Physics

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medium importance~1 Q in Tier 127 formulas⚡ 12 shortcuts6 subtopics

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Motion, gravitation, work-energy, fluids and levers

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⏱ 9 min read🧩 5 question types🎯 18 practice Q

The idea in one minute

Newton'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

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 ( $F = ma$ ); force is also the rate of change of momentum ( $p = mv$ ). 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 $F = Gm_1m_2/r^2$ , 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.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common3 practice Q

Everyday event → Newton's law / momentum

How to spot it:

A daily-life situation (bus braking, rocket, gun recoil, cricketer catching, carpet beating) and options listing the laws.

Method

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 it works:

each law describes one distinct effect, and the story tells you which.

Try this

When a carpet is beaten with a stick, the dust comes out. This is due to —

Show solution

Inertia of rest (Newton's first law). The carpet moves, the dust tends to stay at rest and separates.

Type 2very common4 practice Q

Gravitation, weight and satellites

How to spot it:

Questions on g at poles/equator/centre, weight on the Moon, escape velocity, geostationary orbit, weightlessness.

W = mg,\quad g_{moon}\approx \tfrac{1}{6}g_{earth}

Method

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.

Try this

A body has a mass of 30 kg on Earth. What is its mass on the Moon?

Show solution

30 kg. Mass never changes with place; only weight falls to about one-sixth.

Type 3very common3 practice Q

Fluid principle → application

How to spot it:

A device or effect (hydraulic brake, floating ship, aeroplane lift, lamp wick, round raindrops) and a list of principles.

Method

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.

Try this

The roof of a hut is blown off during a storm. Which principle explains this?

Show solution

Bernoulli's principle — fast wind above the roof lowers the pressure there; the higher pressure inside pushes the roof up.

Type 4common2 practice Q

Class of lever

How to spot it:

A tool (scissors, nutcracker, forceps, wheelbarrow, human arm) and 'is a lever of class …'.

Method

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.

Try this

A bottle opener is a lever of which class?

Show solution

Class 2 — the cap (load) is between the fulcrum (edge resting on the top) and your hand (effort).

Type 5common2 practice Q

Work, energy and power basics

How to spot it:

'If velocity is doubled, KE becomes…', 'work done when …', or a small power calculation.

KE=\tfrac12 mv^2,\quad PE=mgh,\quad P=\frac{W}{t}

Method

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.

Try this

A body's velocity is made three times. Its kinetic energy becomes —

Show solution

KE ∝ v², so 3² = 9 times.

Formula sheet

First equation of motion

v = u + at

Second equation of motion

s = ut + \tfrac{1}{2}at^{2}

Third equation of motion

v^{2} = u^{2} + 2as

Newton's second law

F = ma = \frac{\Delta p}{\Delta t}

p = mv

Law of gravitation

F = \frac{G m_1 m_2}{r^{2}}

G = 6.67 × 10⁻¹¹ N m² kg⁻²

Kinetic and potential energy

KE = \tfrac{1}{2}mv^{2},\quad PE = mgh

Work and power

W = Fs\cos\theta,\quad P = \frac{W}{t}

1 W = 1 J/s

Pressure

P = \frac{F}{A},\quad P_{\text{liquid}} = h\rho g

unit pascal

Shortcuts that save time

⚡ 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?

Show solution

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.

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Saying mass changes on the Moon — only weight changes.

Mistake 02

Believing g is greatest at the equator — it is greatest at the poles.

Mistake 03

Attributing rocket motion to the first law — it is the third law (action–reaction).

Quick revision

Read this the night before the exam.

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.

Practice: 18 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 3 min · wrong answers go to your mistake notebook automatically.

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