;
>> Pentium M with about 1.5GB RAM). So something along the lines of:
>>
>>    http://i.imgur.com/2swfE.png
>>
>> So my questions are... Is this too much for Reduce to handle? Would
>> you recommend the piecemeal method instead?
> 
> In particular, the trouble is that the computation ran over 3 hours
> without completing. When I split up the problem, each piece takes a
> few seconds.
> 
> Ed

I do not know if you will get serious responses if you provide the code 
you used. But I rather suspect you will not get such responses in the 
absence of said code.

Daniel Lichtblau
Wolfram Research