# Axial stress

SenseAO
A steel bar that is 16mm in diameter is resisting a force of 25kN. What is the normal axial stress in the bar(MPa)

Since MPa = N sqmm would it be 25kn/(0.016m x 0.016m)

Im more after the process on how to get the answer than the answer itself.

Staff Emeritus
Homework Helper
Start by looking up the definition of stress.

SenseAO
Yes i have and i get the picture in my head. But i just cant place where the 16mm goes.

Staff Emeritus
Homework Helper
What quantities goes into calculating the stress?

Perhaps you're simply missing the implication of the use of the word diameter.

SenseAO
F/A?

Force being 25kn and the area being 16mmx16mm?

Staff Emeritus
Homework Helper
Good. What precisely does A stand for? It's an area, but the area of what?

SenseAO
The cross sectional area of the beam?

Staff Emeritus
Homework Helper
Right. The cross-sectional area depends on the shape of the cross section. A=0.016m x 0.016m would work if the cross section were square, but is that the case here?

SenseAO
Does not specify. So than we would just do 25kn/0.016m = to give us 1562.5kNm converting this to Nmm would give us 1562 x 10^6 Nmm

am i somewhat right?

Staff Emeritus
Homework Helper
No, that's not correct. For one thing, when you divide, the units divide as well, so you end up with kN/m, not kN m.

What does the 16-mm given correspond to? The problem statement implies what the shape is.

SenseAO
The 16mm corresponds to the bar

Staff Emeritus