Physics
🔒 Log in to trackMotion, gravitation, work-energy, fluids and levers
🔒 Log in to trackNewton's laws of motion
| Law | Statement | Everyday examples |
|---|---|---|
| First (inertia) | A body stays at rest or in uniform motion unless an external force acts | Passengers jerk forward when a bus brakes; dust leaves a beaten carpet |
| Second | Force = rate of change of momentum, F = ma | Catching a fast ball by pulling hands back (more time, less force) |
| Third | Every action has an equal and opposite reaction | Rocket propulsion, recoil of a gun, swimming, walking |
Momentum p = mv is conserved when no external force acts.
Gravitation
- Universal law: F = G m₁m₂ / r² (Newton); G = 6.67 × 10⁻¹¹ N m² kg⁻².
- g = 9.8 m/s² on Earth's surface; it is maximum at the poles, minimum at the equator, and decreases with height and depth (zero at the centre).
- Mass stays the same everywhere; weight (mg) changes. On the Moon weight is about 1/6 of that on Earth.
- Escape velocity from Earth is about 11.2 km/s; orbital velocity near the surface is about 7.9 km/s.
- A geostationary satellite orbits about 36,000 km above the equator with a period of 24 hours.
- Astronauts in orbit feel weightless because they are in free fall with the spacecraft.
Work, energy and power
- Work W = F s cos θ; zero when force ⊥ displacement (e.g. a satellite in circular orbit).
- Kinetic energy = ½mv² (doubling speed → 4 times KE); potential energy = mgh.
- Energy conversions: electric motor electrical → mechanical; dynamo/generator mechanical → electrical; battery chemical → electrical; solar cell light → electrical; microphone sound → electrical; loudspeaker electrical → sound.
Fluids
- Pressure = force/area. Sharp knives and narrow nails work because small area → high pressure.
- Pascal's law — pressure applied to an enclosed liquid is transmitted equally: hydraulic lift, hydraulic brakes, hydraulic press.
- Archimedes' principle — upthrust = weight of liquid displaced; ships float, ice floats (density ~0.92 g/cm³). A body floats if its density is less than the liquid's.
- Bernoulli's principle — faster flow → lower pressure: aeroplane lift, roofs blown off in storms, atomisers.
- Surface tension — liquid drops are spherical; insects walk on water; soap and kerosene lower it (kerosene on ponds kills mosquito larvae).
- Capillarity — rise of oil in a lamp wick, ink in blotting paper, water in plant xylem.
Simple machines — levers
| Class | Arrangement | Examples |
|---|---|---|
| Class 1 | Fulcrum in the middle | Seesaw, scissors, pliers, crowbar, beam balance |
| Class 2 | Load in the middle | Nutcracker, wheelbarrow, bottle opener |
| Class 3 | Effort in the middle | Tongs, tweezers, fishing rod, human forearm |
Detailed notes
Motion in simple words
A body is in motion when its position changes with time. Distance is the total path covered (scalar); displacement is the straight-line gap from start to end with direction (vector). Speed = distance ÷ time; velocity = displacement ÷ time. Acceleration is the rate of change of velocity (m/s²). Walking once round a circular park gives a large distance but zero displacement.
Newton's three laws
- First law (inertia): a body stays at rest or keeps moving in a straight line unless an outside force acts. Examples: passengers fall forward when a moving bus brakes (inertia of motion); they fall backward when a stopped bus starts suddenly (inertia of rest); dust leaves a carpet when it is beaten. Mass is the measure of inertia.
- Second law: force = mass × acceleration (); force is also the rate of change of momentum (). A cricketer pulls his hands back while catching — he increases the time of impact, so the force on his hands is smaller. Cars have crumple zones and air bags for the same reason.
- Third law: every action has an equal and opposite reaction. Rockets and jet planes, recoil of a gun, swimming, walking (we push the ground back, it pushes us forward). Conservation of momentum: without outside force, total momentum stays constant — the gun recoils because bullet and gun share zero total momentum.
Gravitation
Newton's law: every two masses attract with force , where G = 6.67 × 10⁻¹¹ N·m²/kg² (the same everywhere in the universe). Near the Earth this pull gives g ≈ 9.8 m/s².
- g is maximum at the poles and minimum at the equator (the Earth bulges at the equator and spins).
- g decreases with height and with depth; it is zero at the Earth's centre.
- Mass (kg) never changes; weight = mg (newton) changes with place. On the Moon, g is about 1/6 of Earth's, so a 60 kg person still has 60 kg mass but weighs one-sixth as much.
- In a freely falling lift or an orbiting spacecraft, people feel weightless.
- Escape velocity from Earth ≈ 11.2 km/s; it does not depend on the mass of the object. Orbital speed close to the surface ≈ 7.9 km/s.
- A geostationary satellite circles above the equator at about 36,000 km, with a period of 24 hours, so it appears fixed in the sky — used for TV and weather.
Work, energy and power
Work = force × displacement in the direction of force (joule). No displacement means no work — a man holding a heavy box still does no work on it. A force at right angles to motion (like the centripetal force in circular motion) does zero work. Kinetic energy = ½mv² — doubling speed makes it four times. Potential energy = mgh. Energy only changes form: a falling stone turns PE into KE. Power = work ÷ time (watt).
