JEE · Physics
Understand how to convert an interaction into a correct free-body model, choose a frame, apply Newton's laws and friction conditions, and diagnose why a solution fails.
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In short
Laws of Motion connects interactions to changes in motion. A reliable solution has five decisions: choose the body or system, choose the reference frame, draw only the forces acting on that system, write the constraint relations, and then apply momentum or force equations. Most errors happen before algebra begins.
Use this chapter to learn how a situation becomes a model. Do not begin by searching for a familiar formula. Begin by asking, "What exactly am I analysing, and which external interactions act on it?"
Verified against the current JEE Main 2026 syllabus and JEE Advanced 2026 syllabus on 7 September 2026. This is a wording and scope mapping, not a claim about question difficulty or frequency.
This page does not assign chapter weightage or predict how many questions will appear. Sources: JEE Main 2026 syllabus and JEE (Advanced) 2026 syllabus, both linked in the sources section below.
Parent
JEE syllabus
Exam-level official scope for every Physics, Chemistry and Mathematics unit.
Parent
JEE Physics
Subject hub for the Physics chapter set.
Prerequisite
Units and Measurements
Stabilise vectors, units and dimensional reasoning before force problems.
Prerequisite
Kinematics
Signed velocity, acceleration and circular kinematics feed directly into force models.
If these are unstable, revise Units and Measurements and Kinematics first.
The choice of system decides which forces are internal and disappear from the equation.
A block, two connected blocks, a person plus lift, or the entire collection can each be a valid system. Internal forces disappear from the net external-force equation for a combined system, but they remain necessary when an individual contact force is required.
A frame error changes the entire force model, not just one term.
In an inertial frame, use real forces in Newton's second law. In a uniformly accelerating frame, introduce the appropriate pseudo force and keep the frame choice consistent throughout. Do not mix ground-frame acceleration with lift-frame forces.
Weight comes from Earth, normal reaction from contact, tension from a taut connector, friction from a contacting surface, and applied forces from identified agents.
Draw one diagram per chosen body. Newton's third-law partners act on different bodies, so they never cancel on a single-body diagram.
A taut inextensible string, no-slip contact, common acceleration, or geometric contact can relate accelerations. Write that relation separately from the force equations.
Calculate the friction required by the assumed motion before using a limiting value.
Static friction takes the value required to prevent slipping, up to its limit. Use the limiting value only when impending motion is established. Once sliding occurs, use the stated kinetic-friction model.
Check direction, limiting cases, dimensions, and whether the assumed contact state survives. If the calculated static friction exceeds its maximum, the no-slip model is invalid and must be rebuilt.
The sum of external forces equals the rate of change of the system's momentum with respect to time.
External force changes system momentum.
Use when — Any particle or chosen system in an inertial frame.
Common trap — Including internal action-reaction pairs as external forces.
The sum of forces equals mass times acceleration.
Constant-mass form of Newton's second law.
Use when — Constant mass in an inertial frame.
Common trap — Treating one component equation as a scalar law for the whole motion.
Impulse equals the time integral of force, and equals the change in momentum.
Impulse equals momentum change.
Use when — Short-duration interaction.
Common trap — Replacing a varying force by its peak instead of its average.
Static friction is less than or equal to the coefficient of static friction times the normal reaction.
Static friction adjusts up to a limit.
Use when — Contact without slipping.
Common trap — Automatically setting f_s = mu_s N.
Kinetic friction equals the coefficient of kinetic friction times the normal reaction.
Simple kinetic-friction model.
Use when — Sliding contact under the stated model.
Common trap — Using kinetic friction before confirming slip.
The sum of radial forces equals mass times speed squared divided by radius.
Inward resultant supplies radial acceleration.
Use when — Motion on a circular path.
Common trap — Adding a separate "centripetal force" to existing forces.
The pseudo force equals minus mass times the frame's acceleration.
Translational pseudo force in an accelerating frame.
Use when — Uniformly translating non-inertial frame.
Common trap — Using it in an inertial frame or with the wrong direction.
The tangent of the banking angle equals speed squared divided by the product of radius and gravitational acceleration.
Ideal friction-free banking relation.
Use when — Design speed on a banked road with no friction.
Common trap — Using it when friction is active without rebuilding the equations.
Answer: The upper block does not slip while a <= mu_s g; beyond that acceleration, the blocks move independently.
Pairing equal and opposite forces on one free-body diagram.
Knowledge gapWhy it happens
Third-law partners act on different bodies, not the same one.
How it is corrected
Draw a separate diagram for each body and place each partner force on its own diagram.
Choosing friction opposite to velocity by habit.
Decision / selection errorWhy it happens
Friction opposes relative slipping or its tendency at the contact, not velocity in general.
How it is corrected
Identify the surface in contact and the direction of relative slipping tendency before assigning friction direction.
Setting normal reaction equal to m g in every problem.
Execution errorWhy it happens
Normal reaction depends on the forces resolved perpendicular to the actual contact, which changes with incline, applied force or acceleration.
How it is corrected
Resolve forces perpendicular to the contact surface for the specific configuration given.
Treating tension as identical across any pulley system.
Decision / selection errorWhy it happens
Equal tension needs the stated ideal string and pulley assumptions; a massive pulley or string changes this.
How it is corrected
Check whether the string and pulley are stated as ideal before assuming equal tension throughout.
Using the limiting-friction value throughout a problem.
Execution errorWhy it happens
Static friction only reaches its limiting value at the point of impending motion.
How it is corrected
First calculate the friction required by the assumed motion, then compare with the limiting value.
Calling m v^2 / r a new force.
Knowledge gapWhy it happens
It is the required inward resultant, not an additional physical force acting on the body.
How it is corrected
Identify which real forces (tension, normal reaction, friction, gravity component) create the inward resultant.
Check direction, limiting cases, dimensions, and whether the assumed contact state survives. If the calculated static friction exceeds its maximum, the no-slip model is invalid and must be rebuilt.
Use the smallest Preparation Intelligence v1.1 label supported by what the student actually did.
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FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
It is a diagram of one chosen body or system showing only the external forces acting on it.
No. Static friction adjusts from zero up to a limiting magnitude mu_s N.
No. It is the name for the inward resultant required for circular motion.
Prefer momentum or impulse when the interaction is brief, the force varies with time, or internal forces can be eliminated by choosing a larger system.
Scope claims are checked against the current NTA JEE Main 2026 syllabus and the official JEE Advanced 2026 syllabus. Official paper archives are linked for practice only; this page does not assert PYQ counts, frequency, weightage or trend for Laws of Motion.
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