Chapter-6-How-Forces-Affect-Motion.pdf/Grade 9 Science · Curiosity Series · A journey through Newton's Laws Prepared by K Sandeep Swamy (M.Sc, B.Ed) For ONLINE Classes (ACADEMIC,IITJEE & NEET ) Contact 9491878325

Chapter-6-How-Forces-Affect-Motion.pdf/Grade 9 Science · Curiosity Series · A journey through Newton's Laws Prepared by K Sandeep Swamy (M.Sc, B.Ed) For ONLINE Classes (ACADEMIC,IITJEE & NEET ) Contact 9491878325

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  Chapter 6: HowForces Affect Motion Grade 9 Science · Curiosity Series · A journey through Newton's Laws Prepared by K Sandeep Swamy (M.Sc, B.Ed) For ONLINE Classes (ACADEMIC,IITJEE & NEET ) Contact 9491878325
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  Why Does aCanoe Move Forward? Push Water Backward When a canoeist pushes water backwards, the canoe moves forward — an everyday action- reaction pair. Force Affects Speed Pushing harder makes the canoe move faster; a heavier canoe moves slower with the same push. Gateway to Newton's Laws These simple observations lead us directly to Newton's three laws of motion — the foundation of classical mechanics.
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  SECTION 6.1 The Conceptof Force Understanding what a force is, how it is measured, and what it can do to objects around us.
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  What Can aForce Do? A force is a push or pull that can change an object's state in three ways: Start Motion Make a stationary object move — e.g., kicking a football from rest. Change Motion Change the speed or direction of a moving object — e.g., a cricket bat hitting a ball. Change Shape Deform an object — e.g., squeezing a lemon or compressing a spring.
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  Force: Magnitude, Direction& Unit Force is a vector quantity — it requires both a magnitude and a direction to be fully described. SI unit: newton (symbol: N) — named after Isaac Newton Definition: 1 N = force that gives a 1 kg object an acceleration of 1 m s⁻² Instrument: Measured using a spring balance Force = Vector quantity → Always state both size and direction!
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  SECTION 6.2 Balanced andUnbalanced Forces What happens when two forces act on the same object — and what determines whether it moves?
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  Balanced Forces: NoNet Effect When two forces are equal in magnitude but opposite in direction, they cancel out and produce no change in motion. Balanced forces → no acceleration → no change in state of motion. Tug of War If both teams pull with equal force, the rope stays perfectly still — net force = zero. Ball on Water Gravitational force (downward) = Buoyant force (upward) → ball floats without sinking.
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  Unbalanced Forces: NetForce Causes Motion Opposite Directions Net force = difference of the two forces; direction = direction of the larger force. Same Direction Net force = sum of both forces; direction = common direction of both forces. Key Rule An object's motion depends only on the net force acting on it — not individual forces separately.
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  Worked Example: NetForce on a Block Three cases of forces applied to the same block — let's calculate the net force in each: 1 Case (a): Same Direction 10 N + 6 N → Net force = 16 N to the right 2 Case (b): Opposite, 10 N wins 10 N right − 6 N left → Net force = 4 N to the right 3 Case (c): Opposite, 10 N wins 6 N right − 10 N left → Net force = 4 N to the left
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  SECTION 6.3 The Forceof Friction Friction is everywhere — it opposes motion, but it also makes motion possible in the first place.
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  Friction: Always Opposing Motion Frictionis a contact force that acts between surfaces, always directed opposite to the direction of motion. An object starts moving only when applied force exceeds friction Once the applied force is removed, friction gradually brings the object to rest Friction depends on the nature of the surfaces in contact Friction always opposes motion — it never acts in the direction of movement!
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  Friction Depends onSurface Type Smooth Surfaces → Less Friction Objects travel farther before stopping. E.g., a coin on polished marble slides a long way. Rough Surfaces → More Friction Objects stop sooner. E.g., a coin on rough wood stops quickly after a short distance. Activity 6.2: Measure It! A spring balance pulled along different surfaces shows a smaller reading for smoother surfaces — confirming less friction.
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  Galileo's Thought Experiment "Ifall friction could be removed, a moving object would continue moving forever." — Galileo Galilei, 17th century Ancient belief: A continuous force is needed to keep an object moving Galileo's insight: It is friction that stops objects — not the absence of force Remove friction entirely → motion continues indefinitely This revolutionary idea paved the way for Newton's First Law
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  SECTION 6.4 Newton's FirstLaw of Motion Published in 1687 — the law that changed how humanity understands motion forever.
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  The Law ofInertia Newton's First Law: An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon it. Net Force = 0 Acceleration = 0. No change in speed and no change in direction. Inertia The tendency of every object to resist any change in its state of motion — rest or uniform motion. Published 1687 Isaac Newton published all three laws in his landmark work Principia Mathematica.
