Class IX SEBA Science – Chapter 9: Force and Laws of Motion
Class IX SEBA Science – Chapter 9: Force and Laws of Motion
🧲 1. Balanced and Unbalanced Forces
- Force: A push or pull acting on an object.
- Balanced force:
- Equal forces in opposite directions
- Net force = 0
- No change in motion
- Unbalanced force:
- Unequal forces
- Net force ≠ 0
- Causes change in motion
🚀 2. First Law of Motion (Law of Inertia)
An object remains in its state of rest or uniform motion unless acted upon by an unbalanced external force.
- Inertia: Tendency of objects to resist change in state.
- More mass → more inertia.
- Examples:
- Luggage falls forward in a braking bus
- Dust removed from mat by beating
⚖️ 3. Momentum
- Momentum (p) = mass × velocity
- p = mv
- SI unit: kg·m/s
- Vector quantity (depends on direction)
🧪 4. Second Law of Motion
Rate of change of momentum is directly proportional to applied force and takes place in the direction of force.
- Formula:
F = ma
(F: Force, m: mass, a: acceleration) - SI unit of force: Newton (N)
🔁 5. Third Law of Motion
For every action, there is an equal and opposite reaction.
- Acts on different bodies, at the same time.
- Examples:
- Gun recoils when bullet is fired
- Walking (foot pushes ground backward, ground pushes forward)
🧮 6. Conservation of Momentum
Total momentum before and after interaction remains constant, if no external force acts.
- For two objects: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
(u = initial velocity, v = final velocity)
📊 Table: Comparison of Newton’s Laws
| Law | Statement | Example |
|---|---|---|
| 1st (Inertia) | Body continues in rest/motion unless acted upon | Coin falls in glass when card flicked |
| 2nd (Force) | F = ma; acceleration ∝ force | Pushing a heavier object needs more force |
| 3rd (Action-Reaction) | Every action has equal and opposite reaction | Recoil of gun |
📉 Graphical Explanation: F = ma
- Greater the force → greater the acceleration
- For same force → less mass accelerates more
📘 Exercise Questions and Answers
1. What is the momentum of an object of mass 0.1 kg moving with speed 10 m/s?
p = mv = 0.1 × 10 = 1 kg·m/s
2. If action and reaction are equal and opposite, why do they not cancel out?
Because they act on different objects, not the same one.
3. What is inertia? Give two examples.
Tendency of body to remain in same state.
Examples:
- Dusting a mat
- Person jerks forward in braking car
4. What is force required to stop a 20 kg body moving at 2 m/s in 5 seconds?
Initial momentum = 20×2 = 40
Final = 0
Change = 40
F = Δp / t = 40 / 5 = 8 N
5. State and prove law of conservation of momentum.
See section above under heading 6. Conservation of Momentum.
✍️ Extra Important Questions
1. Define Newton.
1 Newton is the force that produces acceleration of 1 m/s² in a body of 1 kg mass.
2. How does a rocket move?
By expelling gas backward (action), rocket moves forward (reaction).
3. What happens when a horse pulls a cart?
Horse applies force on cart → cart moves. Cart pulls horse backward, but horse applies more force on ground than cart resists.
4. A cricket ball of mass 0.15 kg is moving at 40 m/s. What is its momentum?
p = mv = 0.15 × 40 = 6 kg·m/s
⭐ Points to Remember
- Force changes state of motion
- Inertia ∝ mass
- F = ma is the key formula of motion
- Momentum is conserved in absence of external force
- Action and reaction act on different bodies
- SI unit of force = Newton
✅ MCQs for Practice
1. What is the SI unit of force?
a) Joule
b) Newton ✅
c) Pascal
d) Watt
2. Momentum is a:
a) Scalar
b) Vector ✅
c) Constant
d) Speed
3. Which of the following has more inertia?
a) Scooter
b) Car
c) Train ✅
d) Ball
4. What is the force when mass = 5 kg and acceleration = 2 m/s²?
a) 10 N ✅
b) 2.5 N
c) 5 N
d) 7 N
continue to Chapter 10: Gravitation?
