Given data:
* The extension of the steel wire is 0.3 mm.
* The length of the wire is 4 m.
* The area of cross section of wire is,

* The young modulus of the steel is,

Solution:
The young modulus of the steel in terms of the force and extension is,

where F is the force acting on the steel wire,, l is the original length of the wire, dl is the extension of the wire, and A is the area,
Substituting the known values,

Thus, the force which produce the extension of 0.3 mm of the steel wire is 31.5 N.