Physics — Mechanics Basics: Newton's Laws, Gravitation, Work-Energy, Simple Machines

Mechanics is the branch of physics dealing with motion and forces. PSC frequently asks conceptual questions and numerical shortcuts from this topic.

Newton’s Laws of Motion

Law	Statement	Example
First Law (Inertia)	A body at rest stays at rest, and a body in motion stays in uniform motion, unless acted upon by an external force	Passengers jerk forward when a bus suddenly stops
Second Law (F = ma)	Force equals mass multiplied by acceleration	A heavier object needs more force for the same acceleration
Third Law (Action-Reaction)	Every action has an equal and opposite reaction	Rocket propulsion — gases push down, rocket moves up

Key Concepts from Newton’s Laws

Concept	Formula/Details
Momentum	p = mv (mass x velocity); unit: kg m/s
Impulse	J = F x t = change in momentum
Conservation of momentum	In absence of external force, total momentum before = total momentum after collision
Inertia	Depends on mass only (not weight or velocity)

Types of Forces

Force	Details
Gravitational	Attracts all masses; weakest fundamental force but infinite range
Friction	Opposes relative motion between surfaces
Normal force	Perpendicular contact force from a surface
Tension	Force transmitted through a string, rope, or cable
Centripetal force	Directed towards centre of circular motion; F = mv²/r

Gravitation

Aspect	Details
Newton’s Law of Universal Gravitation	F = G(m1 x m2)/r²
G (Gravitational constant)	6.674 x 10⁻¹¹ N m²/kg²
g (acceleration due to gravity)	9.8 m/s² on Earth’s surface
Weight	W = mg (depends on location)
Mass	Constant everywhere; measured in kg

Variation of g

Condition	Effect on g
At poles	Maximum (Earth is flattened at poles)
At equator	Minimum on surface (Earth bulges at equator)
With altitude	Decreases (g decreases as distance from centre increases)
With depth	Decreases (becomes zero at centre of Earth)
On Moon	About 1/6 of Earth’s g

Escape Velocity and Orbital Velocity

Concept	Formula	Value (Earth)
Escape velocity	v = sqrt(2gR)	~11.2 km/s
Orbital velocity (near surface)	v = sqrt(gR)	~7.9 km/s

Work, Energy, and Power

Work

Aspect	Details
Definition	W = F x d x cos(theta)
Unit	Joule (J) = 1 Newton x 1 metre
Work is zero when	Force is perpendicular to displacement (e.g., carrying a bag while walking horizontally — gravity does no work)
Negative work	When force opposes displacement (e.g., friction)

Energy

Type	Formula	Details
Kinetic Energy	KE = (1/2)mv²	Energy of motion
Potential Energy	PE = mgh	Energy due to position (gravitational)
Elastic PE	PE = (1/2)kx²	Energy stored in a spring

Law of Conservation of Energy: Energy can neither be created nor destroyed, only converted from one form to another. Total energy of an isolated system remains constant.

Power

Aspect	Details
Definition	Rate of doing work: P = W/t
Unit	Watt (W) = 1 Joule/second
1 Horsepower (HP)	= 746 Watts
1 kWh	= 3.6 x 10⁶ Joules (unit of energy, not power)

Friction

Type	Details
Static friction	Prevents a body from starting to move; maximum value = limiting friction
Kinetic friction	Acts on a moving body; less than limiting static friction
Rolling friction	Least of all; wheels roll instead of slide

Key Facts	Details
Friction depends on	Nature of surfaces and normal force
Friction does NOT depend on	Area of contact or velocity (for kinetic friction at moderate speeds)
Coefficient of friction	mu = Friction force / Normal force
Advantages of friction	Walking, writing, braking, holding objects
Disadvantages	Wear and tear, energy loss as heat
Reducing friction	Lubrication, ball bearings, polishing, streamlining

Simple Machines

Machine	Principle	Mechanical Advantage (MA)
Lever	Rotates around a fulcrum	MA = Effort arm / Load arm
Pulley	Changes direction of force	Single fixed pulley: MA = 1; Movable: MA = 2
Inclined Plane	Reduces effort by increasing distance	MA = Length / Height
Wheel and Axle	Rotational leverage	MA = Radius of wheel / Radius of axle
Screw	Inclined plane wrapped around a cylinder	MA = 2 x pi x R / Pitch
Wedge	Two inclined planes back to back	Used in knives, axes, nails

Three Classes of Levers

Class	Arrangement	Example
Class 1	Fulcrum between effort and load	See-saw, scissors, crowbar
Class 2	Load between fulcrum and effort	Wheelbarrow, nutcracker, bottle opener
Class 3	Effort between fulcrum and load	Tongs, fishing rod, human forearm

Important Formulas Summary

Quantity	Formula	Unit
Speed	distance / time	m/s
Velocity	displacement / time	m/s
Acceleration	change in velocity / time	m/s²
Force	mass x acceleration	Newton (N)
Momentum	mass x velocity	kg m/s
Work	force x displacement x cos(theta)	Joule (J)
Power	work / time	Watt (W)
Kinetic Energy	(1/2)mv²	Joule
Potential Energy	mgh	Joule
Pressure	Force / Area	Pascal (Pa)

PSC-Focused Quick Recall

Question Pattern	Answer
Newton’s First Law is also called	Law of Inertia
SI unit of force	Newton
1 HP = ? Watts	746
g at centre of Earth	Zero
Escape velocity from Earth	11.2 km/s
Friction is independent of	Area of contact
Rolling friction is ____ than sliding friction	Less
Class 2 lever example	Wheelbarrow
Unit of energy used in electricity bills	kWh (kilowatt hour)
Conservation of energy was stated by	Hermann von Helmholtz (1847)
Weight of a body on Moon compared to Earth	1/6
Rocket propulsion is based on	Newton’s Third Law