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.
