Newton's Laws
Newton's three laws of motion: inertia, force, and action-reaction principle.
General idea of the topic
Newton's three laws form the basis of classical dynamics. The first defines inertia and inertial frames: with no net force, a body keeps its velocity. The second relates the net force to the acceleration through the mass. The third establishes that forces always come in pairs between two different bodies.
The laws are not three formulas to apply in any order but a procedure: the first says where they hold, the second allows the motion to be calculated and the third helps relate forces between interacting bodies without cancelling them within a single diagram.
What the student is expected to understand
By the end, the student should be able to apply the second law by components, interpret equilibrium as zero acceleration and not as rest, and recognise action and reaction pairs without confusing them with forces on a single body that balance each other.
The student should also be able to explain why mass measures the resistance to changing velocity.
Recommended learning route
- Newton's First Law (Inertia): starts from inertia and equilibrium: with no net force there is no change in velocity.
- Newton's Second Law (Force, Mass, and Acceleration): relates net force, mass and acceleration and is applied by components to motion problems.
- Newton's Third Law (Action and Reaction): explains the force pairs between two bodies, each applied to a different body.
Essential physical criterion
The essential criterion is not to mix laws or bodies. The second law is applied to one body with the sum of the forces acting on it; the third relates forces acting on different bodies and therefore they do not cancel each other. Confusing the two is the cause of most conceptual errors in dynamics.
Connection with real problems
- motion of vehicles and pushes.
- systems of connected bodies.
- rockets and reaction propulsion.
- equilibrium and stability of structures.
Final goal of the topic
The final goal is to apply Newton's three laws with judgement in any dynamics problem and to justify which law is used at each step.
Topics in this section
- Newton's First Law (Inertia)
Zero net force, inertial frame, and persistence of rest or constant velocity.
- Newton's Second Law (Force, Mass, and Acceleration)
Relation between net force, mass, and system acceleration. Theory, formulas, worked examples and interactive graphs.
- Newton's Third Law (Action and Reaction)
Every interaction between two systems produces forces equal in magnitude and opposite on different bodies.