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8-Gravel

## Units not Compatible

It's been a while since I've last used Mathcad Prime, and that lack of practice might have caused me to become a bit rusty. Could someone please assist me here

ACCEPTED SOLUTION

Accepted Solutions
25-Diamond I
(To:TR_9781963)

1)

In the expression

"1" is  unit-less, so (d.a/d.b)^4 must be unit,less, too. As d.b is defined as a length (5 cm), d.a must be a length, too.
So your guess value is missing a length unit. You can't choose it to be 5 cm as you would get a division by zero with this start value. I had chosen 4 cm as initial guess.

2) But now we again run into a unit mismatch. We see why when we evaluate the right hand side of your equation with the guess value for d.a. The result could be expressed in units meter time Pascal, but the left hand side (tau) is just of unit Pascal. See the highlighted expression in the following picture.

So you have to check your equation. For example we are missing a length unit in the denominator on the right hand side.

3)

In an attempt to fix this unit mismatch are change the formula so that it reads d.b^3 in the denominator instead of d.b^2.

I have no idea if this makes sense and I guess its awfully wrong to do so. But doing so adds a length unit in the denominator and so "cures" the unit mismatch problem 😉

Now the solve block works, but it's not able to find a solution. Reason is that the right hand side of your equation yields about 8 MPa for d.a=0 and gets larger with increasing d.a. So you can't expect a result of just 56 Pa as you demand.
Its obvious that for values of d.a larger than 5 cm the right hand side of your equation yields just negative results - so again you can achieve +56 Pa.

So obviously my "fix" is not the correct one for the task. Maybe your start value for d.b is too low? Should it really be 0.05 m = 5 cm or shouldn't it

be 5 m ?

Here is the plot of the right hand side of the "fixed" equation. Its clearly seen that the graph starts and stays way over 56 Pa (note that the unit chosen in the plot is MPa!)

2 REPLIES 2
25-Diamond I
(To:TR_9781963)

1)

In the expression

"1" is  unit-less, so (d.a/d.b)^4 must be unit,less, too. As d.b is defined as a length (5 cm), d.a must be a length, too.
So your guess value is missing a length unit. You can't choose it to be 5 cm as you would get a division by zero with this start value. I had chosen 4 cm as initial guess.

2) But now we again run into a unit mismatch. We see why when we evaluate the right hand side of your equation with the guess value for d.a. The result could be expressed in units meter time Pascal, but the left hand side (tau) is just of unit Pascal. See the highlighted expression in the following picture.

So you have to check your equation. For example we are missing a length unit in the denominator on the right hand side.

3)

In an attempt to fix this unit mismatch are change the formula so that it reads d.b^3 in the denominator instead of d.b^2.

I have no idea if this makes sense and I guess its awfully wrong to do so. But doing so adds a length unit in the denominator and so "cures" the unit mismatch problem 😉

Now the solve block works, but it's not able to find a solution. Reason is that the right hand side of your equation yields about 8 MPa for d.a=0 and gets larger with increasing d.a. So you can't expect a result of just 56 Pa as you demand.
Its obvious that for values of d.a larger than 5 cm the right hand side of your equation yields just negative results - so again you can achieve +56 Pa.

So obviously my "fix" is not the correct one for the task. Maybe your start value for d.b is too low? Should it really be 0.05 m = 5 cm or shouldn't it

be 5 m ?

Here is the plot of the right hand side of the "fixed" equation. Its clearly seen that the graph starts and stays way over 56 Pa (note that the unit chosen in the plot is MPa!)

25-Diamond I
(To:Werner_E)

Could it be that you simply used the wrong unit for tau (shear stress?) ?

or

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