Forces and motion: Newton's three laws
The three sentences from 1687 that still run every mechanical calculation, explained with everyday examples.
What a force is
A force is a push or a pull. Gravity pulls you down; the chair pushes you up; friction grips your shoes. Forces are measured in newtons (N) — one newton is roughly the weight of an apple. Engineers care about forces because everything a machine does, and every way a part can break, starts with a force.
The three laws
First law: things keep doing what they are doing unless a force intervenes. A hockey puck glides until friction and the boards stop it. This is inertia — mass resisting change.
Second law: F = m·a. Force equals mass times acceleration. Double the force on the same shopping cart and it accelerates twice as hard; double the load in the cart and the same push accelerates it half as much. This one equation is the engine room of dynamics — it lets engineers compute the forces a machine part must survive.
Third law: forces come in pairs. When a rocket pushes exhaust down, the exhaust pushes the rocket up. When you lean on a wall, the wall pushes back on you exactly as hard. In a laminate, when a stiff ply drags its neighbor along, the neighbor drags back — that is how load actually gets shared inside a composite.
From forces to stress
A 1000 N force is nothing to a bridge cable and everything to a sewing thread. What matters to the material is force divided by the area carrying it — called stress. That single idea, stress = force / area, is the doorway from physics class into real engineering, and it is where the next article picks up.
Newton's second law animated: identical force, three times the mass, one third the acceleration.
1. With the same push (force), you double the mass in a shopping cart. What happens to the acceleration?
2. Newton's third law says forces…
3. Why is 1000 N nothing to a bridge cable but fatal to a sewing thread?