Relationship between thickness, width and length

[Perodamh]

1.A rectangular steelbar of length subjected to tensile force of 80kN. Calculate the change in length if the width and thickness are (40 & 25)mm respectively. (E = 207 GN/m2).

Homework Equations

E= stress/strain
stress = F/A; F =80kN
strain = dL/L
3. I just want to know what to use for the original length. whether its the width or the thickness. And also the relationship between width, thickness and length if anyone can explain. Thank you.[/B]

 

[mjc123]

Neither. The original length appears to have been omitted from your question (by you or someone else?). Without that, you can't calculate the change in length. you can only calculate the strain (dL/L).
There is no "relationship" between width, thickness and length. You can make a steel bar of any L, w and t you like. But you need w and t to calculate the stress from the force.
80 kN is not the stress, it is the force. How do you work out A in the equation stress = F/A?

 

[Perodamh]

Neither. The original length appears to have been omitted from your question (by you or someone else?). Without that, you can't calculate the change in length. you can only calculate the strain (dL/L).
There is no "relationship" between width, thickness and length. You can make a steel bar of any L, w and t you like. But you need w and t to calculate the stress from the force.
80 kN is not the stress, it is the force. How do you work out A in the equation stress = F/A?

The Area is the width * thickness?
I've crosschecked and that's how the question is.

 

[mjc123]

Then the question as it stands is impossible to answer. You can calculate the relative change in length (i.e. the strain).

 

[Perodamh]

o

Then the question as it stands is impossible to answer. You can calculate the relative change in length (i.e. the strain).

my bad, the length is 2.5, just asked the lecturer again, for the area does that mean the length * width or still width * thickness

 

[mjc123]

The relevant area is the cross-sectional area perpendicular to the direction of the force. I assume the force is in the direction of the length, though it doesn't explicitly say so.

 

[Perodamh]

The relevant area is the cross-sectional area perpendicular to the direction of the force. I assume the force is in the direction of the length, though it doesn't explicitly say so.

Okay, thanks