attractive force

Meaning: An attractive force on object A points toward object B, and the force on B points toward A. By Newton's third law the two forces are equal in size and opposite in direction. Gravity between masses is attractive, F = Gm₁m₂/r², with G = 6.67 × 10⁻¹¹ N·m²/kg². The electrostatic force is attractive between opposite charges and repulsive between like charges. Attractions between molecules (dipole, London) are electrostatic in origin.

Equation: F = kq₁q₂/r², where F is the force in newtons (N), k = 8.99 × 10⁹ N·m²/C², q₁ and q₂ are the charges in coulombs (C), and r is the center-to-center separation in meters (m). A negative F means the force is attractive.

On the MCAT: Passages ask what happens to the force when r doubles (it falls to one-fourth) or when one charge or mass doubles (the force doubles). Free-body diagrams require you to point the arrow toward the other object. Orbit questions treat gravity as the centripetal force. Energy questions use U = kq₁q₂/r, which is negative for opposite charges. Work must be done to separate them, and U rises toward zero as r grows.

Common confusion: A negative sign tells you direction (attraction), not a smaller magnitude. Compare strengths by absolute value. Also, the two partners feel equal forces, but a lighter object accelerates more because a = F/m.