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Speed, Distance and Time | Kerala PSC LDC
- Fact 1 of 8🚗
The basic speed formula
Speed = Distance ÷ Time. Example: a bus covers 120 km in 3 hours, so its speed is 120 ÷ 3 = 40 km/h. Rearrange it to get Distance = Speed × Time or Time = Distance ÷ Speed.
- Fact 2 of 8🔄
Converting km/h to m/s
To change km/h to m/s, multiply by 5/18. To change m/s to km/h, multiply by 18/5. Example: 36 km/h = 36 × 5/18 = 10 m/s, a quick shortcut for train and race questions.
- Fact 3 of 8⚖️
Average speed for equal distances
If the same distance is covered at speeds a and b, the average speed is 2ab/(a+b), not the simple average. Example: 40 km/h and 60 km/h give 2×40×60/100 = 48 km/h.
- Fact 4 of 8🚆
Relative speed of two trains
When two objects move in the same direction, relative speed is the difference of their speeds. When they move towards each other, it is the sum. Example: trains at 60 and 40 km/h moving apart in the same direction have a relative speed of 20 km/h.
- Fact 5 of 8🚉
Train crossing a pole or platform
A train passing a pole covers its own length: time = length ÷ speed. Crossing a platform means covering train length plus platform length. Example: a 150 m train at 54 km/h (15 m/s) passes a pole in 150 ÷ 15 = 10 seconds.
- Fact 6 of 8📊
Speed and time are inversely proportional
For the same distance, time is inversely proportional to speed. If speeds are in the ratio 3:4, the times taken are in the ratio 4:3. This shortcut solves many PSC ratio questions without calculating the distance.
- Fact 7 of 8🛶
Upstream and downstream speeds
In boat problems, downstream speed = boat speed + stream speed, and upstream speed = boat speed − stream speed. Example: a boat at 10 km/h in a 2 km/h stream goes 12 km/h downstream and 8 km/h upstream.
- Fact 8 of 8📈
Distance from a speed-time graph
On a speed-time graph, the area under the graph gives the distance travelled. For a constant speed, this is a rectangle, so distance = speed × time. This idea is often tested in graph-based questions.
