# COMPLETE ENGLISH ARTICLE
## TNPSC Group IV – Day 10: Motion, Distance, Displacement, Speed, Velocity and Acceleration
**Category:** TNPSC Grp IV | **Date:** 22 October 2026 | **Session:** 1:00 PM IST | **Section:** General Studies – Physics
Today's topic covers how motion is described, the different types of motion, important formulas, graphs and common examination mistakes. It is part of General Science, Unit I of the Group IV syllabus, which includes scientific laws of motion and everyday mechanics. This is a General Studies lesson, not an Aptitude lesson.
### 1. Motion and Rest An object is **in motion** when its position changes with time relative to a chosen reference point. It is **at rest** when its position does not change relative to that reference point. Rest and motion are relative. A passenger is at rest relative to another passenger inside the bus, but moving relative to a roadside tree.
### 2. Types of Motion - **Linear motion:** Movement along a straight path, such as a car on a straight road. - **Circular motion:** Movement along a circular path, such as the tip of a clock hand. - **Oscillatory motion:** Repeated to-and-fro motion, such as a swing. - **Random motion:** Motion without a fixed predictable path, such as dust moving in turbulent air. - **Uniform motion:** Equal distances in equal intervals of time. - **Non-uniform motion:** Unequal distances in equal intervals of time, such as a bus in heavy traffic.
### 3. Distance and Displacement **Distance** is the total length of the actual path travelled. It is a **scalar** (magnitude only). SI unit: **metre (m)**. **Displacement** is the change from the starting position to the finishing position, including direction. It is a **vector** (magnitude and direction). SI unit: **metre (m)**. If a person walks 3 m east and then 3 m west, the distance is **6 m**, but displacement is **0 m**. The magnitude of displacement cannot exceed the distance travelled.
### 4. Speed and Velocity **Speed = distance travelled / time taken.** It is a scalar. SI unit: **m/s**. **Average velocity = displacement / time taken.** It is a vector. SI unit: **m/s**. **Average speed = total distance / total time.** A vehicle's speedometer displays instantaneous speed, not the average speed of the whole journey. **Unit conversions:** Multiply km/h by **5/18** to obtain m/s; multiply m/s by **18/5** to obtain km/h. Example: **72 km/h = 20 m/s**.
### 5. Acceleration **Acceleration** is the rate of change of velocity with time. **a = (v − u)/t**, where u = initial velocity, v = final velocity, and t = time. SI unit: **m/s²**. Acceleration may be negative when velocity decreases along the chosen positive direction. This is often called **deceleration or retardation**. Acceleration also occurs when only the direction of velocity changes.
### 6. Three Equations of Motion For **constant acceleration in a straight line**: 1. **v = u + at** 2. **s = ut + ½at²** 3. **v² = u² + 2as** Here s means displacement. These equations must not be applied indiscriminately to motion with changing acceleration.
### 7. Motion Graphs - **Slope of distance–time graph:** speed. - **Slope of displacement–time graph:** velocity. - **Slope of velocity–time graph:** acceleration. - **Signed area under a velocity–time graph:** displacement. If velocity is never negative, this area also equals distance.
### 8. Circular Motion and Gravity In **uniform circular motion**, speed can remain constant while direction changes. Therefore, the object has acceleration. **Centripetal acceleration = v²/r**; **centripetal force = mv²/r**. Near Earth's surface, acceleration due to gravity is approximately **9.8 m/s²** (often rounded to 10 m/s² in simple textbook problems). Without air resistance, freely falling objects have the same gravitational acceleration.
### 9. Fully Worked Physics Examples **Example 1:** A student travels 150 m in 30 s. Average speed = 150/30 = **5 m/s**. **Example 2:** A vehicle's velocity increases from 10 m/s to 20 m/s in 5 s. Acceleration = (20−10)/5 = **2 m/s²**. **Example 3:** A vehicle initially moving at 20 m/s stops with constant acceleration −4 m/s². Using v² = u² + 2as: 0 = 20² + 2(−4)s; s = **50 m**.
### 10. Everyday Applications in Tamil Nadu A Chennai suburban train speeds up after leaving a station and slows down before reaching the next station: examples of acceleration and deceleration. A bus turning along a ghat road changes its velocity because its direction changes, even if its speed remains constant. The total road distance between two places is not necessarily equal to their straight-line displacement.
### 11. Quick Exam Revision Distance and speed are **scalars**; displacement and velocity are **vectors**. After one complete circular lap, displacement is **zero**, while distance is **2πr**. Constant speed does not always mean zero acceleration. The SI unit of acceleration is **m/s²**. Remember: **72 km/h = 20 m/s**.
### 12. Textbook References and Book-Back Progress **Original textbook located:** Tamil Nadu SCERT, **Class 9 Science, Unit 2 – Motion**, 2020 edition, printed pages 14–25. Its original chapter PDF has been inspected. **Whether the same exercise numbering appears in the 2026–27 edition remains unverified.** **2020 book-back inventory:** Section I, choose the correct answer **4**; II, fill in the blanks **4**; III, true/false **5**; IV, assertion/reason **3**; V, matching **1 question with 4 pairs**; VI, short answers **7**; VII, detailed answers **2**; VIII, numerical problems **3**. **Total: 29 main numbered questions, or 32 atomic items when the four matching pairs are counted individually.** The chapter also contains six activities, tracked separately. **Verified completed:** **0/29** main book-back questions. **Pending:** **29/29**. **Proposed evening batches:** **1/6 through 6/6**, approximately five main questions per batch; graph-dependent questions will be checked against the original diagrams. **Source integrity:** the **2020 SCERT chapter is verified**, but the **current 2026–27 edition is 'source not yet verified'**. **Supplementary reference:** Class 10 Science, Unit 1 – Laws of Motion. Newton's laws will be treated in a separate lesson later.
### 13. Spaced Revision Review this topic on **23 October (+1), 25 October (+3), 29 October (+7) and 5 November (+14), 2026**. Today's cumulative review: 21 October Acids/Bases/Salts (+1), 19 October lesson (+3), and 15 October lesson (+7), using the notes already delivered. This lesson has no quiz; questions will appear in the 7:00 PM practice session.
**ETN Push Title:** 🚆 TNPSC Group IV | Motion – Day 10! **ETN Push Message:** 📚 Learn distance vs displacement, speed vs velocity, acceleration, three equations of motion and graphs in today's complete Physics lesson on ETN News!
