Physics: How Force Over Time Determines Impulse

Newton's Third Law describes how forces always arise in pairs: when one object pushes another, the second pushes back with equal magnitude and opposite direction, at every instant. These action–reaction forces act on different objects simultaneously, which is why a hand pressing a wall feels resistance rather than passing straight through it. When a force changes over time, its effect is captured by impulse — the area under a force–time graph, J = FΔt for constant force, or the area beneath the curve when force varies. Dividing total impulse by total time gives the average force, F̄ = J/Δt, a single steady value representing the same overall push. This matters because real forces rarely stay constant: a triangular profile peaking briefly delivers far less impulse than its peak suggests, and Newton's Third Law ensures the wall pushes back with that same average force.