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Homework Statement:
 Find the total elongation of the bar, if the bar is subjected to axial forces as shown in the figure. The crosssectional areas is A and modulus of elasticity is E.
Relevant Equations:

##Y = \frac{F/A}{\Delta L/L}##
Newton's Third Law
1. Drawing Free Body Diagrams for all components we get :
2. Following this we can find total elongation using ##\Delta L = \frac {1}{AY}(F_1*L_1 + F_2 *L_2+ F_3 *L_3) ##
My questions :
a) I am assuming that the internal forces (3t) are neglected in the FBD because of Newton's third law whereby the force and the opposing force would cancel one another.(?)
b) But if this is the case then why is the 1t force considered on BC?
c) The FBD shows that for a stretching force of 5t an equal and opposite restoring force of 5t occurs (is this correct)?
Thank you!