Newton’s First Law of Motion
Key Idea: An object stays at rest or moves at a constant speed in a straight line unless a net external force acts on it.
1. Galileo’s Insight
- A ball rolling on a horizontal plane stops due to friction. Without friction, it would keep moving forever!
- Rest and uniform motion (constant speed in a straight line) are equivalent states. Both mean zero net force.
2. The Law Explained
- Statement: “Every body continues in its state of rest or uniform motion unless compelled by an external force to change.”
- Inertia: An object’s “resistance to change” in motion. Example: Standing in a bus – your body lags when the bus starts/stops due to inertia.
- Zero net force → Zero acceleration (\( \vec{a} = 0 \)).
3. Real-Life Examples
- Book on a table: Gravity pulls it down (\( W \)), the table pushes up (\( R \)). \( W = R \) → Net force = 0 → No motion.
- Car moving at constant speed: Engine force cancels friction → Net force = 0 → No acceleration.
- Astronaut in space: Far from stars/spaceship → No forces → Acceleration = 0. He keeps moving at his original speed!
4. Common Misconceptions
- ❌ “Forces cancel, so the object stops.” → Wrong!
✅ Correct: “The object is at rest/moving uniformly, so forces must cancel.” - Friction is key! Without it, objects wouldn’t stop naturally.
5. Inertia in Action
- Bus suddenly starts: You’re thrown backward (body resists motion).
- Bus suddenly stops: You lurch forward (body resists stopping).
High-Yield NEET Topics
- Newton’s First Law Statement: Must memorize the exact wording.
- Inertia: Definition and examples (bus jerk, objects resisting motion changes).
- Net Force & Acceleration: \( \vec{F}_{\text{net}} = 0 \) ↔ \( \vec{a} = 0 \).
- Real-World Applications: Astronaut in space, car motion, book on a table.
- Friction’s Role: Explains why objects stop in daily life despite the First Law.
