Newton’s First Law (Law of Inertia)
An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction, unless acted upon by an unbalanced force.
Key Concept: Inertia is the tendency of an object to resist changes in its state of motion.
Examples
- A book on a table remains at rest until pushed
- A passenger lurches forward when a bus suddenly stops
Newton’s Second Law
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
$$
F = ma
$$
Where:
- $F$ = Net force (in Newtons, N)
- $m$ = Mass (in kg)
- $a$ = Acceleration (in m/s²)
Solved Example
Problem: A 5 kg object is acted upon by a force of 20 N. Find the acceleration.
Solution:
$$
a = \frac{F}{m} = \frac{20}{5} = 4 \text{ m/s}^2
$$
Newton’s Third Law
For every action, there is an equal and opposite reaction.
$$
\vec{F}_{12} = -\vec{F}_{21}
$$
Important Points
- Action and reaction act on different bodies
- They are equal in magnitude but opposite in direction
- They act simultaneously
| Property | Action Force | Reaction Force |
|---|---|---|
| Magnitude | F | F |
| Direction | Forward | Backward |
| Acts on | Body A | Body B |
Practice Questions
- A force of 10 N acts on a mass of 2 kg. What is the acceleration?
- Why do we fall forward when a bus suddenly stops?
- Explain why a rocket can propel in vacuum using Newton’s third law.