Work and energy theorem:

$\overline{){\mathbf{W}}{\mathbf{=}}{\mathbf{-}}{\mathbf{\u2206}}{\mathbf{U}}}$

From the conservation of energy principle:

$\overline{){\mathbf{W}}{\mathbf{=}}{\mathbf{\u2206}}{\mathbf{K}}{\mathbf{+}}{\mathbf{\u2206}}{\mathbf{U}}}$

A system of two objects has Δ*K*_{tot }= 7 J and ΔU_{int} = -3 J.

**Part A**

How much work is done by interaction forces?

**Part B**

How much work is done by external forces?

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