Science & Technology
Physics in Everyday Life
Motion and Force
Newton's laws, momentum, friction, circular motion, gravitation, pressure, buoyancy and Bernoulli with everyday examples for CGPSC Prelims and Mains GS-II.
Prerequisites
- Class 9–10 science vocabulary (speed, mass, force). No algebra beyond one-line formulas is needed.
- Physics in Everyday Life: chapter overview for how the three topics fit together.
- Space and Defence Technology uses this page’s ideas of orbits and rockets, so read this first.
The Hook: A Bus on the Keshkal Ghat
Connect
A bus climbs the Keshkal ghat (केशकाल घाटी) on the Raipur–Jagdalpur highway. On each hairpin bend, standing passengers lurch outwards. When the driver brakes hard, everyone pitches forward. The driver keeps the bus in a low gear on the way down instead of riding the brakes, and on a wet day he slows down well before the bend.
Each of these moments is a law of physics at work: inertia, friction, centripetal force and momentum. Why do we lurch, why do wet roads need slower speeds, and why does a rocket at Sriharikota rise without any air to push against? This page answers those questions.
Why it matters: General science in Prelims Paper I is almost always asked as “which law explains this everyday event?” Mains P-IV (GS-II) asks short explanations of the same phenomena, so one set of notes serves both stages.
Describing Motion (गति)
| Term | Meaning | Everyday check |
|---|---|---|
| Distance | Total path length covered. A scalar (size only) | A bus runs 20 km around a city loop |
| Displacement | Shortest straight-line distance from start to finish, with direction. A vector | After the full loop it is zero |
| Speed | Distance ÷ time (scalar) | The speedometer shows speed |
| Velocity | Displacement ÷ time, with direction (vector) | Turning at constant speed changes velocity |
| Acceleration | Rate of change of velocity. SI unit m/s² | Braking is negative acceleration (retardation) |
Uniform circular motion is accelerated motion
A ceiling fan blade or a satellite moving at constant speed in a circle is still accelerating, because its direction changes every instant. This is a favourite statement-based trap.
Newton’s Laws of Motion
Isaac Newton set out the three laws in the Principia (1687).
| Law | Statement in plain words | Everyday examples |
|---|---|---|
| First law (law of inertia, जड़त्व) | A body stays at rest or in uniform straight-line motion unless an external force acts on it | Passengers fall forward when a bus brakes (inertia of motion) and backward when it starts (inertia of rest); dust leaves a carpet when it is beaten; a coin on a card drops into the glass when the card is flicked |
| Second law | Force = rate of change of momentum; for constant mass \( F = ma \) | A heavier truck needs more force for the same acceleration; a fielder pulls his hands back while catching |
| Third law | To every action there is an equal and opposite reaction, acting on a different body | Walking (foot pushes ground back, ground pushes foot forward); swimming; recoil of a gun; rocket and jet propulsion |
Inertia is the tendency of a body to resist a change in its state. Mass is the measure of inertia: a loaded truck is harder to start and stop than a bicycle.
Momentum and impulse
- Momentum (संवेग) \( p = mv \) (mass × velocity), unit kg m/s.
- Impulse = force × time = change in momentum. For the same change in momentum, a longer stopping time means a smaller force.
| Design or habit | Physics |
|---|---|
| Seat belts and airbags | Stop the body over a longer time, so the force on it is smaller. Seat belts also fight inertia of motion |
| Cricketer drawing hands back while catching | Increases the time to stop the ball, so the sting is less |
| Sand or soft mats in long-jump and high-jump pits | Increase stopping time on landing |
| Thermocol and bubble-wrap packing | Increase the time of impact if the parcel is dropped |
| Karate chop on a slab | The reverse: a very short impact time gives a large force |
Conservation of momentum
When no external force acts, the total momentum of a system stays constant.
- Gun recoil: The bullet moves forward and the gun moves back, so the total momentum stays zero.
- Rocket: Hot gases are pushed out backwards at high speed and the rocket gains equal forward momentum. A rocket does not need air to push against, which is why it works in space.
Friction (घर्षण)
Friction is the force that opposes relative motion between two surfaces in contact.
| Type | Size | Example |
|---|---|---|
| Static (limiting) | Largest | A heavy almirah refuses to move until you push hard enough |
| Sliding (kinetic) | Smaller than static | Once moving, it is easier to keep pushing |
| Rolling | Smallest | Wheels, ball bearings, luggage trolleys |
| Fluid friction (drag) | Depends on speed and shape | Streamlined shapes of aircraft, fish and birds |
| Friction is useful | Friction is a nuisance |
|---|---|
| Walking, writing, braking, holding objects | Wears out tyres, shoe soles and machine parts |
| Tyre treads grip the road; matchsticks light | Wastes energy as heat in engines |
Reducing friction: lubricants (oil, grease), ball bearings, polishing surfaces and streamlining. Increasing friction: treads on tyres and soles, sand on slippery roads, chalk powder on a gymnast’s hands.
