Question

A car travels from P to Q at a uniform speed of 20 ms^-1 and immediately turns back and travels towards Q with a uniform speed of 30 ms^-1. What is the average speed for the whole journey?

Answer 1

let distance between P and Q is S.
Speed of car while travelling from P to Q is 20 m/s.
Let car take time T₁ to travel from P to Q then T₁= S/20.
Speed of car while travelling from Q to P is 30 m/s.
Let car take time T₂ to travel from Q to P then T₂= S/30.
Total time = T₁ + T₂ = S/20 +S/30 =S/12.
So average speed of journey = total distance/ total time = 2S/(S/12) = 24 m/s.
Average speed of journey is 24 m/s.