NCERT Class 9 Science Chapter 7 – Motion

NCERT Class 9 Science Chapter 7 introduces the basic concepts of motion and develops quantitative understanding of how objects move. NCERT Class 9 Science Chapter 7 builds the foundation of kinematics, which is essential for higher physics and competitive examinations.

The chapter explains important physical quantities such as distance, displacement, speed, velocity and acceleration with proper SI units. It introduces graphical representation of motion using distance–time and velocity–time graphs.

NCERT Class 9 Science Chapter 7 also derives the three equations of motion for uniformly accelerated motion:
v = u + at
s = ut + ½at²
v² – u² = 2as

These equations form the backbone of numerical problem-solving in Class 9, Class 10 and higher physics including JEE preparation.

The chapter emphasizes unit conversion such as 1 km/h = 5/18 m/s and introduces the concept of uniform circular motion.

For structured preparation of NCERT Class 9–12 for UPSC, BPSC and State PCS examinations, strengthen your basics with our complete NCERT Book Notes PDF for Class 9-12, available inside the NCERT foundation course level-2.

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1. Introduction

• Motion refers to change in position of an object with respect to time and reference point.
• If position does not change with time, object is said to be at rest.
• Motion is always described with respect to a reference frame.
• For example, a passenger sitting in a moving train appears at rest relative to train but in motion relative to ground.
• The study of motion without considering cause is called kinematics.

2. Motion and rest relative to reference frame

• An object may be at rest for one observer and in motion for another.
• Motion depends on the observer’s point of reference.
• Example: Trees appear moving when viewed from a moving bus.
• Earth rotates and revolves, yet we appear stationary relative to Earth’s surface.

3. Types of motion

• Rectilinear motion – Motion along a straight line.
• Circular motion – Motion along circular path.
• Periodic motion – Motion repeating at regular intervals.
• Oscillatory motion – Motion about mean position.
• Uniform motion – Equal distances in equal intervals of time.
• Non-uniform motion – Unequal distances in equal intervals of time.

4. Distance and displacement

• Distance is total path length travelled by object.
• Distance is a scalar quantity.
• Displacement is shortest distance between initial and final position.
• Displacement is a vector quantity.
• Displacement may be zero even if distance is non-zero.
• SI unit of both is metre (m).

5. Uniform and non-uniform motion

• Uniform motion: Object covers equal distances in equal intervals of time.
• Example: Car moving at constant speed on straight highway.
• Non-uniform motion: Object covers unequal distances in equal intervals.
• Example: A freely falling object.

6. Speed

• Speed is distance travelled per unit time.
• Speed = Distance / Time.
• SI unit is metre per second (m/s).
• Average speed = Total distance / Total time.
• Instantaneous speed is speed at particular instant.
• 1 km/h = 5/18 m/s.

7. Velocity

• Velocity is displacement per unit time.
• Velocity = Displacement / Time.
• It is a vector quantity.
• SI unit is m/s.
• Uniform velocity means constant speed in a fixed direction.
• If direction changes, velocity changes even if speed is constant.

8. Acceleration

• Acceleration is rate of change of velocity.
• Acceleration = (Final velocity – Initial velocity) / Time.
• SI unit is m/s².
• Uniform acceleration means equal change in velocity in equal intervals.
• Negative acceleration is called retardation.

9. Graphical representation of motion

• Distance–time graph shows relationship between distance and time.
• Straight line parallel to time axis indicates object at rest.
• Straight inclined line indicates uniform motion.
• Curve indicates non-uniform motion.
• Slope of distance–time graph gives speed.

10. Velocity–time graph

• Velocity–time graph shows velocity variation with time.
• Straight horizontal line indicates uniform velocity.
• Straight inclined line indicates uniform acceleration.
• Area under velocity–time graph gives displacement.
• Slope of velocity–time graph gives acceleration.

11. Equations of uniformly accelerated motion

• First equation: v = u + at.
• Second equation: s = ut + ½ at².
• Third equation: v² – u² = 2as.
• Where u = initial velocity, v = final velocity, a = acceleration, t = time, s = displacement.
• Derived from velocity–time graph.

12. Derivation from velocity–time graph

• Slope of v–t graph = acceleration = (v – u)/t.
• Area under v–t graph = displacement.
• Area of rectangle = ut.
• Area of triangle = ½ at².
• Total displacement = ut + ½ at².

13. Uniform circular motion

• Motion along circular path with constant speed.
• Direction changes continuously.
• Velocity changes due to change in direction.
• Example: Motion of satellite around Earth.

14. Numerical applications

• If a car accelerates from 5 m/s to 15 m/s in 2 s, acceleration = 5 m/s².
• If initial velocity is 0 and acceleration is 9.8 m/s², after 2 s velocity = 19.6 m/s.
• If u = 0, a = 2 m/s², t = 5 s, displacement = 25 m.
• Conversion example: 72 km/h = 20 m/s.

15. Units and conversions

• SI unit of distance = metre (m).
• SI unit of time = second (s).
• SI unit of speed and velocity = m/s.
• SI unit of acceleration = m/s².
• 1 km = 1000 m.

16. Conclusion

• Motion is relative and depends on reference frame.
• Speed, velocity and acceleration describe motion quantitatively.
• Graphs help visualize motion.
• Equations of motion apply only for uniform acceleration.

17. Exam oriented facts

• Scalar quantity – Quantity with magnitude only (distance, speed).
• Vector quantity – Quantity with magnitude and direction (displacement, velocity).
• Uniform motion – Equal distances in equal intervals.
• Acceleration – Rate of change of velocity.
• Slope of distance–time graph – Speed.
• Slope of velocity–time graph – Acceleration.
• Area under velocity–time graph – Displacement.
• Uniform circular motion – Constant speed but changing velocity.

Understanding NCERT Class 9 Science Chapter 7 is essential for developing clarity about motion and acceleration.

This chapter forms the base for Newton’s Laws of Motion, gravitation, work-energy theorem and advanced mechanics in senior secondary classes.

For school examinations, students must focus on definitions, unit conversions, graphical interpretation and derivations of equations of motion.

For competitive examinations like JEE and NDA, NCERT Class 9 Science Chapter 7 provides fundamental clarity about kinematics and numerical problem-solving.

Continue reading NCERT Class 9 Science Chapter 8 – Force and Laws of Motion to understand how motion is caused and controlled.

FAQs

Q1. What is NCERT Class 9 Science Chapter 7 about?
It explains the concepts of motion, distance, displacement, speed, velocity, acceleration and equations of motion for uniformly accelerated motion.

Q2. What are the three equations of motion?
The three equations are v = u + at, s = ut + ½at², and v² – u² = 2as, which apply to uniformly accelerated motion.

Q3. What is the difference between speed and velocity?
Speed is a scalar quantity representing distance per unit time, while velocity is a vector quantity representing displacement per unit time.

Q4. What does the slope of a velocity–time graph represent?
The slope of a velocity–time graph represents acceleration, and the area under the graph represents displacement.

Q5. Why is NCERT Class 9 Science Chapter 7 important for competitive exams?
It builds the foundation of kinematics and numerical problem-solving, which are essential topics for JEE and other engineering entrance examinations.

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