Newton's Laws of Motion Explained
The three laws in plain language, with the equation F = ma and worked examples.
Isaac Newton's three laws describe how forces affect the motion of objects. They apply well to everyday speeds and sizes.
First law: inertia
An object stays at rest, or keeps moving at constant velocity in a straight line, unless a resultant (net) force acts on it.
Inertia is the tendency of an object to resist changes in its motion. A passenger lurches forward when a car brakes suddenly because the body tends to keep moving at the earlier velocity.
Second law: F = ma
The resultant force on an object equals its mass times its acceleration:
F = m × a
Force is measured in newtons (N), mass in kilograms (kg) and acceleration in metres per second squared (m/s²).
Example 1. A 1500 kg car accelerates at 2 m/s². The resultant force is 1500 × 2 = 3000 N.
Example 2. A resultant force of 20 N acts on a 4 kg trolley. Acceleration = F ÷ m = 20 ÷ 4 = 5 m/s².
Note that "resultant" means the overall force after combining all forces, taking direction into account.
Third law: action and reaction
When object A exerts a force on object B, object B exerts an equal and opposite force on object A. These forces act on different objects, so they do not cancel each other.
Example. When you push on a wall, the wall pushes back on you with equal force. A rocket pushes exhaust gases backward, and the gases push the rocket forward.
Weight and mass
Mass is the amount of matter in an object, measured in kg. Weight is the gravitational force on it, measured in newtons: W = m × g. Near the Earth's surface g is approximately 9.8 N/kg (often rounded to 10 in simple problems). A 5 kg object weighs about 5 × 9.8 = 49 N.
Common mistakes
- Thinking an object needs a constant force to keep moving. Without friction, it would keep moving at constant velocity.
- Believing the equal and opposite forces of the third law cancel. They act on different objects.
- Confusing mass and weight.
- Forgetting to use the resultant force in F = ma.
Quick recap
First law: no resultant force, no change in motion. Second law: F = ma. Third law: forces come in equal and opposite pairs acting on different objects.