Question

Two trains 120m and 80m in length are travelling in opposite directions with velocities

42 kmh–1 and 30 kmh–1 respectively. How much time will they require to completely cross

each other?​

Answer 1

relative vel of train of vel 42 km/h  w r t the other=42-(-30)=72 km/h as they are running opposite to each other.

the distance it (one withvel 42 km/h )have to cross=120+80=200 km to cross each other

will be=200/72  =2 hr 7/9 min

Answer 2

Answer:

let the length of 1st train L1= 120m

length of 2nd train L2= 80m

speed of 1st train S1=42 km/hr

speed of 2nd train S2 = 30 km/hr

now formula is

speed= distance/ time

so,

S1 + S2 = (L1 + L2)/ time

(42 + 30) km/hr = (120+80) m/ time

72 km/hr = 200 m/ time

now convert the speed from km/hr to m/s by multipying 5/18

72*5/18 = 200/ time

20 m/s = 200 m/ time

time= 200/ 20

time = 10 seconds

so the train will cross each other in 10 seconds

Explanation:

use relative motion concept