Motion, Force, Work & Energy
In short: Newton's laws, gravity, friction, pressure, work, energy and simple machines.
General Science › Physics · 36 min read · Free to read · English & Telugu
Bilingual study notes for the Andhra Pradesh Police SI & Constable exams (SLPRB AP) by AP Police Hub, a product of Anvil Academy.
The everyday mechanics an exam loves — why a seatbelt saves you, why a rocket rises, why a ship floats, and the SI units & discoverers behind it all.
A Newton's cradle — momentum and energy passing through a row of steel balls · Wikimedia Commons (CC BY 2.0, Sheila Sund)
1. Rest, Motion & Its Types
A body is at rest if its position does not change with time, and in motion if it does — but both are relative to a chosen reference point. A passenger sitting in a moving train is at rest relative to the train, yet in motion relative to the platform. This is why we say motion and rest are never absolute.
- Rectilinear (translatory) motion — Motion along a straight line — a car on a straight road, a freely falling stone, a marching soldier.
- Circular motion — Motion along a circular path — the moon around the earth, a stone tied to a string and whirled, the tip of a fan blade.
- Rotational motion — A body spinning about a fixed axis — the earth on its axis, a spinning top, a potter's wheel.
- Oscillatory / periodic motion — A to-and-fro motion that repeats in equal time — a pendulum, a swing, a vibrating guitar string. Motion that repeats after a fixed time is called periodic.
- Random motion — Motion with no fixed path or direction — a flying insect, gas molecules, a football during play.
2. Speed, Velocity & Acceleration
These three describe how fast and which way a body moves. The key exam distinction is scalar vs vector: a scalar has only size (magnitude); a vector has size and direction.
- Speed — Distance travelled per unit time — a scalar (no direction). SI unit metre/second (m/s); everyday unit km/h. A car showing 60 km/h on the speedometer tells you speed, not direction.
- Velocity — Displacement per unit time — speed in a stated direction, a vector. Same SI unit m/s. '60 km/h due north' is velocity. If direction changes, velocity changes even at constant speed.
- Acceleration — Rate of change of velocity — a vector. SI unit m/s². Speeding up = positive; slowing down = negative acceleration = retardation / deceleration. A body moving with uniform velocity has zero acceleration.
Uniform vs non-uniform motion| Type | Meaning | Everyday example |
|---|
| Uniform motion | Equal distances in equal time intervals | A car on cruise at steady 50 km/h |
| Non-uniform motion | Unequal distances in equal time intervals | A bus in city traffic, stopping & starting |
3. Newton's First Law — Inertia
Sir Isaac Newton gave three laws of motion (published in his 1687 book *Principia*). The First Law states: a body continues in its state of rest or of uniform motion in a straight line unless acted upon by an external force. This natural resistance to a change of state is called inertia — so the first law is also called the Law of Inertia.
- Inertia of rest — the seatbelt — When a bus suddenly starts, you jerk backward — your body tends to stay at rest. When it suddenly stops, you lurch forward — your body tends to keep moving. The seatbelt / headrest exists precisely to supply the force that stops this forward lurch in a crash.
- More everyday inertia — Dusting a carpet by beating it (dust at rest falls off as the carpet moves), a coin on a card dropping into a glass when the card is flicked, and shaking a wet umbrella to fling off water — all show inertia of rest.
- Inertia depends on mass — The heavier a body, the more inertia it has. It is far harder to push a loaded truck than a bicycle — mass is the measure of inertia.
4. Second Law — Force & Momentum
Momentum (p) is the 'quantity of motion' in a body = mass × velocity. It is a vector, SI unit kg·m/s. A slow-moving truck can carry more momentum than a fast bullet because of its huge mass.
- The law — F = ma — The rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction of the force. In everyday form this becomes Force = mass × acceleration (F = ma). So the same push gives a lighter body more acceleration.
- Unit of force — the newton — SI unit of force = newton (N). 1 N is the force that gives a 1 kg mass an acceleration of 1 m/s². The older CGS unit is the dyne (1 N = 10⁵ dyne).
- Why a cricketer pulls his hands back — A fielder moves his hands backward while catching a fast ball. This increases the time over which the ball's momentum falls to zero, so the force on his hands is reduced — the same reason we use airbags, foam packaging and long jump sand-pits.
5. Third Law — Action & Reaction
Newton's Third Law: *to every action there is an equal and opposite reaction*. The action and reaction are equal in magnitude, opposite in direction, and — crucially — act on two different bodies, so they never cancel out.
- Rocket & jet propulsion — A rocket throws hot gases downward (action); the gases push the rocket upward (reaction). A jet engine and even a garden-sprinkler work the same way. This works in empty space, needing no air to push against.
- Recoil of a gun — When a bullet is fired forward, the gun kicks backward (recoils). Because the gun is far heavier than the bullet, its recoil velocity is small — a direct result of conservation of momentum.
