1 Statement

• By the first law, an object with no resultant force has no acceleration.
• So if a resultant force acts (ΣF ≠ 0) the velocity changes and the object accelerates (a ≠ 0).
• Newton's second law gives the exact relation.

Newton's second law: The resultant force acting on an object is equal to the time rate of change of its momentum.

ΣF = Δp/Δt = (m v_f − m v_i)/Δt

If the mass is constant:

ΣF = m (v_f − v_i)/Δt = m Δv/Δt

and since Δv/Δt is the acceleration,

ΣF = m a or a = ΣF / m