Young's modulus — Finding the change in the length of a metal bar under stress

[chriscarson]

Homework Statement

Finding the change in lenght

Relevant Equations

A steel bar 6.00 m long and with rectangular cross section of 5.00 cm x 2.50 cm supports a mass of 2000 kg.
How much is the bar stretched ? (the young s modulus of steel is 20 x 10 n\m squared)

A steel bar 6.00 m long and with rectangular cross section of 5.00 cm x 2.50 cm supports a mass of 2000 kg.
How much is the bar stretched ? (the young s modulus of steel is 20 x 10 n\m squared)

 

[berkeman]

Welcome to the PF.

You need to show your attempt at the solution before we can offer any tutorial help.

How is Young's Modulus related to the dimensions of the metal bar and how much it stretches?

 

[chriscarson]

Hi Thanks .

ok I will post a picture when it s done

 

[chriscarson]

so this is how I did it and I m sure it s wrong

 

[Chestermiller]

Homework Statement:: Finding the change in lenght
Homework Equations:: A steel bar 6.00 m long and with rectangular cross section of 5.00 cm x 2.50 cm supports a mass of 2000 kg.
How much is the bar stretched ? (the young s modulus of steel is 20 x 10 n\m squared)

A steel bar 6.00 m long and with rectangular cross section of 5.00 cm x 2.50 cm supports a mass of 2000 kg.
How much is the bar stretched ? (the young s modulus of steel is 20 x 10 n\m squared)

That Young's modulus value you give for steel is not right.

What is the equation for the force in terms of Young's modulus, the length of the bar, the change in length of the bar, and the cross sectional area?

 

[chriscarson]

Yeah that s my problem, the value of the young s modulus . the 20 x 10 have to be to the power of 10 but I can t type that right ?

sorry but I did not understand your question

 

[gneill]

At the top of the post edit window there is a toolbar with various icons. A portion of the toolbar looks as follows:

If you want to enter subscripts or superscipts the select the "..." icon:

If you want to insert special symbols such as Greek letters or other mathematical symbols, then select the $\sqrt{x}$ icon.

Better still, learn to use LaTeX syntax to write your equation!

 

[chriscarson]

Ok good

Thanks

 

[haruspex]

sorry but I did not understand your question

What force, F, is required to produce an elongation ΔL in a bar length L, cross sectional area A and Young's modulus E?

 

[chriscarson]

The force is 2000 kg

 

[haruspex]

The force is 2000 kg

That's not what I asked. Put aside all the information in post #1 and answer my question in post #10. There is a standard equation (or combination of equations) that relates those five variables. It/they should be in your course notes. If you can't find them there, g**gle.

(Anyway, the force in the question is not 2000kg. 2000kg is a mass, not a force.)

 

[chriscarson]

I think you mean post #9, but anyway, the equation that I know is

Delta L = force over area and original length over young modulus

Hope you understand my writing

 

[chriscarson]

I think you mean post #9, but anyway, the equation that I know is

Delta L = force over area and original length over young modulus

Hope you understand my writing

My problem is how to work out the original length over the young modulus.

 

[haruspex]

I think you mean post #9, but anyway, the equation that I know is

Delta L = force over area and original length over young modulus

Hope you understand my writing

Closer, but written that way it is highly ambiguous. Please write it as an algebraic expression, using the variables I defined, and with proper use of parentheses.

 

[chriscarson]

It s that formula there , could be that it s called also an equation .

 

[haruspex]

View attachment 254949

It s that formula there , could be that it s called also an equation .

Yes, that's the expression. Sorry, I hadn't looked at your post #4 before, I was only explaining the question in post #5.
Most of your work looks ok to me, and the value for Young's modulus of steel is near enough, but you seem have lost count of the leading zeroes somewhere. Instead of writing those long strings of zeroes use exponents, 10ⁿ.
And there's no point in keeping so many significant figures. Given your modulus value only has two sig figs you can truncate the weight at 20kN, etc.

 

[chriscarson]

Yes, that's the expression. Sorry, I hadn't looked at your post #4 before, I was only explaining the question in post #5.
Most of your work looks ok to me, and the value for Young's modulus of steel is near enough, but you seem have lost count of the leading zeroes somewhere. Instead of writing those long strings of zeroes use exponents, 10ⁿ.
And there's no point in keeping so many significant figures. Given your modulus value only has two sig figs you can truncate the weight at 20kN, etc.

So the value Y in that equation is my problem , how do you have to put it in the calculator ?

by the way , the value for the young s modulus is given in the question .

 

[haruspex]

So the value Y in that equation is my problem , how do you have to put it in the calculator ?

by the way , the value for the young s modulus is given in the question .

Are you asking how you put powers of 10 into a calculator? That depends on the calculator. They usually have a 'scientific' mode. Otherwise, plug all the values in as just one digit before the decimal point and keep track separately of your powers of 10.
(That's what I did in my head when I checked your arithmetic. I did not need a calculator.)

 

[chriscarson]

ok so that I wrote 2000 kn in the calculator was good , I will work to find where I have a mistake in the answer.or maybe you will work it also for yourself.

thanks for now you was helpful .

 

[haruspex]

so that I wrote 2000 kn in the calculator was good

Eh? No, 20kN, or 20000N.

 

[chriscarson]

Eh? No, 20kN, or 20000N.

Oh sorry yes 20000 can t do the N

Although the number is 2000 000 000 000 on my paper that I marked for you.