A cylindrical specimen of material with diameter 1.5x10^-4 has a breaking stress of 1.3GPa. Calculate the tensile force acting on the specimen at breaking point.

We first need to remember that stress is force over area. We can calculate the area using the diameter as the specimen is told to be cylindrical (circular cross-section). The area can be calculated using 0.25 x pi x d^2 where d=1.5x10-4, the answer is 1.767x10^-8. Now we can re-arrange the formula for stress to make force the subject: this gives force equals stress multiplied by area. The stress at breaking is given so the answer is achieved by multiplying 1.767x10^-8 by 1.3 GPa (1.3x10^9) giving roughly 23N.