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Physics Notes > Newton's Laws of Motion
Master Newton’s three laws of motion, force mass acceleration formulas ($F=ma$), inertia examples, and action-reaction pairs with clear diagrams and practice MCQs.
An object at rest stays at rest, and an object in motion continues at constant velocity in a straight line, unless acted upon by a net external force. This is the principle of inertia. Mass is a measure of inertia — the greater the mass, the greater the force needed to change its motion.
Examples: A book on a table stays at rest (normal force and gravity cancel). A hockey puck on frictionless ice continues at constant velocity. A passenger thrown forward when a car brakes (their body resists the change in motion).
Suppose a hockey puck is moving across a smooth surface and the net external force on it is zero. According to the law of inertia, its acceleration is zero, so its velocity remains constant. The mathematical relationship is:
Fnet = 0 → a = 0 → v = constant
In real-world situations, friction and air resistance usually provide external forces that eventually slow moving objects down. This is why an object may appear to stop even though Newton’s First Law says that motion continues when the net external force is zero.
Newton’s Second Law of Motion can be summarized by the equation F = ma. It states that the net force acting on an object equals its mass multiplied by its acceleration. This law explains how a change in force or mass affects an object’s acceleration.
The net force on an object equals its mass times its acceleration: F = ma. Force and acceleration are both vectors — they always point in the same direction. If multiple forces act, sum them as vectors to find the net force.
Worked example: A car of mass 1,200 kg decelerates from 20 m/s to rest in 5 seconds. Acceleration a = (0 − 20)/5 = −4 m/s². Net force F = 1,200 × (−4) = −4,800 N (braking force opposing motion).
For every action force, there is an equal and opposite reaction force acting on a different object. These pairs never cancel because they act on different bodies.
Examples: A rocket pushes exhaust gas backward; gas pushes rocket forward. Your feet push the ground backward; ground pushes you forward. The Earth pulls you down with gravity; you pull the Earth up with equal force.
In microgravity environments, objects continue moving indefinitely along straight paths because resistance forces like friction and atmospheric drag are absent. Spacecraft navigation relies directly on these principles to execute complex orbital adjustments using precise thruster burns.
A free-body diagram visually isolates a single object to analyze all acting vector forces including gravity, normal force, tension, and friction. Drawing these components accurately allows students to calculate net forces and solve complex mechanical problems with clarity.
Essential net force equations ($F=ma$), weight formulas, friction, and kinematic equations with SI units for quick revision
Unit: N (Newtons)
Unit: N (Newtons)
Unit: N (Newtons)
Unit: m/s
Unit: m (metres)
Unit: kg·m/s
Newton’s laws of motion form the core foundation of classical mechanics and physics fundamentals. Concepts like net force, mass, momentum, kinematics, and acceleration allow us to calculate movement in everyday engineering systems. Related mechanics topics—such as projectile motion, friction, spring force, and universal gravitation—build directly on these three laws of motion and are central to physics exams.
Clear answers to key questions about the 3 laws of motion, force formulas, and free body diagrams.
Newton’s First Law (law of inertia) states that an object stays at rest or moves at constant velocity unless a net external force acts on it. It tells us that forces are required to change motion, not to maintain it.
F = ma is Newton’s Second Law. Net force (F) in Newtons equals mass (m) in kg times acceleration (a) in m/s². It is the most used equation in classical mechanics and applies to everything from falling apples to spacecraft.
Newton’s Third Law states that for every force, there is an equal and opposite force acting on a different object. A horse pulls a cart forward; the cart pulls the horse backward with equal force. These forces do not cancel because they act on different objects.
A free body diagram (FBD) shows all forces acting on one object as arrows. It is the essential first step in solving any Newton law problem. Draw the object, draw all force arrows to scale and direction, then apply F = ma to each direction separately.
Explore force acceleration and mass, work, energy, SUVAT equations, and power formulas.
Apply Newton’s laws of motion to parabolic trajectories, initial velocity, and launch angles.
Interactive formula library and calculators for force ($F=ma$), acceleration, and momentum.