NCERT Class 9 Science How Forces Affect Motion – Newton’s Laws Explained

Short Intro:
This chapter explores how forces affect motion, focusing on Newton’s three laws, friction, and real-world applications. Learn how balanced and unbalanced forces influence objects, and practice step-by-step solutions for various problems.

Quick Information Box:

  • Chapter: 6 – How Forces Affect Motion
  • Class: 9
  • Key Topics: Force, Friction, Newton’s Laws of Motion, Acceleration, Mass, Systems of Objects
  • Unit of Force: Newton (N)
  • Key Concept: Force produces acceleration and occurs in action-reaction pairs

Concepts Used (Topics Covered):

  • Definition of Force
  • Measuring Force with Spring Balance
  • Balanced and Unbalanced Forces
  • Friction: Static and Kinetic
  • Newton’s First Law (Law of Inertia)
  • Newton’s Second Law (F = ma)
  • Newton’s Third Law (Action-Reaction)
  • Systems of Objects and Tension Forces

Important Formulas:

  • Net Force: Fnet=maF_\text{net} = maFnet​=ma
  • Gravitational Force: F=mgF = mgF=mg
  • Acceleration from Kinematics: a=v−uta = \frac{v-u}{t}
  • Force from Momentum: F=ΔpΔtF = \frac{\Delta p}{\Delta t}

Questions & Step-by-step Solutions:

  1. Net Force on a Block:

    • Two forces: 10 N and 6 N acting on a block.
    • Case (a): Same direction → Fnet=10+6=16 NF_\text{net} = 10 + 6 = 16\,N (right)
    • Case (b): Opposite direction → Fnet=10−6=4 NF_\text{net} = 10 – 6 = 4\,N (right)
    • Case (c): Opposite direction → Fnet=10−6=4 NF_\text{net} = 10 – 6 = 4\,N (left)

  2. Frictional Force:

    • If applied force < friction → object stationary
    • If applied force > friction → object accelerates
    • Use Fnet=Fapplied−FfrictionF_\text{net} = F_\text{applied} – F_\text{friction}

  3. Newton’s First Law:

    • Object moves with constant velocity if net force = 0.
    • Example: Force on moving box equals friction → box continues at constant speed

  4. Newton’s Second Law Problems:

    • Example: Push a 25 kg block with net force 5 N → a=F/m=0.2 m/s2a = F/m = 0.2\, m/s^2
    • Displacement in 2 s: s=ut+0.5at2=0+0.5∗0.2∗4=0.4 ms = ut + 0.5at^2 = 0 + 0.5*0.2*4 = 0.4\, m

  5. Sports Car Force Calculation:

    • Mass 1500 kg, v=0→10 m/sv = 0 \to 10\, m/s in 5 s → a=2 m/s2a = 2\, m/s^2
    • F=ma=1500∗2=3000 NF = ma = 1500*2 = 3000\,N

  6. Newton’s Third Law Applications:

    • Pushing a table while sitting on a chair → chair moves backward
    • Rocket launches: gas expelled downward → rocket moves upward
    • Forces equal in magnitude but act on different objects

  7. System of Two Boxes:

    • External force FFF on combined mass → a=F/(m1+m2)a = F/(m_1 + m_2)

Common Mistakes:

  • Forgetting friction acts opposite to motion
  • Confusing balanced vs unbalanced forces
  • Neglecting mass when calculating acceleration
  • Misinterpreting action-reaction pairs

Exam Tips:

  • Always draw a free-body diagram
  • Identify all forces before applying Newton’s laws
  • Check the direction of net force
  • Use SI units consistently

Practice MCQs:

  1. Two forces 4 N and 5 N act on block P in opposite directions. Net force? (Answer: 1 N)
  2. A 0.1 kg bullet fired from 5 kg gun → recoil acceleration of gun? (Answer: a=F/m=0.4 m/s2a = F/m = 0.4\, m/s^2
  3. Frictionless surface, net force = 0 → velocity? (Answer: constant)

FAQ Section:

  • Q: Does a stationary object require a force to remain stationary?
    A: No, only to overcome friction or initiate motion.
  • Q: Can acceleration be zero if a force is applied?
    A: Only if net force = 0.