**Day 07 | Monday, 19 October 2026 | 1:00 PM IST** **Category: TNPSC Grp IV | General Studies – Physics**
## 1. Learning objectives Today we study rest and motion, types of motion, distance, displacement, speed, velocity, acceleration, the three equations of motion, motion graphs, free fall and circular motion. These concepts fall under General Science, Unit I of the TNPSC Group IV SSLC syllabus.
## 2. Rest and motion An object is **at rest** when its position does not change over time relative to a chosen reference point. It is **in motion** when its position changes relative to that reference point. Motion is relative: a passenger sitting in a bus is at rest relative to the passenger beside them but moving relative to a roadside tree.
## 3. Types of motion **Linear motion:** movement along a straight line, such as a vehicle on a straight road. **Circular motion:** movement along a circular path, such as the tip of a clock hand. **Oscillatory motion:** repeated back-and-forth movement, such as a swing. **Uniform motion:** equal distances covered in equal time intervals. **Non-uniform motion:** unequal distances in equal time intervals, such as a bus in traffic. **Random motion:** irregular movement with changing directions.
## 4. Distance and displacement **Distance** is the total length of the path travelled, regardless of direction. It is a **scalar**, measured in metres (m). **Displacement** is the change from starting position to final position with direction. It is a **vector**, also measured in metres (m).
**Worked example 1:** A person walks 3 km east and then returns 3 km west along the same route. Total distance = 3 + 3 = **6 km**. Starting and ending positions coincide, so displacement = **0**. The magnitude of displacement cannot exceed the distance travelled.
## 5. Speed and velocity **Average speed = total distance / total time.** Speed is a scalar; its SI unit is m/s. **Average velocity = total displacement / total time.** Velocity is a vector; its SI unit is m/s. For a round trip, average velocity may be zero even when average speed is positive.
**Worked example 2:** A vehicle covers 90 km in 1.5 hours. Average speed = 90 / 1.5 = **60 km/h**. We cannot call this its average velocity unless displacement and direction are known.
## 6. Acceleration **Acceleration** measures how quickly velocity changes: a = (v − u)/t for straight-line motion. Here u is initial velocity, v is final velocity, and t is time. SI unit: **m/s²**. Acceleration may change the magnitude of velocity, its direction or both. Negative acceleration is not always the same as slowing down; it depends on the chosen positive direction and the direction of motion.
**Worked example 3:** A vehicle changes velocity from 5 m/s to 25 m/s in 4 seconds. Acceleration = (25 − 5)/4 = **5 m/s²**.
## 7. Unit conversion To convert **km/h to m/s, multiply by 5/18**. To convert **m/s to km/h, multiply by 18/5**.
**Worked example 4:** 72 km/h = 72 × 5/18 = **20 m/s**. Conversely, 15 m/s = 15 × 18/5 = **54 km/h**. Always use consistent units before calculating.
## 8. Three equations of uniformly accelerated motion For motion in a straight line with **constant acceleration**:
1) **v = u + at** 2) **s = ut + ½at²** 3) **v² = u² + 2as**
Here s is displacement. These equations in this form assume constant acceleration.
**Worked example 5:** A car starts from rest and accelerates at 4 m/s² for 5 seconds. u = 0, a = 4, t = 5. Final velocity v = 0 + 4 × 5 = **20 m/s**. Displacement s = 0 + ½ × 4 × 5² = **50 m**.
## 9. Motion graphs For a **distance–time graph**, the slope gives speed; a straight line with constant slope represents constant speed. For a **displacement–time graph**, the slope gives velocity. For a **velocity–time graph**, the slope gives acceleration, while the signed area between the graph and the time axis gives displacement. If velocity stays positive in one direction, this area also equals distance. **Exam warning:** Area under a velocity–time graph does not represent acceleration.
## 10. Free fall Ignoring air resistance, objects falling near Earth's surface have downward acceleration **g ≈ 9.8 m/s²**. Many basic problems approximate this as 10 m/s². For an object released from rest, v = gt; h = ½gt²; v² = 2gh. Without air resistance, the acceleration does not depend on the object's mass.
**Worked example 6:** An object falls from rest for 2 seconds. Using g = 10 m/s², velocity v = 10 × 2 = **20 m/s** and distance fallen h = ½ × 10 × 2² = **20 m**.
## 11. Uniform circular motion and centripetal force In uniform circular motion, **speed remains constant but direction changes continuously**. Velocity therefore changes and acceleration exists. **Centripetal acceleration = v²/r**; **centripetal force = mv²/r**. Both point towards the centre. The outward **centrifugal force** is an apparent force in a rotating reference frame; it should not be mistaken for the Newton's-third-law reaction to the centripetal force.
## 12. Everyday applications in Tamil Nadu A city bus in Chennai traffic covers unequal distances in equal time intervals: this is non-uniform motion. In a **hypothetical calculation**, a vehicle travelling 60 km towards Erode from Salem in 1.5 hours has an average speed of 40 km/h. A speedometer displays instantaneous speed, not the journey's average speed. A car turning on a curved road accelerates because its direction of velocity changes.
## 13. Common examination mistakes Distance is not the same as displacement, and speed is not the same as velocity. After one complete lap, displacement may be zero while distance is not. Constant speed around a circle still involves acceleration. Acceleration is measured in m/s², not m/s. The slope of a velocity–time graph gives acceleration; its signed area gives displacement. Remember that 72 km/h = 20 m/s.
## 14. One-minute revision Distance = total path length. Displacement = directed change in position. Speed = distance/time. Velocity = displacement/time. Acceleration = (v−u)/t. Equations: v=u+at; s=ut+½at²; v²=u²+2as. Gravity near Earth: g≈9.8 m/s². Circular motion: a=v²/r; F=mv²/r.
## 15. Textbook references and complete book-back progress **Main reference:** Tamil Nadu Samacheer Kalvi, **Class 9 Science, Physics Unit 2 – Motion**, original government textbook reproduced from the **2020 edition**, printed pages 14–25; book-back on printed pages 23–25. Subsections 2.1–2.10 cover the principal concepts.
**Book-back index verified against the original 2020 textbook:** Section I MCQs 4; II fill in the blanks 4; III true/false 5; IV assertion–reason 3; V matching 4 pairs; VI short answers 7; VII detailed answers 2; VIII numerical problems 3. **Total: 32 individually indexed items**, counting each of the four matching pairs separately. Some questions require original diagrams.
**Completed: 0/32 (2020 edition); pending: 32/32.** The original **2026–27 edition has not yet been independently compared**, so the 2026–27 complete exercise total is **source not yet verified**. The count of 32 applies only to the indexed 2020 edition. A provisional eight-batch plan of four items per evening can cover all 32 after diagram and edition checks. No book-back questions are reproduced in this afternoon lesson; they are reserved for separate evening batches.
**TNPSC PYQ verification:** No question in this lesson is presented as a verified previous-year TNPSC question. Such attribution requires the actual examination paper and, where possible, its official final answer key.
## 16. Spaced revision and weekly review Review this topic after 1, 3, 7 and 14 days: **20 October, 22 October, 26 October and 2 November 2026**. Today also briefly revise the Day 06 (+1 day) and Day 04 (+3 days) General Studies topics. A first-week cumulative checkpoint can be placed in the additional section of the 7 PM practice without changing its 15 headline questions.
## 17. ETN push notification **English title:** 📚 TNPSC Group IV | Motion Made Easy! **English message:** 🚍 Learn distance, displacement, speed, velocity, acceleration, motion equations and circular motion with clear examples. Full Day 07 Science notes on ETN News!
