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# Answer to Question #141264 in Mechanical Engineering for Jisha

Question #141264
In a simple jib crane, the vertical post is 2.4m high, the jib is 3.9m long and the tie is long. Find the forces on the jib and tie rod when a weight of 2.5kN is supported at the crane end.
1
2020-11-03T09:11:34-0500

Here it is given vertical post is 2.4 m high ,length of jib is 3.9 m long and tie is 2.1 m long

weight acting is 2.5 kN

since the line of action of all forces acts on a coincident point

so we can apply lami's theorem in it

and we can say that

"\\frac{W}{sin \\beta}=\\frac{J}{sin \\theta}=\\frac{T}{sin \\alpha}"

from above triangle we can find value of all sines

"sin \\beta=\\frac{2.4}{3.9}=0.615,sin \\theta=1,sin\\alpha=\\frac{2.1}{3.9}"

Now we take one pair at time for finding values

so ,

"\\frac{W}{sin \\beta}=\\frac{J}{sin \\theta}"

"\\frac{2.5}{0.615}=\\frac{J}{1}"

J=4.065 kN (compressive load) this is the force on jib

"\\frac{J}{sin \\theta}=\\frac{T}{sin \\alpha}"

"\\frac{4.065}{1}=\\frac{T}{0.538}"

T=2.18697 kN (Tension) this is the force on tie

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