Net force, acceleration and Newton's third law
≈ 48 minNet force, acceleration and Newton's third law
Inertia is resistance to a change in motion and increases with mass. Newton's second law quantifies how a net force changes motion: for a fixed mass, larger net force produces larger acceleration; for a fixed net force, larger mass produces smaller acceleration. Newton's third law explains why contact forces occur in pairs, but it does not replace the second-law analysis of one chosen object. A contact interaction can produce equal forces on unequal masses and still produce different accelerations, because each object's acceleration depends on its own mass.
Work it through
A 5.0 kg crate experiences 20 N right and 5 N left. Fnet = +15 N, so a = Fnet/m = 15/5.0 = 3.0 m s⁻² right. The crate pushes back on the person with 20 N left, but that reaction force acts on the person, not on the crate. It belongs in a different free-body diagram.
Mastery target
Use force direction and system mass to calculate acceleration, and explain why equal third-law forces can produce unequal accelerations.
If net force is fixed and mass increases, acceleration:
Name the key physics term from Net force, acceleration and Newton's third law that best fits the explanation and visual model.
A 5.0 kg crate has net force 15 N right. What is its acceleration in m s⁻²?
Which statement corrects a common misunderstanding in Net force, acceleration and Newton's third law?