Motion
About 7 min · Science · English
Time worked out from the lesson's 453 words at 150 words a minute, plus a minute for each question.
Motion is a foundational topic in physics that appears in NSO Class 9 and 10. Understanding motion means describing how objects move — their position changes over time — using quantities like distance, displacement, speed, velocity, and acceleration. This topic builds the language of kinematics, which is essential for later chapters like Force, Gravitation, and Work & Energy.
Key ideas
- Distance vs Displacement: Distance is the total path length traveled (scalar, always positive). Displacement is the shortest straight-line distance from start to finish with direction (vector, can be zero or negative).
- Speed vs Velocity: Speed is distance/time (scalar). Velocity is displacement/time (vector). Average velocity can be zero even if average speed is not, if the object returns to its starting point.
- Acceleration: Rate of change of velocity. Positive acceleration means speeding up in the positive direction or slowing down in the negative direction. Negative acceleration (retardation) means slowing down in the positive direction.
- Uniform Motion: Object covers equal distances in equal intervals of time — velocity is constant, acceleration is zero.
- Non-Uniform Motion: Velocity changes with time — acceleration is non-zero. The three kinematic equations apply only when acceleration is constant.
Watch out for
- Confusing distance with displacement: Using total path length instead of net change in position. Fix: Always check if the question asks for distance (scalar) or displacement (vector). If an object returns to start, displacement = 0 but distance ≠ 0.
- Wrong sign for acceleration: Treating retardation (deceleration) as positive when velocity decreases. Fix: If velocity decreases in the positive direction, acceleration is negative. Set a consistent direction as positive at the start.
- Misapplying equations when acceleration isn't constant: Using v = u + at for motion with varying acceleration. Fix: The three equations of motion apply only to uniformly accelerated motion. Check the problem statement.
Quick reference
- Scalar quantities in motion: Distance, speed. Vector quantities: Displacement, velocity, acceleration.
- v = u + at for final velocity. s = ut + ½at² for displacement. v² = u² + 2as when time is unknown.
- Distance-time graph: Slope = speed. Horizontal line = rest. Curved line = non-uniform motion.
- Velocity-time graph: Slope = acceleration. Area under curve = displacement. Horizontal line = uniform velocity.
- Retardation: Negative acceleration — object slowing down. Common in braking/stopping problems.
Check yourself: 3 questions
1. A car travels 60 km north and then 80 km east. What is the magnitude of its displacement?
2. A train starting from rest attains a velocity of 72 km/h in 5 minutes. What is its acceleration?
3. A body moving with uniform acceleration covers 20 m in the 3rd second and 32 m in the 5th second. What is its acceleration?
This lesson is cut from the topic's note, not rewritten. Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.
Read the full noteFlashcards for this topic
Saves this lesson and its 3 questions in this browser on this phone. No sign-in; nothing about you is saved or sent. A saved copy is not updated until you save it again. Parts that load from the server when you use them (the AI tutor, adaptive practice, sign-in) still need a connection.