CBSE Class 9 Science Notes Chapter 1 law of Motion
PHYSICS
CBSE Class 9 Science Notes Chapter 1 law of Motion
⚡ Newton’s Laws of Motion (Class 9 – Physics)
1️⃣ First Law of Motion – Law of Inertia
"A body continues to be in its state of rest or of uniform motion in a straight line unless compelled to change that state by an external unbalanced force."
Meaning:
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If nothing pushes or pulls an object (unbalanced force), it will just keep doing what it’s doing.
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If it’s at rest, it stays at rest. If it’s moving straight at a constant speed, it keeps moving.
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This resistance to change in motion is called inertia.
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Example:
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A book on a table stays still unless you push it.
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Passengers jerk forward when a moving bus stops suddenly.
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2️⃣ Second Law of Motion – Force and Acceleration
"The rate of change of momentum of a body is directly proportional to the applied unbalanced force and takes place in the direction of the force."
Formula:
where:
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= Force (N)
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= Mass (kg)
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= Acceleration (m/s²)
Meaning:
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More force means more acceleration.
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Heavier objects need more force to accelerate.
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Example:
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It’s easier to push an empty cart than a loaded cart.
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Kicking a football makes it move faster than kicking a stone (if you dare 😅).
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3️⃣ Third Law of Motion – Action and Reaction
"For every action, there is an equal and opposite reaction."
Meaning:
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Forces always come in pairs: action force and reaction force.
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These forces are equal in magnitude but opposite in direction.
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They act on different objects.
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Example:
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When you jump off a boat, the boat moves backward.
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Rocket propulsion: gases push down, rocket goes up.
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Quick Table for Revision
| Law | Key Idea | Example |
|---|---|---|
| 1st | Inertia (no change without unbalanced force) | Book stays on table |
| 2nd | Force = mass × acceleration | Pushing cart |
| 3rd | Action = –Reaction | Rocket lift-off |