< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > Problem Statement: Case 1: Mass is m; Cross sectional area is uniform A. What will be the stress in the middle section. Case 2: Bar is massless; What will be the stress in the middle section. Case 3: Bar is still assumed massless but 100F is increased to 500F; What will be the stress in the middle section. Relevant equation: F/A; point is to find internal force in each situation. My attempt to the solution: I found the acceleration simply by dividing "m" to the net force "99F". My poor understanding about stress was; it only acts when body in is zero unbalanced force. Simply we have to find the internal force at point which stress need to be calculated. But I realized even after there is unbalanced force situation we have internal force at particular section. So I solved for case I: a=(99F/m) After solving equation F1-F=(m/2)*a; I found the value of F1 i.e. (101/2)F stress will be F1/A. But solution given was 99F/A i.e. stress due to original force configuration at middle section. For Case 2 and Case 3; I am clueless.