📋 This chapter has a new name
Force and Laws of Motion → Chapter 6: How Forces Affect Motion
CBSE Class 9 Science now runs on an entirely new NCERT textbook, Exploration. The physics is the same — Newton's three laws, inertia, momentum — but it's taught with a stronger conceptual framing and real-world Indian-context examples, and basic Gravitation has been folded in here rather than kept as its own chapter. Any resource still using the old chapter name is describing the discontinued textbook.
This guide covers the current, confirmed standard-level syllabus for this chapter — 7 verified numericals and the conceptual questions aligned with the current CBSE curriculum — with clear boundaries on what belongs to the optional Advanced Level track instead.
What's In Scope — And What Isn't
✓ Standard Syllabus (This Guide)
- Force, net force, balanced & unbalanced forces
- Newton's First Law (inertia)
- Newton's Second Law (F = ma)
- Newton's Third Law (action-reaction)
- Momentum and conservation of momentum
- Friction (qualitative + basic force balance)
Optional Advanced Level Only
- Pseudo force
- Variation of g with height & depth
- Torque
- Full gravitation treatment
If you're taking the standard Science course rather than Science at Advanced Level, the topics in the Advanced Level column aren't part of this guide's standard syllabus scope — don't spend time on them unless your school has confirmed that you're following the Advanced Level course.
7 Verified Numericals
F = ma
A 5 kg object accelerates at 4 m/s² when a force is applied. Find the force.
Reveal Solution
F = ma = 5 × 4
✓ F = 20 N
Net Force
Two forces of 15 N and 10 N act on an object. Find the net force when they act (a) in the same direction, and (b) in opposite directions.
Reveal Solution
(a) Same direction: forces add → 15 + 10
(b) Opposite directions: forces subtract → 15 − 10
✓ (a) 25 N, (b) 5 N (in the direction of the larger force)
Momentum
A 2 kg object moves at 10 m/s. Find its momentum.
Reveal Solution
p = mv = 2 × 10
✓ p = 20 kg·m/s
Force from Change in Momentum
A bat strikes a stationary 0.5 kg ball, sending it off at 20 m/s in 0.01 seconds. Find the average force exerted by the bat.
Reveal Solution
Use F = change in momentum / time
Change in momentum = m(v−u) = 0.5 × (20−0) = 10 kg·m/s
F = 10 / 0.01
✓ F = 1000 N
⚠️ This large number is realistic — very short contact times (like a bat hitting a ball) produce very large forces even for a modest change in momentum, since time is in the denominator.
Conservation of Momentum — Collision
A 4 kg object moving at 6 m/s collides with a stationary 2 kg object, and they stick together. Find their common velocity after collision.
Reveal Solution
Total momentum before = total momentum after
m₁u₁ + m₂u₂ = (m₁+m₂)v
v = (4×6 + 2×0) / (4+2) = 24/6
✓ v = 4 m/s
Conservation of Momentum — Recoil
A 0.02 kg bullet is fired from a 4 kg gun at 300 m/s. Find the recoil velocity of the gun.
Reveal Solution
Total momentum before firing = 0 (both at rest)
So momentum of bullet forward = momentum of gun backward
mgun × vgun = mbullet × vbullet → vgun = (0.02 × 300)/4
✓ vgun = 1.5 m/s (in the direction opposite to the bullet)
Friction & Net Force
A 10 kg crate is pushed with a force of 50 N across a floor, with friction opposing it at 20 N. Find the crate's acceleration.
Reveal Solution
Net force = applied force − friction = 50 − 20 = 30 N
a = Fnet/m = 30/10
✓ a = 3 m/s²
Important Conceptual Questions
What is the difference between balanced and unbalanced forces?
When forces on an object combine to a net force of zero, they're balanced, and the object's motion doesn't change. When the net force isn't zero, the forces are unbalanced, and the object accelerates in the net force's direction — the direct link between Newton's first and second laws.
