Compute F1 and F2  
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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

Substitute the numeric values and compute F1...

Substitute this value for F1 into the equilibrium equation to find F2.