Fluids — the principle behind the gadget
| Principle | Idea | Everyday use |
|---|---|---|
| Pascal's law | Pressure on an enclosed liquid spreads equally in all directions | Hydraulic lift, hydraulic brakes, hydraulic press |
| Archimedes' principle | Upthrust = weight of liquid displaced | Ships float, hydrometer, lactometer, submarines |
| Bernoulli's principle | Where a fluid moves faster, its pressure is lower | Lift on aeroplane wings, roofs blown off in storms, atomiser/sprayer |
| Surface tension | Liquid surface acts like a stretched skin | Spherical drops, insects walking on water; soap lowers it |
| Capillarity | Liquid rises in thin tubes | Oil in a lamp wick, blotting paper, towels |
| Viscosity | Internal friction of fluids | Honey flows slowly; liquid viscosity falls when heated |
| Ice floats because it is less dense than water (about 0.92 g/cm³). A ship floats but an iron nail sinks because the ship's shape displaces much more water. |
Levers
A lever has a fulcrum (F), a load (L) and an effort (E). The class is fixed by what sits in the middle:
- Class 1 — F in the middle: seesaw, scissors, pliers, crowbar, beam balance.
- Class 2 — L in the middle: nutcracker, wheelbarrow, bottle opener, lemon squeezer.
- Class 3 — E in the middle: tongs, forceps, fishing rod, human forearm, broom. Class 2 always gives mechanical advantage; class 3 never does but gives speed and control.
Quick revision
- Inertia ↔ mass; F = ma; action = reaction (rocket, recoil).
- g: max at poles, min at equator, zero at the centre; Moon ≈ 1/6.
- Escape velocity 11.2 km/s; geostationary period 24 h.
- KE ∝ v² (double speed → 4× KE); work = 0 if no displacement or force ⟂ motion.
- Pascal → hydraulic; Archimedes → floating; Bernoulli → aeroplane lift.
- Lever middle: F-L-E = class 1-2-3.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Everyday event → Newton's law / momentumvery common3 practice Q
A daily-life situation (bus braking, rocket, gun recoil, cricketer catching, carpet beating) and options listing the laws.
- Body resists a change in its state of rest/motion → first law (inertia).
- Force reduced by taking more time, or F = ma → second law / momentum change.
- Push one way, move the other (rocket, recoil, swimming) → third law / conservation of momentum. Why: each law describes one distinct effect, and the story tells you which.
Example: When a carpet is beaten with a stick, the dust comes out. This is due to —
Inertia of rest (Newton's first law). The carpet moves, the dust tends to stay at rest and separates.
Type 2: Gravitation, weight and satellitesvery common4 practice Q
Questions on g at poles/equator/centre, weight on the Moon, escape velocity, geostationary orbit, weightlessness.
- Mass is fixed; weight changes with g.
- g: poles > equator; decreases up and down; zero at the centre.
- Key numbers: g ≈ 9.8 m/s², escape 11.2 km/s, geostationary 24 h at ~36,000 km.
Example: A body has a mass of 30 kg on Earth. What is its mass on the Moon?
30 kg. Mass never changes with place; only weight falls to about one-sixth.
Type 3: Fluid principle → applicationvery common3 practice Q
A device or effect (hydraulic brake, floating ship, aeroplane lift, lamp wick, round raindrops) and a list of principles.
- Enclosed liquid, force multiplied → Pascal.
- Floating / sinking / upthrust → Archimedes.
- Fast-moving air, low pressure → Bernoulli.
- Drops, skin-like surface → surface tension; rising in thin tubes → capillarity; thick flow → viscosity.
Example: The roof of a hut is blown off during a storm. Which principle explains this?
Bernoulli's principle — fast wind above the roof lowers the pressure there; the higher pressure inside pushes the roof up.
Type 4: Class of levercommon2 practice Q
A tool (scissors, nutcracker, forceps, wheelbarrow, human arm) and 'is a lever of class …'.
- Locate fulcrum, load and effort on the tool.
- See which one is in the middle: F → class 1, L → class 2, E → class 3 (F-L-E = 1-2-3).
- Double-check with the standard list: forceps/tongs/arm are class 3.
Example: A bottle opener is a lever of which class?
Class 2 — the cap (load) is between the fulcrum (edge resting on the top) and your hand (effort).
Type 5: Work, energy and power basicscommon2 practice Q
'If velocity is doubled, KE becomes…', 'work done when …', or a small power calculation.
- KE depends on v², so v × k gives KE × k².
- Work = F × s × cos θ — zero if s = 0 or the force is perpendicular to motion.
- Power = work (mgh for lifting) ÷ time.
Example: A body's velocity is made three times. Its kinetic energy becomes —
KE ∝ v², so 3² = 9 times.
Formulas
p = mv
G = 6.67 × 10⁻¹¹ N m² kg⁻²
1 W = 1 J/s
unit pascal
Shortcut tricks
⚡ Lever class by the middle item: 'FLE = 1-2-3'
Whatever sits in the middle decides the class: Fulcrum → 1, Load → 2, Effort → 3. Nutcracker has the load (nut) in the middle → class 2; tongs are squeezed in the middle → class 3.
Example: A wheelbarrow is which class of lever?
Class 2 — the load lies between the wheel (fulcrum) and the handles (effort).
⚡ Squares in energy
KE ∝ v²: double the speed → 4× KE; triple → 9×. Momentum ∝ v: double speed → 2× momentum.
Where students lose marks
Saying mass changes on the Moon — only weight changes.
Believing g is greatest at the equator — it is greatest at the poles.
Attributing rocket motion to the first law — it is the third law (action–reaction).
Practice sets — 18 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.