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  First Law: Graphs& Examples The First Law shows up clearly on motion graphs: Object at Rest Position-time graph: flat horizontal line. Velocity-time graph: flat line at zero. Constant Velocity Position-time: straight sloped line. Velocity-time: flat line at constant value. Practical Example A box pushed with force exactly equal to friction → net force = 0 → moves at constant velocity.
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  SECTION 6.5 Newton's SecondLaw of Motion How does the amount of force — and the mass of an object — determine how quickly it accelerates?
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  Force, Mass &Acceleration Newton's Second Law When a net force acts on an object, it accelerates in the direction of the net force. Acceleration is proportional to force and inversely proportional to mass. F = ma Force (N) = Mass (kg) × Acceleration (m s⁻²) What This Means More Force → More Acceleration For the same mass, doubling the net force doubles the acceleration. More Mass → Less Acceleration For the same force, doubling the mass halves the acceleration.
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  Defining the Newton 1N One Newton Force that produces 1 m s⁻² acceleration on a 1 kg object. This is the SI unit of force. 9.8 g (m s⁻²) Acceleration due to gravity near Earth's surface. Used in F = mg for gravitational force. ~1 N Practical Feel Holding a 100 g mass in your palm requires approximately 1 N of upward force. Key insight: g does NOT depend on the mass of the falling object — all objects fall with the same gravitational acceleration in the absence of air resistance.
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  Worked Example: SportsCar (Example 6.6) Mass of sports car = 1500 kg. Use the velocity-time graph to find the net force at each stage. 1 0–5 s: Accelerating v: 0→10 m s⁻¹ → a = 2 m s⁻² → F = 1500 × 2 = 3000 N east 2 5–10 s: Constant Velocity Velocity unchanged → a = 0 → No net force 3 10–15 s: Decelerating v: 10→0 m s⁻¹ → a = −2 m s⁻² → F = 3000 N west
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  Second Law inReal Life: Reducing Injury F = ma tells us: if we increase the time over which momentum changes, the force is reduced. This principle saves lives. 🏏Cricket Fielder Pulls hands back while catching — increases stopping time → force on hands is greatly reduced. 🚗Airbags Inflate on collision → increase stopping time → reduce the force on the passenger's body. 🥥Cracking Coconut Brought down fast → stops in very short time → ground exerts huge force, breaking the shell.
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  SECTION 6.6 Newton's ThirdLaw of Motion For every action, there is an equal and opposite reaction — perhaps the most surprising law of all.
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  Every Action Hasan Equal & Opposite Reaction Newton's Third Law: Whenever one object exerts a force on a second object, the second simultaneously exerts an equal and opposite force on the first. Forces always occur in pairs — but act on two different objects The two forces are equal in magnitude and opposite in direction Example: Kicking a ball — foot pushes ball forward; ball pushes foot backward Action and reaction forces NEVER cancel each other — they act on different objects!
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  Third Law: Walking,Cycling & Climbing Walking Foot pushes ground backward → friction pushes person forward. Cycling Feet push ground back → ground pushes bicycle forward. Climbing a Tree Person pushes trunk down → friction pushes person up. Rowing a Canoe Paddle pushes water back → water pushes canoe forward.
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  Third Law: Rockets& Balloons The Third Law operates even without any physical contact between the propelling object and the medium: No wheels, no legs, no paddles needed — action-reaction works in open space! 🚀Rocket Engine expels hot gas downward at great speed → gas pushes rocket upward with equal force. 🎈Balloon on a Thread Air rushes out backward from the nozzle → balloon moves forward along the thread.
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  Third Law: Non-Contact Forces Action-reactionpairs exist even when objects never touch each other: 🧲Magnets Two bar magnets repel each other with equal and opposite forces — even through empty space. ⚡Charged Balloons Like charges repel — each balloon experiences an equal force pushing it away from the other. 🍎Gravity Earth pulls fruit downward; fruit pulls Earth upward — equal forces, but Earth barely moves due to its enormous mass.
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  Chapter Summary: Newton'sThree Laws Together, these three laws explain virtually all motion we observe in daily life. Law Statement Key Idea Formula 1st Law (Inertia) No net force → no change in motion. Rest stays at rest; motion stays constant. Objects resist changes to their state of motion — this resistance is called inertia. F_net = 0 → a = 0 2nd Law (F = ma) Net force causes acceleration in the direction of the force. Acceleration is proportional to force and inversely proportional to mass. F = ma 3rd Law (Action- Reaction) Every force has an equal and opposite reaction force on the other object. Forces always come in pairs acting on two different objects — never the same one. F₁₂ = −F₂₁ Master these three laws and you hold the keys to understanding ALL of classical mechanics! 🎓