The two wires shown in figure (14-E2) are made of the same material which has a breaking stress of 8 × 108 N m-2. The area of cross section of the upper wire is 0.006 cm2 and that of the lower wire is 0.003 cm2. The mass m1 = 10 kg, m2 =20 kg and the hanger is light. (a) Find the maximum load that can be put on the hanger without breaking a wire. Which wire will break first if the load is increased? (b) Repeat the above part if m1 = 10 kg and m2 = 36 kg.
(a) 14 kg, lower
(b) 2 kg, upper
Given, The maximum stress the wire can hold, 
Area of cross section of upper wire, 
Area of cross section of upper wire, 
(a)mass of the block, 
and 
.
Let the weight of the load be 
Now, we analyze the weight on each wire.
The lower wire has a total weight of 
Hence, stress on the lower wire is 
For maximum load, we take in the maximum stress that the wire can withstand.
The upper wire has a total weight of 
Hence, stress on the lower wire is 
For maximum load, we take in the maximum stress that the wire can withstand.
As the lower wire will break when the weight exceeds 14 kg, it will break before the upper wire, which can withstand weights up to 18 kg.
(b)mass of the block, 
and 
.
Hence, the lower wire has a total weight of 
Hence, stress on the lower wire is 
For maximum load, we take in the maximum stress that the wire can withstand.
Which is the same as previous case.
The upper wire has a total weight of 
Hence, stress on the lower wire is 
For maximum load, we take in the maximum stress that the wire can withstand.
Here, the upper wire will break when the weight exceeds 2 kg, it will break before the lower wire, which can withstand weights upto 14 kg.