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CBSE Class 11 · Physics

Motion in a Plane

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 “Motion in a Plane” 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

1A definition is a test you can run

Motion in a plane is a test: a change that needs two directions (across and up) plus a vector add. A displacement 3 m east and 4 m north is 5 m at a taught angle, not 7 m. One axis is the last chapter.

Adding 3+4 as a plane-distance is the walk-steal.

Figure. A position in the plane is a test: name origin O, then read both coordinates. The arrow from O to P is the position vector — magnitude and direction together, not a lone length.

How it works

  1. Agree two axesThe plane.
  2. Add the two parts as vectors (or a right-triangle hypotenuse)The test.
  3. Then the word plane-motion has contentThe definition ran.

2Name the given before the unknown

The given are the two components (or u, angle, g). The unknown is the resultant or a projectile part. Copy “east” and “north” before you add as scalars.

Using 10 m/s as already the vertical when 10 m/s was the launch speed at 30° is a silent given.

Figure. Write the given first: magnitude A and the angle from the x-axis. The unknowns are the axis components A_x = A\cos\theta and A_y = A\sin\theta, not a new arrow you invent.

How it works

  1. Copy the two parts or u and θThe given.
  2. Name the resultant or the peak/range as taughtThe unknown.
  3. Then resolve and addGiven first.

3 m east, 4 m north

Find the plane-displacement magnitude.

  • Parts3 and 4
  • Hypotenuse5 m
  • Not7 m

Pro tip. Two directions; the plane-distance is the hypotenuse.

3One worked case is enough at this class

One two-part add or one projectile-sketch is enough. A page of the same 3-4-5 clone does not add a new relative-velocity.

Ten twins of 5 m are still one idea.

Figure. One worked flight is enough: a parabola from launch to landing. Time comes from the vertical motion; range is that time times the untouched horizontal speed. H marks the apex; R marks the landing.

How it works

  1. Run one vector-add or one resolveThe case.
  2. A second only as a checkOptional.
  3. StopThis class.

4A miss: swapping the school name for the picture

A throw-photo is a setting. The science is two components. A smile is not uy.

A ball still needs u and θ named.

Figure. The bent polyline is the path. The straight arrow from A to B is displacement. Swapping those names is the school miss — path length is a scalar; displacement is the vector you can put on a free-body or a range formula.

How it works

  1. Keep the throw as a storyA setting.
  2. Write the two partsThe science.
  3. Then the resultantDo not swap.

5Check by the opposite action or the opposite test

Check a resultant by swapping add-order (vectors commute) or by a 3-4-5 rebuild. Check a projectile’s time-up by v_y=0 = uy − gt.

A 1-d check that ignores the second part is not the opposite test.

Figure. Add in the plane with a parallelogram. The opposite test: from the resultant, take away B and you must land back on A. If you do not, a component was swapped.

How it works

  1. State 5 m or the time-upThe claim.
  2. Add in the other order or rebuild uyThe check.
  3. Mend if they disagreeHonest.

6Keep the claim at this chapter, not the next

This chapter’s claim stops at two directions. It does not steal Newton’s third-law pairs, and it does not start 3-d motion.

A page that already “does torque” here misses laws of motion / rotation.

Figure. Motion in a plane is two coordinates and the vectors built from them. Leave Newton’s laws and three-dimensional frames for later chapters — the test here is still r, v, a in the xy-plane.

How it works

  1. Keep two axes and a projectile-splitThis chapter.
  2. Leave F=ma and pairs for laws of motionThe next map.
  3. Do not treat the map as already answeredClaim-size.

7The story is the hook; the test is the idea

A river-story (swim across, current down) is the hook. The test is still two components added as vectors. A story with one number only is a poster.

“The river is fast” with no across/down split is the miss.

Figure. The throw is the hook. The test is independence: the horizontal component stays the same at every station, while the vertical component is spent by gravity and is zero at the apex — never the whole speed.

How it works

  1. Hear the riverThe hook.
  2. Split across and downThe test.
  3. Then the resultantThe idea.

8Units or labels must match before you compare

Units or labels must match before you compare: 3 m east and 4 m north are both lengths; 3 m and 4 m/s are not addable. A component labelled m/s is not a displacement.

Adding a speed to a length because both were “4” is a lying compare.

Figure. A plane vector sum needs matching labels: metre with metre, metre-per-second with metre-per-second. Five metres plus three metres per second is not a resultant, even if the arrows look addable.

How it works

  1. Write each part with its unit and directionThe labels.
  2. Add only same-kind partsThe match.
  3. Then name the resultant’s unitHonest.
3 m east and 4 m north have magnitude
  1. 5 m
  2. 7 m
  3. 1 m

Hypotenuse, not 3+4.

Notes

  • Mapped to the official NCERT chapter “Motion in a Plane”. 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

  • resultant magnitude √(Ax²+Ay²)
  • projectile: resolve u into ux, uy

Recap

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

A definition is a test you can run
Motion in a plane is a test: a change that needs two directions (across and up) plus a vector add.
Name the given before the unknown
The given are the two components (or u, angle, g).
One worked case is enough at this class
One two-part add or one projectile-sketch is enough.
A miss: swapping the school name for the picture
A throw-photo is a setting. The science is two components. A smile is not uy.
Check by the opposite action or the opposite test
Check a resultant by swapping add-order (vectors commute) or by a 3-4-5 rebuild.
Keep the claim at this chapter, not the next
This chapter’s claim stops at two directions.

Practise Motion in a Plane

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