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Ask a friend why a cycle slows down when you stop pedalling, and you may hear "because the force runs out". That answer feels right and is wrong, and it sits behind many mistakes in this chapter. This post sets out what the 2026–27 syllabus covers, explains each law with numbers, works through three problems and finishes with a practice routine.
What does the new Class 9 chapter cover?
In the new NCERT book *Exploration*, this topic is Chapter 6, How Forces Affect Motion. The CBSE Class IX syllabus for 2026–27 lists balanced and unbalanced forces, the force of friction and Newton's three laws, and asks you to calculate force from F = ma and to define the newton.
One change from older books: the 2025–26 syllabus listed inertia and mass, and momentum, as separate items. In the new chapter, momentum (mass × velocity) appears only in a "Ready to Go Beyond" box, as the more complete form of the second law. Read it, but ask your teacher how it will be assessed this year.
Why does a moving object slow down?
Because of friction, not because a force "runs out". NCERT's coin-stack activity shows this: a stack of coins flicked by a rubber band slides further on a polished floor than on a wooden table, because friction is smaller there. Galileo argued with thought experiments that if every obstacle to motion were removed, an object would keep moving. Newton's first law states it: an object at rest stays at rest, and an object in motion keeps moving with constant velocity, unless a net force acts on it. The tendency to resist a change in motion is called inertia.
The wrong idea is stubborn. Clement (1982) used written tests and interviews with physics students and found that many held a stable, alternative view of how force and acceleration are related. He called it "highly resistant to change" and showed it was close to a view Galileo had argued against. Our post on the Force Concept Inventory shows how common these ideas are.
How do you use F = ma?
The second law says the acceleration is in the direction of the net force, proportional to it and inversely proportional to mass: a = F/m. One newton is the force that gives 1 kg an acceleration of 1 m/s². Weight is a force too: F = mg, with g = 9.8 m/s² (10 m/s² for quick estimates).
Example 1: pushing against friction. You push a 20 kg box with 60 N. Friction is 40 N.
Net force = 60 − 40 = 20 N forward.
a = 20/20 = 1 m/s².
From rest, after 3 s: v = u + at = 3 m/s and s = ½at² = ½ × 1 × 9 = 4.5 m.
If you push with exactly 40 N, the net force is zero. A box at rest stays at rest; a box already moving keeps its velocity.
Example 2: connected boxes. Boxes of 3 kg and 2 kg are tied by a string on a frictionless floor. You pull the 3 kg box with 15 N.
Treat both as one system: a = 15/(3 + 2) = 3 m/s².
Tension pulling the 2 kg box: T = 2 × 3 = 6 N.
Check on the 3 kg box: net force = 15 − 6 = 9 N, and 3 × 3 = 9 N.
NCERT uses this "system" idea in Section 6.7: internal forces like the tension cancel out when you look at the whole.
Why don't action and reaction cancel?
Newton's third law says that when one object pushes on a second, the second pushes back on the first with an equal and opposite force at the same time. The two forces act on *different* objects, so they cannot cancel each other. Balanced forces act on the *same* object.
Example 3: two skaters. A 40 kg student and a 60 kg student stand on skates and push each other apart with a force of 120 N.
Each feels 120 N, in opposite directions.
Accelerations: 120/40 = 3 m/s² and 120/60 = 2 m/s².
Equal forces, unequal accelerations, because the masses differ. This is why a gun recoils far less than its bullet moves, and why the Earth does not visibly move towards a falling fruit.
For the original wording of the three laws, read our post on what Newton's laws said in the Principia.
Which mistakes come up most?
"A force keeps it moving." A net force changes motion. Zero net force means constant velocity, which includes being at rest.
Cancelling action-reaction pairs. Check which object each force acts on before adding them.
Mass in newtons, weight in kilograms. Mass is in kg, weight is a force in N. A 50 kg student weighs about 490 N.
Forgetting friction's direction. Friction opposes sliding. When you walk, your foot pushes the ground backwards, so friction on your foot points forwards.
Using F = ma with one force instead of the net force.
If you keep choosing the wrong force to put into F = ma, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can compare your free-body sketch with each step.
A practice routine
For every problem, draw the object and every force on it with arrows before writing an equation.
Write "net force = ..." as a separate line each time.
Do the chapter-end questions on snake-boat rowers and the bullet in the wooden block, then make up one similar problem of your own.
Explain to someone at home why the canoe moves forward when the paddle pushes water back. If you can say which object each force acts on, you have the third law.
Today, pick any moving object around you, list every force on it, and decide whether the forces are balanced.