Why slow down on a wet road?
A film of water acts as a lubricant and reduces the friction between tyre and road. Less friction means less braking force, so the stopping distance increases. Worn-out treads cannot clear the water, which is why tyre treads matter in the monsoon.
Circular Motion
To move in a circle, a body needs a centripetal force (अभिकेंद्र बल) acting towards the centre.
| Situation | What provides the centripetal force |
|---|---|
| Car turning on a flat road | Friction between tyres and road |
| Banking of roads (outer edge raised on curves) | A part of the road’s normal reaction, so a vehicle can turn safely with less dependence on friction |
| Cyclist leaning inwards on a turn | The same idea as banking |
| Moon around Earth; satellites | Gravity |
| Stone whirled on a string | Tension in the string |
The outward pull passengers “feel” on a turn is centrifugal force, a pseudo force felt only inside the turning vehicle. It is really the body’s inertia trying to continue in a straight line.
Applications of the centrifuge idea: the dryer of a washing machine, cream separators in dairies, and laboratory centrifuges that separate blood components.
Gravitation (गुरुत्वाकर्षण)
Newton’s law of gravitation: Every two bodies attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Acceleration due to gravity, g
- At Earth’s surface, \( g \approx 9.8 \) m/s². On the Moon it is about one-sixth of Earth’s value.
- g is greatest at the poles and least at the equator. Earth is slightly flattened at the poles, so the poles are closer to its centre, and Earth’s rotation reduces the effective g most at the equator.
- g decreases with height above the surface and also with depth below it. It is zero at Earth’s centre.
Mass vs weight
| Mass (द्रव्यमान) | Weight (भार) |
|---|---|
| Amount of matter; unit kg | Force of gravity on the body, \( W = mg \); unit newton (N) |
| Same everywhere: Earth, Moon, space | Changes with g: less on the Moon, slightly less at the equator |
| Measured with a beam (pan) balance | Measured with a spring balance |
Weight in a lift, and weightlessness
| Lift | Apparent weight |
|---|---|
| Accelerating upwards | More than the true weight |
| Moving at constant speed | Equal to the true weight |
| Accelerating downwards | Less than the true weight |
| Falling freely (cable snaps) | Zero: weightlessness |
Astronauts float in an orbiting spacecraft not because gravity is absent there, but because the spacecraft and everything in it are falling freely around Earth together.
Escape and orbital velocity
| Quantity | Value for Earth | Note |
|---|---|---|
| Escape velocity | About 11.2 km/s | Minimum launch speed to escape Earth’s gravity; independent of the mass of the body |
| Orbital velocity close to the surface | About 7.9 km/s | Decreases for higher orbits |
| Geostationary orbit | About 35,786 km above the equator (often rounded to 36,000 km) | Period equals one rotation of Earth, so the satellite appears fixed. Used for communication and weather satellites |
The Moon has a much lower escape velocity than Earth, which is one reason it has kept almost no atmosphere.
Pressure and Fluids
Pressure (दाब) = force ÷ area. SI unit: pascal (Pa) = 1 N/m².
| Smaller area, more pressure | Larger area, less pressure |
|---|---|
| A sharp knife cuts easily; nails and needles have pointed tips | School-bag straps are wide; tractors and tanks use broad tracks |
| Camels walk on sand on broad feet; foundations of buildings are wide |
Pressure in liquids and the atmosphere
- Liquid pressure increases with depth. That is why dam walls are thicker at the base, and why water spurts farthest from the lowest hole in a tank.
- Atmospheric pressure at sea level is about \( 1.013 \times 10^{5} \) Pa. It decreases with altitude: ears “pop” in a plane or on a ghat road, and fountain pens may leak in aircraft.
- Boiling point depends on pressure. At high altitudes water boils below 100 °C, so food takes longer to cook. A pressure cooker raises the pressure, raising the boiling point and cooking food faster.
- A drinking straw and a syringe work because atmospheric pressure pushes the liquid up when the pressure inside is lowered.
Three principles behind everyday machines
| Principle | Statement | Applications |
|---|---|---|
| Pascal’s law | Pressure applied to an enclosed fluid is transmitted equally in all directions | Hydraulic brakes, hydraulic lift and jack, hydraulic press, earth-movers |
| Archimedes’ principle | A body immersed in a fluid experiences an upward force (buoyancy, उत्प्लावन बल) equal to the weight of fluid it displaces | Ships float because their hollow shape displaces a lot of water; submarines dive and surface by filling and emptying ballast tanks; it is easier to float in sea water than in river water; a lactometer checks the density of milk and a hydrometer the density of liquids |
| Bernoulli’s principle | Where a fluid moves faster, its pressure is lower | Lift on an aeroplane wing; roofs blown off in storms (fast wind above, still air below); a person standing too close to a fast train can be pulled towards it; sprayers and atomisers |
Floating and sinking in one line
A body floats if its average density is less than that of the fluid, and sinks if it is more. Ice (density about 0.92 g/cm³) floats on water (about 1 g/cm³), with roughly nine-tenths of an iceberg under water. An iron nail sinks, but an iron ship floats because, with the air it encloses, its average density is less than that of water.
