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RRB Group D Relative Speed

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This page covers RRB Group D Relative Speed with complete concept notes, 4 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Relative Speed— Rules & Concept

1.Core Concept

Read this first — the foundation of the topic

When two objects move, their relative speed depends on their direction. If they move towards each other, speeds add up. If they move in the same direction, speeds subtract. Imagine two trains - one coming towards you at 60 kmph while you travel at 40 kmph.

The relative speed is 100 kmph

Key Rules

For objects moving towards each other: Relative Speed = Speed1 + Speed2. For objects moving in same direction: Relative Speed = |Speed1 - Speed2|. For objects moving in opposite directions: Relative Speed = Speed1 + Speed2.

2.Formula Block

Memorise — at least one formula appears in every paper
• When approaching: Relative Speed = S1 + S2
• When separating/same direction: Relative Speed = |S1 - S2|
• Time to meet = Total Distance / Relative Speed
• Time to cross = (L1 + L2) / Relative Speed (for trains)

3.Exam Patterns

What examiners ask — read before attempting PYQs

SSC loves asking about trains crossing each other, people walking towards/away from each other, and boats in rivers. Questions often involve calculating meeting time, crossing time, or finding individual speeds when relative speed is given.

4.Shortcuts

Use these to save 30–60 seconds per question

#1: For train problems, always add train lengths when calculating crossing time. The faster train needs to cover the combined length of both trains to completely cross.

5.Worked Example

Solve this step-by-step before moving on
1
Step 1

Convert speeds to m/s: 54 kmph = 54 × 5/18 = 15 m/s, 36 kmph = 36 × 5/18 = 10 m/s

2
Step 2

Relative speed = 15 + 10 = 25 m/s (opposite directions)

3
Step 3

Combined length = 120 + 80 = 200m

4
Step 4

Time = Distance/Speed = 200/25 = 8 seconds Shortcut Trick #2: Speed conversion hack: kmph to m/s, multiply by 5/18. m/s to kmph, multiply by 18/5. Remember: 5/18 ≈ 0.278, 18/5 = 3.6 Worked Example 2: A man walks at 5 kmph. A car travels at 45 kmph in same direction. If car is initially 2 km behind, when will it catch up?

1
Step 1

Relative speed = 45 - 5 = 40 kmph (same direction, so subtract)

2
Step 2

Distance to cover = 2 km

3
Step 3

Time = Distance/Relative Speed = 2/40 = 0.05 hours = 3 minutes Shortcut Trick #3: For meeting point problems, use the ratio method. If two objects start from opposite ends with speeds S1 and S2, they meet at a point dividing the distance in ratio S1:S2. Most

6.Exam Traps

Common mistakes students make — avoid these

Students forget to add lengths when trains cross each other. They only use train speeds but ignore that crossing means one train must travel its own length plus the other train's length. Always remember: crossing distance = sum of lengths, not just individual train length. Another frequent error is wrong direction calculation.

When objects move towards each other, always add speeds. When in same direction, always subtract. Never confuse these basic rules, as they form the foundation of every relative speed problem in SSC CGL.

Key Points to Remember

  • Relative speed = S1 + S2 when objects move towards each other
  • Relative speed = |S1 - S2| when objects move in same direction
  • For train crossing: Time = (L1 + L2) / Relative Speed
  • Speed conversion: kmph to m/s multiply by 5/18
  • Meeting time = Total distance / Relative speed
  • When trains cross, always add both train lengths to find distance
  • Objects starting from opposite ends meet at distance ratio S1:S2
  • In river problems: Downstream speed = Boat speed + River speed
  • Upstream speed = Boat speed - River speed
  • Relative speed is always positive, use absolute value for same direction

Exam-Specific Tips

  • Speed conversion factor from kmph to m/s is exactly 5/18
  • Speed conversion factor from m/s to kmph is exactly 18/5 or 3.6
  • When two trains cross each other, distance = sum of both train lengths
  • For circular track problems, relative speed determines lap completion time
  • Meeting point divides total distance in ratio of individual speeds
  • In boat problems, still water speed = (Downstream + Upstream)/2
  • River current speed = (Downstream - Upstream)/2
  • Two objects starting together separate at their relative speed rate
Practice MCQs

Relative Speed — Practice Questions

4graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

Two trains are running towards each other on parallel tracks. Train A is moving at 60 km/h and Train B is moving at 40 km/h. If they are currently 500 km apart, in how many hours will they meet?

Practice 2medium

Two trains are running on parallel tracks in opposite directions. Train A travels at 60 km/h and Train B travels at 40 km/h. If they are initially 500 km apart, how long will it take for them to meet?

Practice 3medium

Two trains start simultaneously from stations A and B, which are 450 km apart. Train X travels from A towards B at 60 km/h, while Train Y travels from B towards A at 90 km/h. At what time will the two trains meet?

Practice 4hard

Two trains start simultaneously from stations A and B, which are 480 km apart. Train X travels from A towards B at 60 km/h, while Train Y travels from B towards A at 40 km/h. After how much time will they meet, and at what distance from station A will they meet?

60-Second Revision — Relative Speed

  • Formula: Towards each other = S1 + S2, Same direction = |S1 - S2|
  • Remember: Train crossing always needs combined length of both trains
  • Trap: Never forget to convert units - kmph to m/s multiply by 5/18
  • Quick check: Relative speed should make logical sense with given scenario
  • Meeting time = Total distance divided by relative speed
  • Boat speed formulas: Still water = (Down+Up)/2, Current = (Down-Up)/2
  • Always use absolute value when calculating same direction relative speed
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