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CBSE Class 9 · Science

How Forces Affect Motion

Official NCERT chapter from Science (Grade 9, 2026 edition) (book code iesc1). ExamMaster notes are original teaching at CBSE Class 9 depth.

This lesson follows the official chapter “How Forces Affect Motion” in Science (Grade 9, 2026 edition). 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 9
  • Medium level
  • 8 concepts

1Newton’s First Law of Motion

Newton’s first law: a body stays at rest, or keeps uniform straight motion, unless a net force changes that. Inertia is that staying-habit. A book on a table staying put is the rest-case; a puck that keeps sliding (as a thought) is the motion-case. A force is what changes the habit, not what “keeps it moving” as a push-story you never named.

“Force is needed to keep moving” without friction named is the everyday miss.

Figure. First law: if the net force is zero, the block keeps its velocity — rest stays rest, a slide stays a slide. Force is not required to keep moving.

How it works

  1. Name rest or uniform straight motionThe habit.
  2. Ask whether a net force is presentThe unless.
  3. Call the staying-habit inertiaThe first law.

2Newton’s Second Law of Motion

Newton’s second law at this class: a net force changes velocity; for a given mass, a larger net force gives a larger acceleration, a = F/m (or F = ma) as taught. 12 N on 3 kg is 4 m/s². Direction of a follows the net force.

Using F = ma with a force that is not the net force is a stolen F.

Figure. Second law: for the same mass, double the force doubles the acceleration. The 2F arrow is drawn twice the F arrow; 2a is twice a.

How it works

  1. Name the net force and the massThe given.
  2. Write a = F/mThe change of motion.
  3. Keep the unit N, kg, m/s²The second law.

12 N on 3 kg

A net force of 12 N acts on a 3 kg body. Find the acceleration.

  • a = F/m12/3
  • a4 m/s²
  • DirectionAlong the net force

Pro tip. Net force, not any one named push you forgot to add.

3Newton’s Third Law of Motion

Newton’s third law: forces come in a pair on two bodies — equal size, opposite way, same kind, as taught. You push the wall; the wall pushes you. The pair is not two forces on the same body cancelling.

Drawing both arrows on one block and calling it “third law cancelled” is the miss.

Figure. Third law: A’s push on B and B’s push on A are one pair — equal length, opposite arrows, on two bodies. They do not cancel on one body.

How it works

  1. Name body A and body BTwo objects.
  2. Name A-on-B and B-on-AThe pair.
  3. Equal, opposite, different bodiesThe third law.

4Forces Acting on a System of Objects

A system of objects is a boundary you choose: two blocks, or a cart plus a person. Forces inside the boundary are internal; a push from outside is external. The heading is “name the system”, not “more maths”. One pair of blocks is enough.

A universe-wide system with no boundary is not this class.

Figure. On a system of A and B, the internal pair cancels. Only the external force on the system changes the motion of the pair together.

How it works

  1. Draw a dashed boundaryThe system.
  2. Tag each force as in or outInternal versus external.
  3. Apply the laws to the system you namedHonest boundary.

5A definition is a test you can run

Force / inertia / pair is a test: name what would change the motion, or name the two bodies of a pair. If you only say “push”, you have a heading.

A superhero poster is not the test.

Figure. The definition is a test: add every force on the body. Zero net force means velocity stays; a leftover net force means velocity changes.

How it works

  1. Name the body and the net force or the pairThe object.
  2. Say what motion-habit changes, or that two bodies share a pairThe test.
  3. Then the word has contentThe definition ran.

6Name the given before the unknown

The given is the named push, mass, or the two bodies. The unknown is a, or “which law”. Copy the net force before you divide.

Using 12 N when 4 N friction was also given and unused is a silent given.

Figure. Name F and m before you ask for a. Second law is a = F/m; writing a first is naming the unknown as if it were given.

How it works

  1. Copy F_net and m, or the two namesThe given.
  2. Name a or the lawThe unknown.
  3. Then divide or pairGiven first.

7One worked case is enough at this class

One F = ma or one pair is enough. A page of the same 12-on-3 clone does not add a new third-law pair.

Ten twins of 4 m/s² are still one idea.

Figure. One worked case: 10 N on 2 kg gives a = 5 m/s^2. That is enough to see F = ma at this class; a second number set is not a new law.

How it works

  1. Run one net-force case or one A-on-B pairThe case.
  2. A second only as a checkOptional.
  3. StopThis class.

8A miss: swapping the school name for the picture

A tug-of-war photo is a setting. The science is two bodies and a pair, or a net force on one system. A cheer is not F.

A rope advert is not the third law.

Figure. Velocity is not a force. The force is the push that can change v. Swapping those school names for the arrow on the page is the miss.

How it works

  1. Keep the tug as a storyA setting.
  2. Name the system or the pairThe science.
  3. Then a or the opposite arrowDo not swap.
12 N net on 3 kg gives a
  1. 4 m/s²
  2. 36 m/s²
  3. 9 m/s²

a = F/m = 12/3.

Notes

  • Mapped to the official NCERT chapter “How Forces Affect 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

  • a = F_net / m
  • third law: pair on two bodies, equal and opposite

Recap

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

Newton’s First Law of Motion
Newton’s first law: a body stays at rest, or keeps uniform straight motion, unless a net force changes that.
Newton’s Second Law of Motion
Newton’s second law at this class: a net force changes velocity; for a given mass, a larger net force gives a larger acceleration, a = F/m (or F = ma) as taught.
Newton’s Third Law of Motion
Newton’s third law: forces come in a pair on two bodies — equal size, opposite way, same kind, as taught.
Forces Acting on a System of Objects
A system of objects is a boundary you choose: two blocks, or a cart plus a person.
A definition is a test you can run
Force / inertia / pair is a test: name what would change the motion, or name the two bodies of a pair.
Name the given before the unknown
The given is the named push, mass, or the two bodies.

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