Memory Tricks
Newton's laws as a bus ride
- 1st law, “Lazy”: the body wants to keep doing what it was doing (passenger lurches).
- 2nd law, “Push harder”: more mass needs more force (\( F = ma \)).
- 3rd law, “Push back”: you push the ground, the ground pushes you. Action and reaction act on two different bodies.
Friction order: 'S-S-R, big to small'
Static > Sliding > Rolling. That is why we “roll” heavy things on wheels.
Where is g biggest? 'Poles are Plus'
Poles = Plus (maximum g), Equator = Empty-ish (minimum g), Centre = zero.
Three fluid laws: PAB
Pascal = Press (hydraulic brakes) · Archimedes = Afloat (ships) · Bernoulli = Blow (wings, roofs in storms).
Don’t Confuse
| This | vs | That |
|---|---|---|
| Mass: constant, kg, beam balance | vs | Weight: varies with g, newton, spring balance |
| Centripetal force: real, towards the centre | vs | Centrifugal force: pseudo, felt outward inside the turning vehicle |
| Escape velocity (11.2 km/s): leave Earth’s pull | vs | Orbital velocity (about 7.9 km/s near the surface): circle Earth |
| Pascal’s law: pressure transmitted in enclosed fluid | vs | Archimedes’ principle: upthrust equal to the weight of displaced fluid |
| Inertia of rest: fall back when a bus starts | vs | Inertia of motion: fall forward when it brakes |
| Speed: scalar | vs | Velocity: vector, changes even when only direction changes |
Common Mistakes
MCQ traps
- “Astronauts are weightless because there is no gravity in space.” False. Gravity at the height of a space station is still strong. They are weightless because they are in free fall with their spacecraft.
- “Action and reaction cancel each other, so nothing moves.” False. They act on different bodies, so they cannot cancel.
- “A rocket needs air to push against.” False. It pushes its own exhaust gases backwards and works best in the vacuum of space.
- “Escape velocity is less for a lighter object.” False. It does not depend on the mass of the object, only on the planet’s mass and radius.
- “g is maximum at the equator because Earth bulges there.” Reversed. g is maximum at the poles.
- “A body moving in a circle at constant speed has no acceleration.” False. Its direction changes, so it accelerates towards the centre.
Practice
All questions below are original practice questions written in CGPSC style. They are not previous-year questions.
Level 1: Foundation
- The tendency of a body to resist a change in its state of rest or motion is called ______.
- The SI unit of pressure is the ______.
- A seat belt reduces injury by increasing the ______ over which the passenger’s momentum changes.
- The escape velocity from Earth is about ______ km/s.
Answers
- Inertia.
- Pascal (N/m²).
- Time.
- 11.2 km/s.
Level 2: Prelims MCQs
Q1. Consider the following statements:
- The weight of a body is greater at the poles than at the equator.
- The mass of a body on the Moon is about one-sixth of its mass on Earth.
- Astronauts inside an orbiting spacecraft feel weightless because they and the spacecraft are in free fall.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 1 and 3 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer and explanation
(b) 1 and 3 only. g is greatest at the poles, so weight is greater there. Statement 2 is wrong: mass never changes; it is the weight on the Moon that is about one-sixth. Statement 3 is the correct explanation of weightlessness in orbit.
Q2. Match List I (Device or event) with List II (Principle):
| List I | List II |
|---|---|
| A. Hydraulic brakes of a car | 1. Archimedes’ principle |
| B. A steel ship floating | 2. Bernoulli’s principle |
| C. Lift on an aeroplane wing | 3. Pascal’s law |
| D. Recoil of a gun | 4. Conservation of momentum |
- (a) A-3, B-1, C-2, D-4
- (b) A-1, B-3, C-2, D-4
- (c) A-3, B-2, C-1, D-4
- (d) A-3, B-1, C-4, D-2
Answer and explanation
(a) A-3, B-1, C-2, D-4. Hydraulic brakes transmit pressure through an enclosed fluid (Pascal). A ship floats on buoyancy (Archimedes). Faster air over the curved upper surface of a wing means lower pressure above it (Bernoulli). In gun recoil, the total momentum of gun and bullet stays zero (conservation of momentum).