- Walking & swimming — We push the ground backward with our feet; the ground pushes us forward. A swimmer pushes water backward; the water pushes the swimmer forward. On smooth ice or a frictionless floor, walking becomes almost impossible.
A rocket at liftoff — hot gas is thrown downward (action) and the rocket is pushed upward (reaction). This is Newton's Third Law in action. Space Shuttle Atlantis liftoff (STS-125), NASA · Wikimedia Commons (Public Domain)
6. Gravitation, g, Mass vs Weight
Every object in the universe attracts every other object — the Universal Law of Gravitation, given by Newton (the falling-apple story). The earth's pull is what makes objects fall downward and gives them weight.
- Acceleration due to gravity (g) — All bodies fall with the same acceleration g ≈ 9.8 m/s² near the earth's surface, regardless of mass (in vacuum a feather and a coin fall together). g is greatest at the poles, least at the equator, and decreases with altitude and depth.
- Mass vs weight — Mass is the quantity of matter in a body — constant everywhere, a scalar, measured in kg. Weight is the force of gravity on it (W = mg) — a vector, measured in newton (N), and it changes with location. On the moon, weight is about 1/6 of that on earth, but mass stays the same.
- Weightlessness — An astronaut in an orbiting station feels weightless because the station and the astronaut are in free fall together around the earth — there is no supporting reaction, so no sensation of weight. The same brief feeling comes in a free-falling lift.
7. Friction — A Necessary Evil
Friction is the force that opposes motion between two surfaces in contact. It is called a 'necessary evil' because we cannot live without it, yet it wastes energy. It arises from the tiny roughness (interlocking) of surfaces.
Friction — where it helps and where it harms| Uses (advantages) | Drawbacks (disadvantages) |
|---|
| Walking, gripping, holding objects | Wears out machine parts, tyres, shoes |
| Brakes of vehicles stop the wheels | Wastes energy as heat; lowers efficiency |
| Writing, lighting a matchstick, nails hold | Machines need lubrication & cooling |
- Reducing friction — Use lubricants (oil, grease), ball-bearings (which turn sliding into rolling — rolling friction is much less than sliding friction), polishing, and streamlined shapes for cars, planes and fish.
- Increasing friction — Tyres and shoe soles are given treads (grooves); sand is spread on slippery roads — more grip means safer stopping.
8. Pressure & Atmospheric Pressure
Pressure = force per unit area (P = F/A). SI unit pascal (Pa) = N/m², named after Blaise Pascal. For the same force, a smaller area gives more pressure — which is why a sharp knife cuts better and a camel's broad feet stop it sinking into sand.
- Atmospheric pressure & straws — The air above us presses with atmospheric pressure ≈ 101,325 Pa (760 mm of mercury) at sea level, measured by a barometer (invented by Torricelli). When you suck a straw, you lower the pressure inside; the higher outside atmospheric pressure pushes the drink up the straw. On the moon (no air) a straw would not work.
- Why dams are thick at the base — Liquid pressure increases with depth (P = hρg). Water at the bottom of a dam presses hardest, so the wall is built thick at the base and thinner at the top. The same reason: a tank's lower holes squirt water farthest.
- Pascal's Law — Pressure applied to an enclosed liquid is transmitted equally in all directions. This runs the hydraulic brake, hydraulic press and hydraulic lift/jack, letting a small force lift a car. Also why pressure-cookers cook faster and it is harder to breathe at high altitude.
9. Buoyancy, Archimedes & Density
Any object placed in a liquid feels an upward push called buoyancy (upthrust) — this is why a bucket of water feels lighter under water and why we can lift a heavy person in a pool. The idea was discovered by Archimedes (the famous *'Eureka!'* story).
- Archimedes' Principle — A body immersed in a fluid is buoyed up by a force equal to the weight of the fluid it displaces. If this upthrust equals the body's weight, it floats; if less, it sinks.
- Why iron sinks but a ship floats — A nail sinks because iron is denser than water. A ship, though made of iron, is hollow — its large shape displaces enough water to equal its weight, so it floats. The same reason a person floats more easily in the salty, denser Dead Sea.
- Density & relative density — Density = mass ÷ volume, SI unit kg/m³. Relative density (specific gravity) = density of a substance ÷ density of water; it has no unit (a pure ratio). Water is the reference; ice (RD < 1) floats, and this is measured with a hydrometer.
10. Surface Tension, Viscosity & Capillarity
The surface of a liquid behaves like a stretched elastic skin — a property called surface tension. It is caused by the mutual attraction (cohesion) between the liquid's own molecules: a molecule deep inside is pulled equally on all sides, but one at the surface is pulled only inward and sideways, so the surface tightens into the smallest possible area. Its SI unit is newton/metre (N/m).
