Newton’s First Law of Motion (Law of Inertia)
Key Ideas
- 🔄 Inertia: A body resists changes to its state of rest or motion.
- 🚫 Aristotle’s Mistake: He thought forces are needed to keep objects moving. Example: A toy car stops when you stop pulling it.
- 🔧 Galileo’s Insight: Friction is the hidden force that stops motion. Without friction, objects move forever!
What Aristotle Got Wrong
Aristotle believed objects need a continuous force to stay in motion. For example, he thought air pushes an arrow to keep it flying. But Galileo proved this wrong using experiments:
- On a frictionless horizontal plane, objects move at constant speed with no force.
- Inclined plane experiments showed that objects naturally maintain motion unless friction slows them.
Galileo’s Experiments
- Double Inclined Plane: A ball rolls down one slope and up another. Without friction, it reaches the same height every time. If the second slope is flat, the ball rolls forever!
- Horizontal Motion: With no friction, a moving object keeps its speed forever. In real life, friction makes it stop.
Newton’s First Law
Galileo’s ideas led Newton to state:
“Every body stays at rest or moves at constant speed in a straight line unless a net external force acts on it.”
Mathematically:
If \( \sum F_{\text{ext}} = 0 \), then acceleration \( a = 0 \).
Example: A spaceship in deep space (no forces) moves at constant velocity without engine power.
Inertia in Daily Life
- 🚗 A car stops suddenly, but you lurch forward (your body resists change in motion).
- 📚 A book stays on the table until you push it.
Ancient Indian Contributions
- Concept of Vega: “Tendency to move straight” (similar to inertia).
- Bhaskara (1150 A.D.) introduced instantaneous motion, like modern velocity.
NEET High-Yield Concepts
- Newton’s First Law: Zero net force ⇒ constant velocity.
- Inertia: Resistance to change in motion.
- Role of friction in stopping motion.
- Difference between Aristotle’s fallacy and Galileo’s correction.
- Examples of inertia (e.g., spaceship, sudden braking).
