Newton's Laws of Motion
Experience all three laws of Newton through interactive demonstrations — from inertia to action-reaction pairs.
Key Notes
Newton's First Law (Inertia): A body at rest stays at rest, and a body in motion continues in uniform motion, unless acted on by a net external force.
Newton's Second Law: F = ma — the net force on a body equals its mass times acceleration. This is the workhorse of JEE mechanics.
Newton's Third Law: For every action, there is an equal and opposite reaction. The forces act on DIFFERENT bodies.
The concept of inertial frames is fundamental — Newton's laws hold only in non-accelerating reference frames.
Free Body Diagrams (FBDs) are the most important tool. Always isolate the body and draw ALL forces acting on it.
In connected body problems (Atwood machine, pulleys), use constraint equations: acceleration of connected bodies is related.
Pseudo forces appear in non-inertial frames. In a frame accelerating at 'a', a pseudo force = −ma acts on every body.
The normal force is NOT always equal to mg. It depends on the situation (inclines, lifts, circular motion).
Formulas
Newton's Second Law
Net force equals mass times acceleration.
Weight
Force due to gravity on a mass m.
Atwood Machine Acceleration
Acceleration when two masses hang from a pulley.
Atwood Machine Tension
Tension in the string of an Atwood machine.
Apparent Weight in Lift
+ for upward acceleration, − for downward.
Connected Bodies
Common acceleration of connected masses under force F.
Important Points
Action and reaction NEVER cancel each other because they act on different bodies.
In a lift accelerating upward, you feel heavier (N > mg). Accelerating downward, you feel lighter (N < mg).
In free fall (like a broken lift cable), N = 0 — you experience weightlessness.
For Atwood machines: if m₁ = m₂, acceleration = 0 and tension = mg (system is in equilibrium).
String tension is the same throughout an ideal (massless, inextensible) string.
When solving problems, choose the direction of expected motion as positive and be consistent.
Constraint relations: for pulleys, if one end moves by x, the other adjusts to keep string length constant.
Newton's Laws of Motion notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.