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General Science: Force and Laws of Motion (Physics)
📅 Date: 16 December 2025 (Tuesday)
This chapter, "Force and Laws of Motion," is the cornerstone of Mechanics. In competitive exams, questions from this chapter are almost always application-based. They won't ask you to state the law; they will give you a real-life situation (like a passenger falling in a bus) and ask which law applies.
1. What is Force?
Definition: It is a push or a pull that changes or tends to change the state of rest or motion of an object.
Effects: Can change speed, direction, or shape.
- Balanced Forces: Resultant force = 0. No change in motion.
- Unbalanced Forces: Resultant force is NOT 0. Causes acceleration/motion.
2. Newton’s First Law of Motion (The Law of Inertia)
The Law: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Inertia is the natural tendency of an object to resist change.
Relation to Mass: Mass is the measure of Inertia. (Heavier object → More Mass → More Inertia → Harder to move).
Exam Favorites (Practical Applications):
- Bus Starts Suddenly (Passengers fall backward): Feet move with the bus, but the upper body wants to stay at rest (Inertia of Rest).
- Bus Stops Suddenly (Passengers fall forward): Feet stop with the bus, but the upper body wants to keep moving (Inertia of Motion).
- Beating a Carpet (Dust comes out): Carpet moves away, dust stays at rest due to inertia and falls down.
- Coin on a Card: Flick the card, the coin falls into the glass. (Inertia of Rest).
3. Newton’s Second Law of Motion (Measurement of Force)
Concept: Momentum (p): The "power" of a moving object.
Formula: p = m × v (Mass × Velocity).
The Law: The rate of change of momentum is directly proportional to the applied force.
Formula: F = m × a (Force = Mass × Acceleration).
To increase the time taken to stop the ball. Increasing time reduces the rate of change of momentum, which reduces the force impact on the hands (Less pain).
Other Applications:
- Karate Chop: Hand strikes very fast (Time is small) → Creates Huge Force on bricks.
- Seatbelts: Stretch slightly to increase impact time → Reduces force on passenger.
4. Newton’s Third Law of Motion (Action-Reaction)
The Law: To every action, there is an equal and opposite reaction.
Crucial Condition: Action and Reaction forces act on two different bodies. (This is why they don't cancel each other out).
Exam Favorites:
- Walking: You push ground backward (Action); Ground pushes you forward (Reaction).
- Recoil of a Gun: Bullet goes forward; Gun moves backward. (Gun moves slower due to higher mass).
- Rocket Propulsion: Gas ejected down; Rocket goes up.
5. Conservation of Momentum
The Law: In an isolated system (no external force), total momentum before collision = total momentum after collision.
Equation: m1u1 + m2u2 = m1v1 + m2v2.
Example: When a bullet is fired, the momentum gained by the bullet is equal and opposite to the momentum gained by the gun.
6. Mentor’s Final Drill (Exam-Ready Questions)
A: Newton’s First Law (Inertia of Rest).
Q: Why does a fireman struggle to hold a hose pipe ejecting water at high speed?
A: Newton’s Third Law. The water rushing forward exerts a backward reaction force.
Q: If a constant force acts on a body, what stays constant: Velocity or Acceleration?
A: Acceleration (Since F=ma, if F and m are constant, a is constant). Velocity keeps increasing.
Q: Which physical quantity is defined as the product of mass and velocity?
A: Momentum.
Q: An object of mass 2 kg is sliding on a frictionless surface with a constant velocity of 4 m/s. How much force is required to keep it moving at the same speed?
A: Zero. (Newton's First Law: No friction = No force needed to maintain uniform motion).
Key Takeaway: Don't memorize definitions. Visualize the Bus (First Law), the Cricket Catch (Second Law), and the Gun/Rocket (Third Law). These three images will help you answer 90% of the questions from this chapter.