The breaking stress for a metal is 7.8×109Nm−2 . The density of the metal is 7800kgm−3. If g=10Nkg−1, Find the maximum length of the wire (in km) made of this metal which may be suspended without breaking.

If L is the maximum length of the wire that can be suspended.Then, F=mg, where m=ρLA (where A- cross-sectional area)∴ Maximum stress =mgA=g⋅ρLAA=ρLg Given, breaking stress ρb=7.8×109Nm−2∴ ρLg=7.8×109⇒L=7.8×109×17800×10 or  L=105m=100km