Newton's three laws

Science

Definition

Newton's three laws of motion describe the relationship between forces and the motion of objects. The first law (inertia) states an object stays at rest or in uniform motion unless acted on by a net force. The second law states that net force equals mass times acceleration. The third law states that every action has an equal and opposite reaction.

Examples

  • A book on a table stays at rest until pushed (1st law); the harder you push, the faster it accelerates (2nd law)
  • A rocket propels forward by expelling exhaust gases backward (3rd law)
  • Wearing a seatbelt protects you because your body tends to keep moving forward during a sudden stop (1st law)
Key Fact

1st: F_net = 0 → constant velocity; 2nd: F_net = ma; 3rd: F_AB = −F_BA

Related Terms

Net force Acceleration Free-body diagrams Friction

Study This Concept

Practice Newton's three laws with free review games in these units:

AP Physics 1 Unit 2: Forces and Newton's Laws Physics Unit 2: Forces and Newton's Laws