Newton's Three Laws of Motion
First Law — Law of Inertia
An object remains at rest, or continues to move in a straight line at constant velocity, unless acted upon by an external unbalanced force.
Inertia is the resistance of an object to change in its state of motion. A heavier (more massive) object has greater inertia.
Example: A passenger lurches forward when a bus suddenly stops — inertia keeps them moving.
Second Law — Law of Force
The acceleration of an object is directly proportional to the net force applied and inversely proportional to its mass.
Units: Force in Newtons (N), mass in kg, acceleration in m/s²
Example: A force of 20 N acts on a 4 kg object. a = F/m = 20/4 = 5 m/s²
Third Law — Law of Action and Reaction
For every action, there is an equal and opposite reaction.
Forces always come in pairs. When you push a wall, the wall pushes back on you with the same force.
Examples: Rocket propulsion, swimming (push water backward → move forward), recoil of a gun.
Weight vs Mass
| Mass | Weight |
|---|---|
| Amount of matter | Force of gravity on mass |
| Measured in kg | Measured in Newtons (N) |
| Same everywhere | Varies with gravity |