Newton's Laws of Motion
Newton's laws of motion are three fundamental laws given by Sir Isaac Newton. They describe the relationship between the motion of an object and the forces acting on it.
1. Newton's First Law — Law of Inertia
An object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an external unbalanced force.
Fnet = 0 ⇒ v = constant
Example
When a moving bus suddenly stops, passengers tend to move
forward due to inertia.
Types of Inertia
- Inertia of rest
- Inertia of motion
- Inertia of direction
2. Newton's Second Law — Law of Acceleration
The rate of change of momentum of an object is directly proportional to the applied external force and takes place in the direction of the force.
Momentum
p = mv
Therefore,
F = dp/dt
For constant mass:
F = ma
Where:
- F = Force in newtons (N)
- m = Mass in kilograms (kg)
- a = Acceleration in m/s²
Example
A football accelerates more when kicked with greater force.
For the same force, a lighter ball accelerates more than a
heavier ball.
3. Newton's Third Law — Law of Action and Reaction
For every action, there is an equal and opposite reaction.
FAB = −FBA
Characteristics
- The two forces are equal in magnitude.
- The forces are opposite in direction.
- They act on different objects.
- They occur simultaneously.
Examples
- A rocket moves upward by expelling gases downward.
- While walking, our feet push the ground backward and the ground pushes us forward.
- A swimmer pushes water backward and water pushes the swimmer forward.
Quick Summary
| Law | Main Concept | Equation |
|---|---|---|
| First Law | Inertia | Fnet = 0 ⇒ v = constant |
| Second Law | Acceleration | F = ma |
| Third Law | Action–Reaction | FAB = −FBA |
In Simple Words
- First Law: Objects resist changes in motion.
- Second Law: Force produces acceleration.
- Third Law: Forces always occur in equal and opposite pairs.