Q3. A cricketer draws his hands backwards while catching a fast ball. This is done to:
- (a) increase the momentum of the ball
- (b) reduce the change in momentum of the ball
- (c) increase the time of impact so that the force on the hands is reduced
- (d) increase the friction between the ball and the hands
Answer and explanation
(c). The ball’s change in momentum is fixed (from its speed to zero). Spreading that change over a longer time reduces the force, because force equals change in momentum divided by time. Seat belts, airbags and the sand in a long-jump pit use the same idea.
Q4. Roads are banked (the outer edge raised) at curves mainly to:
- (a) drain rainwater off the road
- (b) provide the necessary centripetal force without depending only on friction
- (c) reduce the weight of vehicles on the curve
- (d) increase the speed of vehicles by reducing friction
Answer and explanation
(b). On a flat curve, only friction supplies the centripetal force, and on a wet road friction may not be enough. Banking tilts the road so that part of the road’s push on the vehicle points towards the centre of the curve. Drainage is a side benefit, not the reason.
Q5. Which of the following is the smallest for the same pair of surfaces?
- (a) Static friction
- (b) Sliding friction
- (c) Rolling friction
- (d) Limiting friction
Answer and explanation
(c) Rolling friction. The order is static (limiting) > sliding > rolling. This is why wheels and ball bearings save so much effort. Limiting friction is the maximum value of static friction, so (a) and (d) are the largest.
Level 3: Mains Answer Writing
Q1. (60 words) Explain, with two everyday examples, how Newton’s third law of motion works. Why can a rocket move in outer space?
Model answer outline
- Law: Every action has an equal and opposite reaction, acting on a different body.
- Examples: walking (foot pushes the ground back, the ground pushes the foot forward); recoil of a gun; swimming.
- Rocket: It pushes exhaust gases backwards at high speed and the gases push the rocket forward. No air is needed, so it works in space. The same principle lifts ISRO’s launch vehicles from Sriharikota.
Q2. (150–200 words) Explain the principles of physics behind the safety features of modern road vehicles. How can this knowledge make roads safer in a state like Chhattisgarh?
Model answer outline
- Intro: Road safety depends on managing inertia, momentum and friction.
- Body:
- Inertia: In a sudden stop, an unbelted body keeps moving forward. Seat belts hold it to the vehicle.
- Impulse: Airbags, crumple zones and helmet padding increase the impact time, which reduces the force.
- Friction: Tyre treads and anti-lock brakes keep grip. On wet roads, friction falls and stopping distances rise.
- Circular motion: Banked curves and speed limits on bends supply and respect the needed centripetal force.
- Momentum: Heavier, faster vehicles carry more momentum and need more distance and force to stop. Overloaded trucks are dangerous for this reason.
- Chhattisgarh angle: Ghat sections such as Keshkal on the Raipur–Jagdalpur route have sharp hairpin bends where speed control, banking and engine braking matter. Heavy coal and mineral traffic in districts such as Korba also makes speed and load limits important.
- Conclusion: Physics explains why rules on seat belts, helmets, speed and loading save lives. Awareness backed by enforcement and good road engineering is the way forward.
Quick Revision Table
| Fact | Detail |
|---|---|
| Newton’s laws | Principia, 1687 |
| First law | Inertia; mass is the measure of inertia |
| Second law | \( F = ma \); force = rate of change of momentum |
| Third law | Action and reaction on different bodies; rocket, walking, gun recoil |
| Momentum | \( p = mv \); conserved when no external force acts |
| Impulse | Force × time; longer time means smaller force (seat belts, airbags) |
| Friction order | Static > sliding > rolling |
| Centripetal force | Towards the centre; banking of roads supplies it |
| g on Earth | About 9.8 m/s²; maximum at poles, minimum at equator, zero at centre |
| Moon’s g | About one-sixth of Earth’s |
| Escape velocity (Earth) | About 11.2 km/s; independent of the object’s mass |
| Geostationary orbit | About 35,786 km above the equator |
| Pressure | Force ÷ area; unit pascal |
| Pascal’s law | Hydraulic brakes, lift, press |
| Archimedes | Buoyancy; ships, submarines, lactometer |
| Bernoulli | Aeroplane lift; roofs in storms |
Key Takeaways
Key Takeaways
- Inertia explains most “jerk” questions. Impulse explains every “increase the time of impact” safety device.
- Action and reaction act on different bodies. A rocket needs no air.
- Weight changes, mass does not. g is highest at the poles, and orbiting astronauts are in free fall, not in zero gravity.
- Pascal, Archimedes, Bernoulli: brakes, ships, wings.
- Uniform circular motion is accelerated motion. The centripetal force points inwards.
What’s Next
- Light and Sound, the next topic in this chapter
- Electricity and Magnetism
- Space and Defence Technology, where orbits, escape velocity and rockets are applied
- Physics in Everyday Life: Quick Revision
Advertisement