Newton's Laws, Gravity & Energy

🎯 Learning Objective

Understand Newton's three laws with examples, the value of g, and the units of work, energy and power.

💡 Concept

• Newton's 1st Law (Inertia): a body stays at rest or in uniform motion unless an external force acts — e.g. you jerk forward when a bus suddenly brakes
• Newton's 2nd Law: Force = mass × acceleration (F = ma); force equals rate of change of momentum
• Newton's 3rd Law: every action has an equal and opposite reaction — e.g. rocket launch, gun recoil, walking
• Acceleration due to gravity g ≈ 9.8 m/s² and is the SAME for all masses (in vacuum a feather and a stone fall together)
• Mass stays constant everywhere, but weight = mass × g changes — the Moon's gravity is about 1/6th of Earth's
• Work = Force × displacement (in the force's direction); SI unit = joule (J)
• Work is zero when displacement is perpendicular to the force (e.g. carrying a bag horizontally)
• Energy = capacity to do work (unit joule); Kinetic energy = ½mv², Potential energy = mgh
• Power = work done / time taken; SI unit = watt (W); 1 horsepower ≈ 746 watts
• Momentum = mass × velocity (a vector), conserved when no external force acts

🧮 Key Formulas

F = m × a (newton)

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Weight W = m × g (g ≈ 9.8 m/s²)

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Work = Force × displacement (joule)

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Power = Work / time (watt); Momentum = m × v

✏️ Easy Example

Q. When a bus suddenly stops, passengers fall forward. This is due to: (a) gravity (b) inertia of motion (c) friction (d) momentum of the bus

1. The body was moving and 'wants' to keep moving
2. This tendency to resist change in motion = inertia (Newton's 1st law)

Answer: (b) inertia of motion

🇮🇳 Real-Life Example

A rocket rising into the sky is Newton's 3rd law you can watch on TV — it pushes hot gas DOWN, and the gas pushes the rocket UP with an equal, opposite force. No air needed; that's why it works in space.

📝 Exam-Level Example

Q. The SI unit of power is: (a) joule (b) newton (c) watt (d) pascal

1. Power = work done per unit time = joule/second
2. One joule per second = one watt

Answer: (c) watt

📝 Exam-Level Example

Q. If a person holds a heavy suitcase and stands still, the work done on the suitcase is:

1. Work = force × displacement
2. There is no displacement, so work = 0

Answer: Zero

🪄 Memory Trick

Newton's laws in one line: 1st = 'lazy' (inertia), 2nd = 'F = ma' (formula), 3rd = 'react' (action–reaction). Lazy, Formula, React → 1-2-3.

⚠️ Common Mistakes

• ❌ Confusing mass (constant) with weight (changes with gravity)
• ❌ Thinking heavier objects fall faster in vacuum — g is the same for all masses
• ❌ Saying work is done while just holding a weight still (displacement = 0 → work = 0)

🏆 Exam Tips

• ✅ F = ma is the most-used physics formula in RRB — memorise it cold
• ✅ Weight on Moon = (Earth weight)/6, but mass is unchanged

📌 Summary

• 1st law = inertia, 2nd law = F = ma, 3rd law = action–reaction
• g ≈ 9.8 m/s², same for all masses; weight = mg
• Work = force × displacement (joule); Power = work/time (watt)
• 1 HP ≈ 746 W; momentum = mass × velocity