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NCERT Solutions for Class 9th Science Chapter 6 How Forces Affect Motion

Updated on 2026-09-19

About this chapter

A force has magnitude and direction . Its SI unit is the newton (N), measured with a spring balance. One newton is the force that gives a 1 kg object an acceleration of 1 m s –2 . Forces equal in magnitude and opposite in direction are balanced ; the net force is zero. Unbalanced forces give a non-zero net force — added when the forces point the same way, subtracted when they oppose. Motion depends only on the net force. Friction always acts opposite to the direction of motion (or attempted motion). It depends on the nature of the surfaces in contact. Without it, a moving object would never slow down on its own. Newton's first law: an object at rest stays at rest and an object in motion keeps moving with constant velocity unless a net force acts on it. Zero net force means zero acceleratio

  • Chapter opener
  • .2 Balanced and Unbalanced Forces
  • .4 Newton's First Law of Motion
  • .5 Newton's Second Law of Motion
  • .6 Newton's Third Law of Motion
  • .7 Forces Acting on a System of Objects
  • End-of-chapter exercise
  • Project work
Quick revision
TermWhat it meansFormula or key ruleWhere it appears in the chapter
ForceA push or pull; a vector quantity — magnitude, direction and unit must all be statedSI unit newton (N); 1 N = 1 kg m s–2Section 6.1; Fig. 6.1 kicking, striking, squeezing
Spring balanceInstrument that measures the magnitude of a force from the stretch of a springReading in N; can measure weight or any pullFig. 6.2; Activities 6.2 and 6.6
Balanced forcesEqual in magnitude, opposite in direction; net force zeroNet force = 0 → acceleration = 0Tug of war, Fig. 6.4a; weight and normal force
Net forceThe single force that has the same effect as all the forces acting togetherSame direction: F1 + F2; opposite: |F1 – F2| along the largerExample 6.1; Fig. 6.5, Fig. 6.6
Force of frictionContact force opposing relative motion between two surfacesDepends on the nature of the surfaces; measured by the spring balance reading just as motion startsSection 6.3; Activities 6.1, 6.2
Normal forcePush of a surface on the object resting on it, perpendicular to the surfaceBalances the weight on a horizontal surfaceFig. 6.11
InertiaTendency of an object to resist a change in its state of rest or of uniform motionNewton's word; the idea behind the first lawMeet a Scientist, page 100
Newton's first lawNo net force → no change in velocitya = 0 when net F = 0Section 6.4; Examples 6.2, 6.3
Newton's second lawNet force produces acceleration along itselfa = F/m, i.e. F = maSection 6.5; Eqs. 6.1, 6.2
WeightGravitational force with which the Earth pulls an objectF = mg, g = 9.8 m s–2 (10 m s–2 for quick estimates)Eq. 6.3; Example 6.4
Newton's third lawForces occur in pairs on two different objects, equal and oppositeFAB = – FBA, acting on different bodiesSection 6.6; Activities 6.5, 6.6, 6.7
MomentumMass × velocity; the fuller form of the second law uses its rate of changeDirection same as velocity; works even when mass changesReady to Go Beyond, page 104
System of objectsTwo or more connected bodies treated as one objecta = F ÷ (m1 + m2); internal forces ignoredSection 6.7; Eq. 6.4, Figs. 6.34, 6.35
Increasing the collision timeLonger stopping time → smaller acceleration → smaller forceF = ma with a = (v – u)/tCatching a ball, airbags, landing mats, bubble wrap
Read the chapter

Exercises

  1. Chapter opener — Think It Over Page 94
  2. .2 Balanced and Unbalanced Forces — In-text Questions Page 956
  3. .2 Balanced and Unbalanced Forces — Pause and Ponder Page 976
  4. .3 The Force of Friction: Often Overlooked but Always Present — In-text Questions Page 976
  5. .3 The Force of Friction: Often Overlooked but Always Present — What if … Page 986
  6. .3 The Force of Friction: Often Overlooked but Always Present — Activity 6.1: Let us investigate Page 986
  7. .3 The Force of Friction: Often Overlooked but Always Present — In-text Questions Page 996
  8. .3 The Force of Friction: Often Overlooked but Always Present — Activity 6.2: Let us measure Page 996
  9. .3 The Force of Friction: Often Overlooked but Always Present — Think as a Scientist Page 1006
  10. .4 Newton's First Law of Motion — Pause and Ponder Page 1016
  11. .5 Newton's Second Law of Motion — Think as a Scientist Page 1026
  12. .5 Newton's Second Law of Motion — Activity 6.3: Let us experiment (Demonstration activity) Page 1026
  13. .5 Newton's Second Law of Motion — Think as a Scientist Page 1036
  14. .5 Newton's Second Law of Motion — Activity 6.4: Let us experiment (Demonstration activity) Page 1036
  15. .5 Newton's Second Law of Motion — Threads of Curiosity Page 1046
  16. .5 Newton's Second Law of Motion — Pause and Ponder Page 1066
  17. .6 Newton's Third Law of Motion — Activity 6.5: Let us explore Page 1076
  18. .6 Newton's Third Law of Motion — Activity 6.6: Let us verify Page 1086
  19. .6 Newton's Third Law of Motion — Activity 6.7: Let us understand Page 1096
  20. .6 Newton's Third Law of Motion — In-text Questions Page 1096
  21. .6 Newton's Third Law of Motion — Pause and Ponder Page 1106
  22. .7 Forces Acting on a System of Objects — In-text Questions Page 1116
  23. End-of-chapter exercise — Revise, Reflect, Refine Page 112 – 114
  24. Project work — The Journey Beyond Page 115
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