Speed, Time and Distance

5 min

Master unit conversions, average speed, and relative motion for exams.

The Golden Formula

Distance is the product of speed and time. Use 5/18 to convert km/h to m/s. Relative speed depends on whether objects move in the same or opposite direction.

Why This Topic Matters

Forms the base for complex Train and Boat problems.
Directly asked in SSC CGL, CHSL, and Railway exams.
High-scoring section with standard formula applications.

Quick Memory Hooks

Big to Small (km/h to m/s): Small fraction 5/18.
Same Direction: Subtract speeds (S for Same, S for Subtract).
Opposite Direction: Add speeds together.

Formula Summary Table

Concept	Formula	Condition
Conversion	x * 5/18	km/h to m/s
Avg Speed	2xy / (x+y)	Equal Distances
Rel. Speed	S1 + S2	Opposite Dir.
Rel. Speed	S1 - S2	Same Dir.

Visualizing Motion

5/18 Factor

(Unit Bridge)

Unit Bridge

Convert speed instantly.

2xy / (x+y)

(Round Trip)

Round Trip

Use for equal trips.

V1 - V2

(Chasing Cars)

Chasing Cars

Same direction motion.

V1 + V2

(Head-on)

Head-on

Moving towards each other.

Common Exam Traps

Average speed = (x + y) / 2Average speed = 2xy / (x + y)

Arithmetic mean is wrong for speeds over same distance.

Speed in km/h, Time in secondsConvert all to same unit

Never calculate without checking unit consistency.

Add speeds in same directionSubtract speeds (S1 - S2)

Relative speed decreases in the same direction.

Check Your Progress

Q1.A car travels at 72 km/h. What is its speed in m/s?

Q2.A man goes from A to B at 20 km/h and returns at 30 km/h. Find average speed.

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