🌱 1. Force
✅ Key Definition
Force is a push or a pull that can change or try to change the state of motion or shape of an object.
🌱 Explanation
• A force can make a stationary object move.
• It can stop a moving object.
• It can change the speed of an object.
• It can change the direction of motion.
• It can change the shape of an object.
📚 Effects of Force
- Change in speed
- Change in direction
- Change in shape
🖊 Diagram Description
Draw a box on a surface.
• Show one arrow pushing it to the right (Force).
• Show the box moving in the same direction.
Label: Applied Force, Direction of Motion.
🧠 Important Point
Force has both magnitude and direction. It is a vector quantity.
🌱 2. Balanced and Unbalanced Forces
✅ Key Definition (Balanced Forces)
When two or more forces acting on an object cancel each other and the net force is zero, they are called balanced forces.
✅ Key Definition (Unbalanced Forces)
When forces acting on an object do not cancel each other, the net force is not zero and motion changes.
🌱 Explanation
• Balanced forces do not change the state of motion.
• Unbalanced forces change speed or direction.
• Motion changes only when net force ≠ 0.
📊 Difference Between Balanced and Unbalanced Forces
| Balanced Forces | Unbalanced Forces |
|---|---|
| Net force = 0 | Net force ≠ 0 |
| No change in motion | Change in motion |
| Equal and opposite | Unequal forces |
🧠 Memory Trick
Balanced = No motion change
Unbalanced = Motion change
🌱 3. First Law of Motion (Law of Inertia)
This law was given by .
✅ Key Definition
An object remains at rest or in uniform motion in a straight line unless acted upon by an unbalanced external force.
🌱 Explanation
• A stationary object stays stationary.
• A moving object continues moving with same speed and direction.
• Change occurs only if an external force acts.
📚 Inertia
✅ Key Definition
The tendency of an object to resist any change in its state of rest or motion is called inertia.
📚 Types of Inertia
- Inertia of Rest
- Inertia of Motion
- Inertia of Direction
🖊 Diagram Description
Draw a bus and a standing passenger.
• Show bus suddenly starting forward.
• Show passenger tilting backward.
Label: Direction of bus, Inertia of rest.
🧠 Memory Trick
"Inertia = Resistance to change"
🌱 4. Mass and Inertia
✅ Key Definition
Mass is the quantity of matter in a body.
🌱 Explanation
• Mass measures inertia.
• Greater mass → greater inertia.
• SI unit = kilogram (kg).
• Mass remains constant everywhere.
🧠 Important Point
Heavier objects are harder to move because they have more inertia.
🌱 5. Momentum
✅ Key Definition
Momentum is the product of mass and velocity of an object.
Formula:
p = m × v
SI unit = kg m/s
🌱 Explanation
• Greater mass → greater momentum.
• Greater velocity → greater momentum.
• Momentum is a vector quantity (has direction).
🧠 Memory Trick
Momentum = “Mass in Motion”
🌱 6. Second Law of Motion
✅ Key Definition
The rate of change of momentum of an object is directly proportional to the applied force and occurs in the direction of force.
🌱 Explanation
• More force → more acceleration.
• More mass → less acceleration (for same force).
• Mathematical form:
F = m × a
SI unit of force = Newton (N)
📚 Definition of 1 Newton
1 Newton is the force required to produce an acceleration of 1 m/s² in a body of mass 1 kg.
🖊 Diagram Description
Draw a football.
• Show a person kicking it.
• Draw arrow in direction of kick.
Label: Applied Force (F), Acceleration (a).
🧠 Memory Trick
F = ma → Very important formula 🔥
🌱 7. Third Law of Motion
✅ Key Definition
For every action, there is an equal and opposite reaction.
🌱 Explanation
• Forces always act in pairs.
• Action and reaction act on different bodies.
• They are equal in magnitude and opposite in direction.
📚 Examples
• Walking
• Swimming
• Recoil of gun
• Rocket launching
🖊 Diagram Description
Draw a person pushing a wall.
• Arrow from hand to wall (Action).
• Arrow from wall to hand (Reaction).
Label clearly.
🧠 Memory Trick
Action = Reaction (AR)
🌱 8. Law of Conservation of Momentum
✅ Key Definition
When no external force acts on a system, the total momentum before collision is equal to the total momentum after collision.
🌱 Explanation
• Applies during collisions.
• Total momentum remains constant.
• Based on Third Law of Motion.
🖊 Diagram Description
Draw two balls moving towards each other.
• Label masses m₁ and m₂.
• Show velocities v₁ and v₂ before collision.
• Show final velocities after collision.
🧠 Memory Trick
Momentum: Before = After
🌟 Quick Revision Summary
🔥 Force is a push or pull.
🔥 Balanced forces → No change in motion.
🔥 Unbalanced forces → Change in motion.
🔥 First Law → Law of Inertia.
🔥 Inertia depends on mass.
🔥 Momentum = m × v.
🔥 Second Law → F = ma.
🔥 1 Newton = 1 kg × 1 m/s².
🔥 Third Law → Action = Reaction.
🔥 Momentum is conserved when no external force acts.
