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CBSE Class 11 · Physics

Laws of Motion

Official NCERT chapter from Physics Part I–II (book code keph1). ExamMaster notes are original teaching at CBSE Class 11 depth.

This lesson follows the official chapter “Laws of Motion” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 11
  • Medium level
  • 8 concepts

1Conservation of momentum

Conservation of momentum: if the net external force on a system is zero, the total p = m1v1+m2v2 (1-d as taught) stays. A 4 kg at 3 m/s hitting a 2 kg at rest and sticking: 12 = 6 v, v=2 m/s. Internal bangs cancel as pairs; an outside push does not.

Using conservation when a taught external force (friction, a wall) sits on the system is a miss.

Figure. An isolated pair keeps total momentum. Equal masses that stick share the first cart's p, so the joined speed is half.

How it works

  1. Name the system and ask if F_ext net is zeroThe gate.
  2. Write total p before = total p afterThe conserve.
  3. Solve; keep the unit kg·m/sThe check.

4 kg at 3 m/s sticks to 2 kg at rest

Find the common speed if F_ext=0.

  • p before12 kg·m/s
  • p after6 v
  • v2 m/s

Pro tip. Same system; no net outside push.

2Equilibrium of a particle

Equilibrium of a particle: net force is zero (and, if the lesson added it, no leftover spin-story on a particle). 5 N east and 5 N west on one bead is equilibrium; 5 N and 3 N is not. Equilibrium is a vector-sum test, not “it looks still in a photo”.

A still photo of a flying ball at the peak is not F_net=0 if gravity is the leftover.

Figure. A particle is in equilibrium when every force has an equal opposite partner on that same free-body. N cancels mg; friction cancels the pull. Net force is zero, so acceleration is zero.

How it works

  1. Draw every force on that one particleThe free-body.
  2. Add as vectorsThe net.
  3. Zero net → equilibriumThe test.

3Common forces in mechanics

Common forces: weight mg down, a normal from a surface, tension along a string, friction opposite the slip-or-tend as taught. A force is a named arrow, not “the motion”. Friction has a static cap μN if that write was taught.

Drawing weight and a “force of motion” in the same direction double-counts.

Figure. Weight, normal, friction and tension are the four forces this chapter keeps naming. List them on one object before you add numbers.

How it works

  1. Name the object and each contact or fieldThe list.
  2. Draw weight, normal, tension, friction as taughtThe commons.
  3. Refuse a mystery “go-force”Honest arrows.

4Circular motion

Circular motion at this class: a net force toward the centre (centripetal) of size mv²/r as taught. The force is toward the centre; the velocity is tangent. A “centrifugal on the body in an inertial story” is a later-or-aside caution if the lesson gave one.

Drawing the centre-force along the velocity is a miss.

Figure. A circle is not drawn. At one instant the velocity is along the tangent and the centripetal force points at the centre along the radius.

How it works

  1. Name m, v, rThe given.
  2. Write F_c = mv²/r toward the centreThe need.
  3. Keep v tangentThe circle.

5Solving problems in mechanics

Solving mechanics problems: draw the system, free-body each body, pick axes, write F_net=ma (or p-conserve if F_ext=0), then the unknown. The method is the list, not a memorised answer. One block on a table is enough.

Jumping to a formula with no free-body is a poster.

Figure. Mechanics problems are one chain: name the body, draw every force on that body, write F = ma, then solve. Skipping the FBD is how a sign goes missing.

How it works

  1. Draw the body and the arrowsThe prepare.
  2. Write F_net=ma along a named axisThe law.
  3. Solve for the unknownThe method.

6A definition is a test you can run

Momentum / equilibrium / F is a test: a named system plus a net-force or a p-sum. If you only say “Newton”, you have a heading.

A rocket-sticker is not the test.

Figure. A definition here is a test you can run. If the net force is zero the velocity stays. If a net force is present the velocity changes. Inertia names that stubbornness; it is not an extra arrow on the free-body.

How it works

  1. Name the system and the arrows or the p-sumThe object.
  2. Give F_net=0, or p-conserve, or mv²/rThe test.
  3. Then the word has contentThe definition ran.

7Name the given before the unknown

The given are the masses and speeds (or the force-list). The unknown is v or a. Copy “sticks” before you use an elastic write.

Using 3 m/s as the finish because it was the start is a silent given.

Figure. Write the given mass and acceleration before the unknown force. Here m = 3 kg and a = 4 m/s², so F = ma = 12 N.

How it works

  1. Copy m, v, and the contact-storyThe given.
  2. Name the after-speed or aThe unknown.
  3. Then p-sum or F=maGiven first.

8One worked case is enough at this class

One p-conserve or one free-body is enough. A page of the same 2 m/s clone does not add a new circle.

Ten twins of 12 kg·m/s are still one idea.

Figure. One honest F = ma case is enough at this class: 4 kg at 5 m/s² needs 20 N. Extra invented carts do not teach a new law.

How it works

  1. Run one collide or one F_net=0The case.
  2. A second only as a checkOptional.
  3. StopThis class.
4 kg at 3 m/s sticks to 2 kg at rest (F_ext=0). Common v is
  1. 2 m/s
  2. 3 m/s
  3. 1.5 m/s

12 = 6v.

Notes

  • Mapped to the official NCERT chapter “Laws of Motion”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • p = mv; Σp conserved if F_ext=0
  • F_c = mv²/r toward centre
  • equilibrium: F_net=0

Recap

Hold these pegs from the official chapter “Laws of Motion”. The wording is ExamMaster’s teaching, not a textbook recap.

Conservation of momentum
Conservation of momentum: if the net external force on a system is zero, the total p = m1v1+m2v2 (1-d as taught) stays.
Equilibrium of a particle
Equilibrium of a particle: net force is zero (and, if the lesson added it, no leftover spin-story on a particle).
Common forces in mechanics
Common forces: weight mg down, a normal from a surface, tension along a string, friction opposite the slip-or-tend as taught.
Circular motion
Circular motion at this class: a net force toward the centre (centripetal) of size mv²/r as taught.
Solving problems in mechanics
Solving mechanics problems: draw the system, free-body each body, pick axes, write F_net=ma (or p-conserve if F_ext=0), then the unknown.
A definition is a test you can run
Momentum / equilibrium / F is a test: a named system plus a net-force or a p-sum.

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