Question

The two wires A and B shown in the figure are made of the same material which has breaking stress of $8\times {10}^{8}N{m}^{-2}$. The area of cross section of the upper wire A is $0.008c{m}^2$ and that of the lower wire B is $0.004c{m}^2$. The mass $m_1=40kg$ and mass $m_2=20kg$ and the hanger is light. The maximum load that can be put on the hanger without breaking any of the wires is
a. Find the maximum load (in kg) that can be put on the hanger without breaking a wire.

• A. $6$ kg
• B. $4$ kg
• C. $8$ kg
• D. $20$ kg

Answer 1

The correct option is B $4$ kg

Breaking load of wire A will be: Breaking stress times area = $8\times {10}^8\times 0.008\times {10}^{-4}=640N=64kg$, approximately
Breaking load of wire B: $8\times {10}^8\times 0.004\times {10}^{-4}=320N=32kg$, approximately.
Now, load already present on upper wire $=40+20=60kg$. Thus it can take only $4kg$ more.
Load already present on lower $=20kg$. Thus it can take $12kg$ more. But if more than $4kg$ is applied, wire A will break.Thus maximum load is $4kg$