A girl stands on a sofa. identify the number of the action/reaction pairs of forces between the girl and the sofa.

Refer to the diagram shown below.

The girl exerts her weight on the sofa with her two legs so that her total weight, W, is

W = W₁ + W₂

R₁ and R₂ are normal reaction forces exerted on the girl by the sofa. R₁ reacts to W₁, and R₂ reacts to W₂.

Although forces W₁ and R₁ are shown as non-colinear in the left diagram, they are actually colinear, as shown in the diagram on the right.

Likewise, W₂ and R₂ are colinear.

Furthermore, Newton's 3rd Law of Motion (to every action, there is an equal and opposite reaction) requires that

W₁ = R₁ and W₂ = R₂.