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CBSE Class 11 · Physics

Motion in a Straight Line

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 Straight Line” 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

Straight-line motion is a test: a change of position on one agreed axis plus a signed displacement. Distance is the walk; displacement is start-to-now with a sign. 4 m forward then 4 m back is 8 m distance and 0 displacement.

Calling every walk “displacement 8 m” is the sign-steal.

Figure. One cart on a straight track, one net force. The test is F = ma: 6 kg and 4 m/s² demand 24 N. The force and acceleration arrows are labelled, not drawn on one scale.

How it works

  1. Agree the line and the positive wayThe axis.
  2. Name walk-length versus signed changeThe two measures.
  3. Then the word motion has contentThe definition ran.

2Name the given before the unknown

The given are u, a, t or the two positions. The unknown is v or s. Copy the signs before you add. For u=5 m/s, a=2 m/s², t=4 s: v=13 m/s, s=36 m if a is constant.

Taking 5 m/s as already the finish is a silent given.

Figure. Write the given mass and acceleration before asking for F. 2 kg and 5 m/s² go in first; F = ma is last and equals 10 N.

How it works

  1. Copy u, a, t or the positionsThe given.
  2. Name v or sThe unknown.
  3. Then pick v=u+at or s=ut+½at²Given first.

u=5 m/s, a=2 m/s², t=4 s

Constant a on a line. Find v and s.

  • v=u+at5+8=13 m/s
  • s=ut+½at²20+16=36 m
  • Check v²=u²+2as169=25+144

Pro tip. Constant a; pick the equation that holds the unknown.

3One worked case is enough at this class

One constant-a case is enough. A page of the same 5-2-4 clone does not add a new sign-story.

Ten twins of 36 m are still one idea.

Figure. A v–t graph of constant a = 4 m/s² from rest. The series starts at the origin (t = 0, v = 0) and reaches v = 12 m/s at t = 3 s. Slope is rise 12 over run 3. The 3 kg cart that makes this slope needs F = 12 N. Arrows in other figures are not this graph.

How it works

  1. Run one line-caseThe case.
  2. A second only as a checkOptional.
  3. StopThis class.

4A miss: swapping the school name for the picture

A race-photo is a setting. The science is a line, signs, and a constant-a pack. A cheer is not v.

A curved track is the next chapter’s plane, not this line.

Figure. Action and reaction sit on two objects and have equal length here. Drawing F and −F on one free-body as if they cancel is the miss. The two pair arrows are to scale with each other; they are not to scale with the one-body arrows.

How it works

  1. Keep the race as a storyA setting.
  2. Write the axis and u, a, tThe science.
  3. Then v or sDo not swap.

5Check by the opposite action or the opposite test

Check v with a second kinematic equation, or check displacement by walking the signed changes back. Adding 5+2+4 as if they were one kind is not a check.

A distance that ignores the return is not the opposite test for displacement.

Figure. The opposite test of F = ma is a = F/m. 12 N on 3 kg gives a = 4; multiplying back by 3 kg must rebuild 12 N. A check that fails means the first step was wrong.

How it works

  1. State 13 m/s or 0 displacementThe claim.
  2. Rebuild with the other equation or the signed walkThe check.
  3. Mend if they disagreeHonest.

6Keep the claim at this chapter, not the next

This chapter’s claim stops at one axis. It does not steal plane-vectors or projectiles, and it does not start Newton’s laws.

A page that already “adds two perpendicular vs” here misses motion in a plane.

Figure. This chapter’s motion is along one straight line. Position is a mark on that line; x = 3 m is the displacement from the origin, not a path in a plane. The next chapter is the plane.

How it works

  1. Keep one line and the kinematic packThis chapter.
  2. Leave two directions for the plane chapterThe next map.
  3. Do not treat the map as already answeredClaim-size.

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

A bus-story is the hook (it sped up). The test is still constant-a equations or a signed displacement. A story with no axis is a poster.

“It went fast” with no u, a, t is the miss.

Figure. The story (someone pushed) is the hook. The test is the measured pair: 2 kg and 6 m/s², which demand 12 N. A push with no numbers is not yet the idea.

How it works

  1. Hear the busThe hook.
  2. Write the axis and the numbersThe test.
  3. Then v or sThe idea.

8Units or labels must match before you compare

Units or labels must match before you compare: 13 m/s is not “13” next to a 36 m. v-labels and s-labels are different kinds. A graph’s slope is m/s only if the axes are position and time as taught.

Calling the 36 a speed because it is larger is a unit-swap.

Figure. 12 N may be compared with kg m/s² because those labels name the same unit. Comparing 12 N with 2 kg mixes force and mass. Match labels before you compare.

How it works

  1. Write v with m/s and s with mThe labels.
  2. Refuse a raw-digit compare across kindsThe match.
  3. Then read a graph only after axes are namedHonest.
5 m/s with a=2 m/s² for 4 s gives v
  1. 13 m/s
  2. 36 m/s
  3. 9 m/s

u+at=5+8.

Notes

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

  • v=u+at
  • s=ut+½at²
  • v²=u²+2as (constant a, one line)

Recap

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

A definition is a test you can run
Straight-line motion is a test: a change of position on one agreed axis plus a signed displacement.
Name the given before the unknown
The given are u, a, t or the two positions.
One worked case is enough at this class
One constant-a case is enough. A page of the same 5-2-4 clone does not add a new sign-story.
A miss: swapping the school name for the picture
A race-photo is a setting. The science is a line, signs, and a constant-a pack. A cheer is not v.
Check by the opposite action or the opposite test
Check v with a second kinematic equation, or check displacement by walking the signed changes back.
Keep the claim at this chapter, not the next
This chapter’s claim stops at one axis. It does not steal plane-vectors or projectiles, and it does not start Newton’s laws.

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