Why does a passenger lurch forward when a bus suddenly brakes?
By Newton's first law, the passenger's body tends to continue in its state of motion (moving forward) due to inertia, even though the bus itself has stopped. There's no force acting directly on the passenger to stop them at the same rate as the bus, so they lurch forward relative to the now-stationary bus.
Why does a gun recoil when a bullet is fired?
Before firing, the gun-bullet system's total momentum is zero. When the bullet gains forward momentum, the gun must gain exactly equal and opposite momentum for the total to stay zero, so it recoils backward. Since the gun's mass is far larger than the bullet's, its recoil velocity is correspondingly much smaller.
Why is it harder to push a loaded cart than an empty one, for the same acceleration?
By Newton's second law, F = ma — a greater mass requires a proportionally greater force to produce the same acceleration. The loaded cart's larger mass directly demands more force for identical acceleration.
Give an everyday example of Newton's third law and explain the force pair.
When swimming, a swimmer pushes water backward with their hands and legs; by Newton's third law, the water pushes the swimmer forward with an equal and opposite force, propelling them through the water. The two forces act on different objects (swimmer pushes water; water pushes swimmer) and never cancel each other.
Numericals and Concepts Are Different Skills
A student who can solve F = ma correctly might still struggle to explain why a passenger lurches forward on sudden braking, or vice versa. Since this chapter blends calculation with conceptual reasoning more than most, tracking both separately matters.
What a Genelis weak area map looks like for this chapter
Next session: conservation of momentum (38%) — not more basic F=ma drilling. Genelis tracks calculation and conceptual understanding as separate skills.
Genelis is an AI-powered personalized learning platform built on Adaptive Personalized Intelligence. The Genelis learning system tracks your accuracy against the current 2026-27 Exploration syllabus specifically — not the discontinued old chapters — and logs every wrong answer to your wrong-question notebook for reattempt.
Learn smarter. Practice deeper. Improve continuously.
Genelis combines Adaptive Personalized Intelligence, AI-generated notes, targeted practice, mock tests, analytics, and personalised revision to help students improve every study session.
Questions Students Commonly Ask
Quick answers to the most common questions related to this guide.
Is Force and Laws of Motion still a chapter in Class 9 Science?
Not under that name. CBSE Class 9 Science now uses an entirely new NCERT textbook called Exploration, and the content previously taught as Force and Laws of Motion is now Chapter 6, titled How Forces Affect Motion. The core physics — Newton's three laws, inertia, momentum, balanced and unbalanced forces — is the same material, taught with a stronger conceptual framing and more real-world examples, but any resource still labelled with the old chapter name is referring to the discontinued textbook.
Is Gravitation still part of this chapter for Class 9?
Only partially, and not as a separate named topic. Gravitation used to be its own standalone chapter in the old Class 9 syllabus; under the new structure, basic gravitational concepts are woven into How Forces Affect Motion rather than given a dedicated chapter. Deeper gravitation content — including how g varies with height and depth — along with pseudo force and torque, is covered in Science at Advanced Level rather than the standard Science syllabus.
What is the difference between balanced and unbalanced forces?
When the forces acting on an object combine to a net force of zero, they are balanced, and the object's state of motion doesn't change — it stays at rest or continues moving at constant velocity. When the net force is not zero, the forces are unbalanced, and the object accelerates in the direction of the net force. This distinction is the direct bridge between Newton's first law (no change in motion without a net force) and Newton's second law (a net force produces a proportional acceleration).
Why does a gun recoil when a bullet is fired?
This follows from conservation of momentum. Before firing, the gun and bullet together are at rest, so their total momentum is zero. When the bullet is fired forward, it gains momentum in one direction — for the total momentum of the system to remain zero, the gun must gain exactly equal and opposite momentum, which it does by recoiling backward. Because the gun's mass is much larger than the bullet's, its recoil velocity is correspondingly much smaller than the bullet's forward velocity.