This page covers SSC GD Constable Relative Speed with complete concept notes, 12 graded practice MCQs, key points and exam-specific tips. Free to study.
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.
🔢
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)
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 Common Mistake
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.
Two trains are moving towards each other on parallel tracks. Train A travels at 60 km/h and Train B travels at 40 km/h. If they are currently 500 km apart, how long will it take for them to meet?
Practice 2easy
A car and a motorcycle are travelling in the same direction on a highway. The car travels at 80 km/h and the motorcycle at 50 km/h. If the motorcycle is currently 90 km ahead of the car, how long will it take for the car to catch up?
Practice 3easy
Two cyclists start from the same point and travel in opposite directions. Cyclist A travels at 15 km/h and Cyclist B at 25 km/h. After how many hours will they be 160 km apart?
Practice 4easy
A boat travels upstream at 12 km/h and downstream at 20 km/h. What is the speed of the current?
Practice 5easy
Two runners start from opposite ends of a 400-meter track and run towards each other. Runner X runs at 8 m/s and Runner Y at 12 m/s. In how many seconds will they meet?
Practice 6easy
A person walks at 5 km/h and a bus travels at 45 km/h in the same direction. If the bus is 80 km behind the person, how long will it take for the bus to catch up?
Practice 7medium
Two trains are moving towards each other on parallel tracks. Train A travels at 60 km/h and Train B travels at 40 km/h. If they are initially 500 km apart, in how many hours will they meet?
Practice 8medium
A boat travels upstream at 12 km/h and downstream at 20 km/h. What is the speed of the boat in still water?
Practice 9medium
A car and a motorcycle start from the same point and travel in the same direction. The car travels at 80 km/h and the motorcycle at 50 km/h. After how many hours will the car be 90 km ahead of the motorcycle?
Practice 10hard
Two trains A and B start simultaneously from stations X and Y respectively, which are 480 km apart. Train A travels towards Y at 60 km/h, while train B travels towards X at 40 km/h. After how many hours will they meet, and at what distance from station X?
Practice 11hard
A boat travels 120 km downstream in 4 hours and the same distance upstream in 6 hours. A swimmer swims at the same speed as the boat's speed in still water. How long will the swimmer take to cover 45 km in still water?
Practice 12hard
Two runners, P and Q, run on a circular track of length 400 metres. P runs at 8 m/s and Q at 6 m/s, both starting from the same point in the same direction. After how many seconds will P lap Q (i.e., be exactly one full lap ahead)?
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