Physics: Units, Motion, Light & Electricity

🎯 Learning Objective

Recall SI units, Newton's laws, the basics of light and sound, and Ohm's law with common electricity facts.

💡 Concept

• The SI system has seven base units: metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (substance) and candela (luminous intensity)
• Derived units: Force is measured in newton (N), Work/Energy in joule (J), Power in watt (W), Pressure in pascal (Pa) and Frequency in hertz (Hz)
• Newton's First Law is the law of inertia; the Second Law gives Force = mass x acceleration (F = ma); the Third Law says every action has an equal and opposite reaction
• The acceleration due to gravity (g) is about 9.8 m/s2; mass stays constant everywhere but weight changes with g
• Energy can neither be created nor destroyed (Law of Conservation of Energy); kinetic energy is energy of motion and potential energy is energy of position
• Light travels in a straight line at about 3 x 10^8 m/s in vacuum and is the fastest thing known
• A concave mirror converges light; a convex mirror diverges it and is used as a vehicle rear-view mirror because it gives a wider view
• Myopia (short-sightedness) is corrected with a concave lens; hypermetropia (long-sightedness) is corrected with a convex lens
• Sound needs a medium and cannot travel in vacuum; its speed in air is about 343 m/s and it is faster in solids than in liquids or gases
• Ohm's Law states V = IR (Voltage = Current x Resistance); India's household supply is about 220-230 volts at 50 Hz

🧮 Key Formulas

F = ma | V = IR (Ohm's Law) | Power P = VI

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g approx 9.8 m/s2 | Speed of light approx 3 x 10^8 m/s | Speed of sound in air approx 343 m/s

✏️ Easy Example

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

1. Force = mass x acceleration
2. Its SI unit is the newton (N)

Answer: (c) newton

🇮🇳 Real-Life Example

The convex mirror on your scooter or car is pure physics — its diverging surface shrinks the image but shows a much wider road behind you, which is exactly why 'objects are closer than they appear'.

📝 Exam-Level Example

Q. Which lens is used to correct myopia (short-sightedness)?

1. In myopia the image forms in front of the retina
2. A concave (diverging) lens corrects it

Answer: Concave lens

📝 Exam-Level Example

Q. Sound cannot travel through which medium?

1. Sound needs particles of a medium to travel
2. It cannot travel through vacuum (empty space)

Answer: Vacuum

🪄 Memory Trick

Mirrors: Concave = Caves in = Converges; Convex = bulges out = diverges (wider view). Lenses for eyes: myopia-concave, hypermetropia-convex.

⚠️ Common Mistakes

• ❌ Confusing mass (constant) with weight (changes with gravity)
• ❌ Saying sound travels faster in gases — it is actually fastest in solids
• ❌ Mixing up the lenses: myopia needs a concave lens, hypermetropia needs a convex lens

🏆 Exam Tips

• ✅ Remember F = ma and V = IR — these two formulas appear again and again
• ✅ A convex mirror always gives an erect, diminished image with a wide field of view

📌 Summary

• 7 SI base units; force = newton, energy = joule, power = watt
• Newton's laws: inertia - F = ma - action/reaction; g approx 9.8 m/s2
• Concave mirror converges; convex mirror diverges (rear-view); myopia-concave, hypermetropia-convex
• Sound needs a medium (approx 343 m/s in air); Ohm's Law V = IR; home supply 220-230 V, 50 Hz