Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 6. How Forces Affect Motion

CBSE Notes New Syllabus: Class 9

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Chapter 6. How Forces Affect Motion

This chapter explains how forces affect the motion of objects. It introduces the concept of force, balanced and unbalanced forces, friction, and Newton's laws of motion. Students learn how force changes the state of motion of an object and how these principles explain many everyday events such as walking, riding a bicycle, catching a cricket ball and using airbags in vehicles.

Chapter Review

• Force – A force can start or stop motion, change the speed or direction of an object, and even change its shape. 
• Force is a Vector Quantity – Every force has both magnitude and direction. A change in either changes its effect.
• SI Unit of Force – The SI unit of force is the newton (N).
• Measuring Force – The magnitude of force can be measured using a spring balance. 
• Multiple Forces – More than one force can act on an object at the same time, but the object's motion depends only on the net force. 
• Balanced Forces – Equal and opposite forces cancel each other, producing zero net force.
• Unbalanced Forces – Unequal forces produce a non-zero net force that changes the motion of an object. 
• Net Force – Forces acting in the same direction are added, while opposite forces are subtracted to obtain the resultant force. 
• Direction of Net Force – The net force always acts in the direction of the larger force.
• Force of Friction – Friction always acts opposite to the direction of motion and opposes movement. 
• Role of Friction – A moving object gradually slows down and finally stops because of friction if no external force is applied. 
• Factors Affecting Friction – The force of friction depends on the nature of the surfaces in contact. Smoother surfaces produce less friction. 
• Normal Force – A surface exerts an upward force called the normal force, which balances the weight of an object placed on it. 
• Newton's First Law of Motion – An object remains at rest or continues moving with constant velocity unless acted upon by a net force. 
• Constant Velocity – If the net force on an object is zero, its velocity remains constant.
• Inertia – Inertia is the tendency of an object to resist any change in its state of rest or uniform motion. 
• Galileo's Contribution – Galileo explained that in the absence of friction, a moving object would continue moving indefinitely. 
• Newton's Second Law of Motion – When a net force acts on an object, it accelerates in the direction of the force.
• Relationship Between Force and Acceleration – Greater force produces greater acceleration, whereas greater mass produces smaller acceleration. 
• Mathematical Formula – Force = Mass × Acceleration (F = ma). 
• One Newton – One newton is the force required to produce an acceleration of 1 m s⁻² in a body of mass 1 kg. 
• Weight of an Object – Weight is the gravitational force acting on an object and is given by F = mg. 
• Acceleration Due to Gravity – Near the Earth's surface, g ≈ 9.8 m s⁻² (approximately 10 m s⁻² for calculations). 
• Applications of Newton's Second Law – Catching a cricket ball by moving the hands backward and using airbags in vehicles reduce the force of impact by increasing the stopping time. 
• Everyday Applications of Force – Walking, pushing objects, cycling, rowing a canoe, arm wrestling, tug of war and driving vehicles all demonstrate the effects of force. 
• Important Formulae
  • F = ma
  • F = mg
  • 1 N = 1 kg m s⁻²
• CBSE Exam Booster – Focus on identifying balanced and unbalanced forces, calculating net force, applying Newton's First and Second Laws, solving numerical problems using F = ma, and explaining real-life examples involving friction and inertia.