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# Units of energy

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## Units of energy

(OP)
Hey, hopefully someone can help me with this. My problem is this, i have calculated the kinetic impact energy of a 272 tonne structure travelling at 0.05m/s to be 341 J.

I am then using conservation of energy to define that strain energy = kinetic energy.  Using a spring rate (k) of 50000 N/mm I should output a deflection of 0.117mm.  However, Mathcad is outputting 3.7mm.

The only reasons I can find for this Mathcad is using 341000 J (341kJ).  My energy is defiantly defined as joules & I have repeated the calculation in a new worksheet; with the same results.

Has anyone else experienced this or give me any explanation?

### RE: Units of energy

(OP)
Solved this, was using millimetres throughout but my hand calcs using J (N/m) instead of J (N/mm).  Always the units..

### RE: Units of energy

I find it useful in problems like this to stick the numbers (without units) into the equation and see what the answer is without MathCAD actually getting the units right for me.  That lets me see where I divide psi by ft and get ft^3 and the answer is off by 144.  I tend to do that about once a day in Excel (please, don't tell me that if I worked in SI that wouldn't happen, just refer back to the OP).

David

### RE: Units of energy

hmmm...  I avoid division by 144 errors by not using Excel at all for doing those types of calculations.

### RE: Units of energy

Me too whenever I can, but none of my clients have MathCad and if I give them some calcs they usually want to be able to change parameters.  Too much work to put it into a program.

David

### RE: Units of energy

#### Quote:

(please, don't tell me that if I worked in SI that wouldn't happen, just refer back to the OP).

The OP wasn't using SI.  The units of J are N.m, not N/m and certainly not N/mm.

But if you use consistent units it doesn't matter whether you use SI or not, or even whether you use Excel.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

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