- Walking on water & spherical drops — Because the surface acts like a skin, water striders and light insects walk on water, a carefully placed steel needle or coin can float even though iron is denser than water, and free raindrops and mercury drops become spherical — a sphere has the least surface area for a given volume.
- Soap, detergent & temperature — Soap and detergents lower the surface tension of water, so it spreads out, wets cloth and seeps into dirt far better — this is why warm, soapy water cleans better than plain water. Surface tension also falls as temperature rises, so hot water washes more effectively.
- Capillary action — cohesion vs adhesion — Capillary action is the rise or fall of a liquid in a very narrow tube, decided by a tug-of-war between cohesion (liquid–liquid attraction) and adhesion (liquid–tube attraction). When adhesion wins, water rises and wets the tube; when cohesion wins, mercury is pushed down.
- Everyday capillarity — Oil rises up a lamp or candle wick, water climbs through soil and up the stems of tall plants, a blotting paper or towel soaks up water, and ink spreads through a piece of chalk — all thanks to capillary action.
- Viscosity — resistance to flow — Viscosity is the internal friction of a fluid — its resistance to flow. Honey and thick oil are highly viscous and pour slowly, while water flows easily (low viscosity). Lubricants (oils, grease) rely on viscosity to keep machine parts apart. A liquid's viscosity decreases as temperature rises — that is why engine oil turns thinner when the engine heats up and honey flows faster when warmed.
Everyday effects of these liquid properties| Property | Everyday effect |
|---|
| Surface tension | Insects walk on water; drops are spherical; a needle floats; soap improves cleaning |
| Capillary action | Oil rises in a wick; water climbs in soil, plants and blotting paper |
| Viscosity | Honey pours slowly; lubricants protect machines; oil thins when hot |
11. Work, Energy & Its Conservation
In science, work is done only when a force moves a body through a distance (W = force × distance). Holding a heavy suitcase still or pushing a wall that does not move is zero work, however tiring. SI unit of work = joule (J); 1 J = 1 N × 1 m.
- Energy = capacity to do work — Energy is the capacity to do work; its SI unit is the joule (J), same as work. The two great mechanical forms are kinetic and potential energy.
- Kinetic vs potential energy — Kinetic energy is energy of motion — a moving car, flowing water, a flying arrow. Potential energy is stored energy of position or state — water in a dam, a stretched bow, a wound spring, a raised hammer.
- Law of conservation of energy — Energy can neither be created nor destroyed; it only changes from one form to another — the total energy stays constant. A falling stone converts potential → kinetic; a bulb converts electrical → light + heat. In a simple pendulum, energy swings endlessly between potential and kinetic.
- Power & its units — Power = rate of doing work = work ÷ time. SI unit watt (W) = 1 joule/second, named after James Watt. 1 horsepower (hp) ≈ 746 watts. The electricity we pay for is billed in kilowatt-hour (kWh) = 1 unit, which is a unit of energy, not power.
12. Simple Machines & Mechanical Advantage
A simple machine is a device that lets us do work more easily — usually by letting a small effort move a large load. The number of times a machine multiplies the effort is its Mechanical Advantage (MA) = load ÷ effort. A machine does not create energy; it only makes the task convenient.
The common simple machines| Machine | How it helps | Everyday example |
|---|
| Lever | A rigid bar turning on a fulcrum; multiplies force | Crowbar, seesaw, scissors, bottle-opener, forearm |
| Pulley | A grooved wheel; changes direction of effort, eases lifting | Drawing water from a well, flag hoisting, cranes |
| Inclined plane | A slope; lets a load rise with less effort over more distance | Ramp, staircase, winding hill road, screw, wedge |
The six simple machines — lever, wheel & axle, pulley, inclined plane, wedge and screw — with everyday examples of each. Simple machines in everyday life, Christinelmiller · Wikimedia Commons (CC BY-SA 4.0)
13. At a Glance — SI Units & Discoverers
The most-tested one-liners of this chapter. If you memorise nothing else, fix these two tables — units and discoverers.
SI units of key quantities| Quantity | SI unit | Scalar / Vector |
|---|
| Speed / Velocity | metre/second (m/s) | Scalar / Vector |
| Acceleration | m/s² | Vector |
| Force | newton (N) | Vector |
| Momentum | kg·m/s | Vector |
| Pressure | pascal (Pa) = N/m² | Scalar |
| Work / Energy | joule (J) | Scalar |
| Power | watt (W) = J/s | Scalar |
| Density | kg/m³ | Scalar |
Discoverers & landmark ideas| Idea / Law | Associated with |
|---|
| Three Laws of Motion; Gravitation | Isaac Newton |
| Buoyancy / Upthrust principle | Archimedes |
| Pressure in enclosed fluids; barometer idea | Blaise Pascal |
| Barometer (atmospheric pressure) | Evangelista Torricelli |
| Falling bodies & inertia (early idea) | Galileo Galilei |
| Unit of power (steam engine) | James Watt |