Figure 30-9 Balance. A scale is in balance when the forces are equal and in opposite directions. The force of gravity on the unknown mass of fruit is balanced by the known standard masses on the opposite side of the balance. F₁ = F₂ , and hence m₁ a_g = m₂ a_g ; because a_g = a_g , m₁ = m₂ . a_g , acceleration caused by gravity.