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Motion — Question 3

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Question 3

Abdul, while driving to school, computes the average speed for his trip to be 20 kmh-1. On his return trip along the same route, there is less traffic and the average speed is 30 kmh-1. What is the average speed for Abdul's trip?

Answer

Distance travelled to reach the school = distance travelled to reach home = d

Time taken to reach school = t1

Time taken to reach home = t2

Average speed = total distance travelledtotal time taken\dfrac{\text{total distance travelled}}{\text{total time taken}}

On going to school :

20 = dt1\dfrac{\text{d}}{\text{t}_1} or

t1 = d20\dfrac{\text{d}}{20}

On return trip :

30 = dt2\dfrac{\text{d}}{\text{t}_2} or

t2 = d30\dfrac{\text{d}}{30}

Now, the average speed for the entire trip

= d+dt1+t2\dfrac{\text{d+d}}{\text{t}_1 + \text{t}_2}

= 2dd20+d30\dfrac{\text{2d}}{\dfrac{\text{d}}{20} + \dfrac{\text{d}}{30}}

= 2d3d + 2d60\dfrac{\text{2d}}{\dfrac{\text{3d + 2d}}{60}}

= 2d×605d\dfrac{\text{2d} \times {60}}{5\text{d}}

= 24 kmh-1

∴ Abdul's average speed for the entire trip is 24 kmh-1.

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Science | Chapter 7: MotionWeb Content — Quick Reference

Chapter 7: Motion — Quick Reference

distance displacement speed velocity acceleration equations of motion graphs circular motion

Quick Revision Points

  • Distance (scalar, total path) vs Displacement (vector, shortest path)
  • Speed = distance/time (scalar); Velocity = displacement/time (vector)
  • Acceleration a = (v−u)/t; unit: m/s²
  • Equations: v = u + at; s = ut + ½at²; v² = u² + 2as
  • d-t graph slope = speed; v-t graph slope = acceleration; area under v-t = distance
  • 1 km/h = 5/18 m/s; Free fall: u = 0, a = g = 9.8 m/s²
  • Uniform circular motion: constant speed, changing velocity (direction changes)
Exam Tips for Chapter 7
  • Read the detailed chapter notes for complete coverage of all NCERT topics.
  • Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
  • Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
  • For numericals (if applicable), practice at least 20 problems of varying difficulty.
  • Refer to the practice question bank (200+ questions) for thorough preparation.
Related Resources
  • Detailed Notes: ch07-motion.html
  • Practice Questions: 100+ questions with answers in 05-practice-questions/
  • Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
  • Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html
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