Two trains on opposite tracks leave the same station at the same time. one train travels at an average speed of 80 km/hr and the other travels at an average speed of 70 km/hr. how long after they leave the station will they be 50 km apart?

We can write the law of motion for both trains, calling them train A and train B. Since they both move by uniform motion, so with constant velocity, we can write their position at time t as:

where  and  are the average speeds of the two trains. For the train B, we put a negative sign, since it is going in the opposite direction.

We want to know the time t after which the distance between the two trains is 50 km. In equations, this means finding the time t after which

And solving, we find:

Which means after 20 minutes.

wavelength divides in half and speed stays the same.