How to calculate elastic modulus

[SteamKing]

So its got to be 184.375 (Knm) / 120 Mpa = 1.5365

What are the units of 1.5365? These are just as important as the number.

 

[John54321]

Kn/m^2

 

[SteamKing]

Kn/m^2

Don't guess. Write out the units for the numerator and the denominator and cancel as required.

Remember that a pascal is a derived unit. Be sure to write it in terms of basic units.

 

[John54321]

Correct that was a guess. But to be honest I'm not sure how the units would work out.

 

[SteamKing]

Correct that was a guess. But to be honest I'm not sure how the units would work out.

That's why I asked you to write them out, just like you were making a calculation with numbers, only using units in place of the numbers.

This is how you train yourself to figure out if the units of the answers to your calculation make sense.

 

[John54321]

I don't know that is why I guessed ? I'm not sure what the units would be this is why I'm using this forum for help.

 

[SteamKing]

I don't know that is why I guessed ? I'm not sure what the units would be this is why I'm using this forum for help.

The Rules in the HW Forums at PF allow helpers to guide those with questions. We are not supposed to do your work for you.
You must do some work in order to learn the material.

I asked you to write out the units in the calculation quoted below:

So its got to be 184.375 (Knm) / 120 Mpa = 1.5365

You have kN-m / MPa. I also told you that pascals (Pa) are derived units. Do you know the definition of a pascal?

 

[John54321]

I understand that this is why I have showed all of my workings for the first 4 parts of the question.

Mpa is a unit measure of pressure.

 

[SteamKing]

I understand that this is why I have showed all of my workings for the first 4 parts of the question.

Mpa is a unit measure of pressure.

Yes, that's what it measures, but the pascal is a derived unit, which means it is made up of a combination of other units. What are those units?

 

[John54321]

Yes internal stress, young modulus and ulltimate tensile strength.

 

[SteamKing]

Yes internal stress, young modulus and ulltimate tensile strength.

No. You keep describing things which Pascals are used to measure.

A pascal is made up of other units, things like meters, kilograms, and whatnot. Which other units are used to make pascals?

Hint: it's OK to google "pascal" if you don't know the answer off the top of your head.

 

[John54321]

It's made up of N for Newton M for metre kg kilogram s second

 

[SteamKing]

It's made up of N for Newton M for metre kg kilogram s second

Yes, but how? What is the particular combination of these units which make up a Pascal but not something else?

Complete, this sentence: 1 pascal = 1 ?

 

[John54321]

Hi SteamKing

1 pascal = 1 m ^-1 kg s^-2

 

[SteamKing]

Hi SteamKing

1 pascal = 1 m ^-1 kg s^-2

That's better.

A pascal is also expressed as a force / unit area. For our purposes, 1 Pa = 1 N / m²

Getting back to your last calculation:

So its got to be 184.375 (Knm) / 120 Mpa = 1.5365

if you divided 10³ N-m by (10⁶ N/m²), what's left over?

 

[John54321]

10^3N-m / 10^6N/^2 = 0.001n-m 0r 0.000001n-m^2 after its sqt

Not sure this is correct not done nothing like this before

 

[SteamKing]

10^3N-m / 10^6N/^2 = 0.001n-m 0r 0.000001n-m^2 after its sqt

Not sure this is correct not done nothing like this before

Sure you have, if you've studied algebra. Except instead of dividing units like N and m², you were working with things like x and y. It's the same principle.

Now, in your calculation above, you are close, but there's a small error. I'm going to switch to Latex and make it easier to see:

$\frac{10^3 N⋅m}{10^6\frac{N}{m^2}}$

Can you reduce the expression above, cancelling out the units which can cancel?

 

[John54321]

Morning SteamKing

Are you talking about seeing it like this 1/1000 n-m^2 ?

If not I am not sure what to do if this isn't correct.
I

 

[SteamKing]

Morning SteamKing

Are you talking about seeing it like this 1/1000 n-m^2 ?

If not I am not sure what to do if this isn't correct.

I

Yes, but you're still not cancelling the units properly.

Remember, if you have a fraction like this: $\frac{a}{\frac{a}{b}}$

you invert the denominator and multiply by the numerator, like so: $\frac{a}{\frac{a}{b}} = a ⋅ \frac{b}{a}=b$

Do this with your expression for the units: $\frac{10^3 N⋅m}{10^6\frac{N}{m^2}}$ and see what's left.

 

[John54321]

So N-m being the denominator makes it be multiplied to N-M^2 end up as the numerator.

Sorry I don't get this ? maybe I need to sit down with someone and get this covered as I am running out of time.

Thanks for your help and patience.

 

[SteamKing]

So N-m being the denominator makes it be multiplied to N-M^2 end up as the numerator.

Sorry I don't get this ? maybe I need to sit down with someone and get this covered as I am running out of time.

Thanks for your help and patience.

If you start with $\frac{10^3 N⋅m}{10^6\frac{N}{m^2}}$, then you get $\frac{10^3 N⋅m}{10^6}⋅\frac{m^2}{N}$ . Can you carry it the rest of the way, now?

 

[John54321]

So what your saying is the units just change over ie n-m = m-n

 

[SteamKing]

So what your saying is the units just change over ie n-m = m-n

It's not clear what you mean here.

Newtons multiplied by meters (N-m) is the same as meters multiplied by Newtons (m-N). That's the same as saying a * b = b * a.

And it's also not the point of writing $\frac{10^3 N⋅m}{10^6}⋅\frac{m^2}{N}$

If you simplify the indicated multiplication, you'll see that the Newtons cancel entirely.

Sorry I don't get this ? maybe I need to sit down with someone and get this covered as I am running out of time.

Thanks for your help and patience.

This may be a better use of your time. You have much remedial work in basic arithmetic and algebra which you need to catch up on. I'm not sure that studying beam problems is going to provide this catch up work, and the work will only get harder.

We are constrained by the rules of PF as to how much help we can give posters, and PF is not an efficient way to teach someone the basics of what they need to know.

 

[John54321]

Hi steamking

If it's against Pf forum rules you can email me its vinto2@aol.com thanks