Parallel forces in equilibrium require unequal magnitudes for an odd number

Steps:

• Equilibrium demands zero net force (translational) and zero net torque (rotational).
• For parallel forces, net force is the algebraic sum considering directions.
• Three equal-magnitude forces cannot sum to zero, as an odd number prevents equal opposition (e.g., two in one direction, one in opposite requires different magnitudes).
• Different magnitudes allow sum to zero; positions ensure torque balance if not collinear.

Why D is correct:

• Newton's first law requires net force = 0; equal magnitudes for three parallel forces violate this, as vectors cannot cancel evenly (odd count).

Why the others are wrong:

• A: Collinear action is possible with unequal magnitudes but not required; non-collinear setups work if torques balance.
• B: Equal magnitudes prevent net force = 0 for three forces, making equilibrium impossible.
• C: Non-collinear action is unnecessary; collinear forces equilibrate if magnitudes sum to zero.

Final answer: D