solving velocity problem for time How long does it take a train moving at 20 m/s to cross completely a trestle 75m long if the train is 335m long?
I honestly don't get how both the distances tie into the velocity equation....help please!
 A: If I under stand the question right you have a train that is 375m long from the nose of the engine to the end of the last car. So, in order for the entire train to pass the trestle completely, the engine must move 335m + 75m= 410m. We have that The train moves at a velocity of 20 m/s, So if we divide 410 m by 20 m/s  we get 
$$\frac{410 m}{20 m/s}=\frac{410}{20}\cdot\frac{m}{m/s}= 20.5\cdot \frac{m}{1}\cdot \frac{s}{m}=20.5 s$$
So it should take 20.5 seconds for the train to completely cross the trestle. 
A: Ok, the length of the train is 335m and it needs to cross a distance that is 75m. 
So picture a train crossing some marked distance of 75m, the front of the train is at the very end of the 75m, but the back part of the train, has still not cross that 75m because the train is really long. So we now have to consider that even if the front part of the train crosses that distance, the entire train still has not cross the distance. You need to find the distance the train needs to travel for the entire train to cross the 75m distance. This distance would 335m + the extra 75m. 
