4. Newton's Third Law, Gravity & Friction

Newton's Third Law says every action force has an equal and opposite reaction force. A rocket pushes exhaust gas down; the gas pushes the rocket up.

Gravity pulls masses together. Near Earth's surface it accelerates falling objects at about \(9.8\text{ m/s}^2\) if air resistance is ignored. Friction resists motion between surfaces and often turns mechanical energy into heat.

Deep explanation. Newton's Third Law says forces come in pairs: if object A pushes on object B, object B pushes back on object A with equal strength in the opposite direction. These forces do not cancel each other because they act on different objects. A swimmer pushes water backward; the water pushes the swimmer forward.

Gravity and friction are common forces in GED® items. Gravity pulls masses together. Near Earth's surface, ignoring air resistance, falling objects accelerate at about \(9.8\text{ m/s}^2\). Friction resists sliding or rolling motion and often changes mechanical energy into thermal energy.

Careful distinction. The normal force from a table holding up a book and the book's weight can balance on the same object. An action-reaction pair acts on two different objects, such as the book pushing down on the table and the table pushing up on the book.

⚠️ Common misconception: heavier objects fall faster. Without air resistance, objects fall with the same acceleration.

💡 Tip: action-reaction forces act on different objects.