Compute Strain in Copper Bars  
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The figure shows the free body diagram of the aluminum bar sandwiched between copper bars. The free body diagram cuts across the three bars. The forces on the copper bars at the top and bottom are F subscript 1 directed rightwards. The force on the aluminum bar is F subscript 2 directed rightwards.

Temperature change

ΔT = +65°C

Copper bars (1)

A1 = 360 mm2
E1 = 120 GPa
α1 = 16.0 × 10-6/°C

Aluminum bar (2)

A2 = 720 mm2
E2 = 70 GPa
α2 = 23.0 × 10-6/°C

Next, the normal strain in the copper bars will be calculated. A common mistake would be to compute the normal strain using Hooke's Law in this manner:

Why is this calculation not correct?