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ptc-67009261-Visitor
1-Visitor
Oct 16, 2014 9:58 PM UTC
Solved

Problem with give/find

Hi, I am using mathcad 15 and am having an issue when using a given/find statement to solve a system of equations. I've constructed my equations and set them into the statement but keep getting a error that says "this variable is undefined". I traced the error to k5 but I checked and it seems to be defined. Can anyone see what my problem is? Thanks for the help.

Werner_E
Werner_EAnswer
25-Diamond I
Oct 16, 2014 11:31 PM UTC

1) You have a unit mismatch. m.i*g is force (N, or kg*m/s^2) but k.i is kg/(m^2*s^2) and x.i are dimensionless.

So either let the vector x be m^3 or set the unit for k.i to N.

2) The last line in your solve block is nonsense. How can a row vector (1x5 matrix) equal a scalar (sum of the five vector elements of x)? Furthermore you already have five equation for your five variables.

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3 replies

Werner_E
Werner_E25-Diamond IAnswer
25-Diamond I
Oct 16, 2014 11:31 PM UTC

1) You have a unit mismatch. m.i*g is force (N, or kg*m/s^2) but k.i is kg/(m^2*s^2) and x.i are dimensionless.

So either let the vector x be m^3 or set the unit for k.i to N.

2) The last line in your solve block is nonsense. How can a row vector (1x5 matrix) equal a scalar (sum of the five vector elements of x)? Furthermore you already have five equation for your five variables.

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ptc-67009261-VisitorAuthor
1-Visitor
Oct 17, 2014 12:38 AM UTC

Thank you, I got it working now.

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MikeArmstrong1-Visitor
1-Visitor
Oct 17, 2014 7:23 AM UTC

When I receive a problem with a solve block I break it down and evaluate both side (if required) to see what the issue is, see attached. As Mathcad's error message do not really tell us much, I find this the easiest way to check if unit compatibility is the issue.

As x is dimensionless, the unit error clearly lies with k.

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