Engineering Mechanics

Question

A railroad car that weights 20,000 lb is traveling eastward with a velocity whose magnitude is 5 fps. A second ar, on the same track, that weight 40,000 lb is also traveling in an easterly direction with a velocity of 7.81 fps. When the  car collided, they became coupled together and then both cars moved with the same velocity. If friction between the cars and the railsis neglected, what were the magnitude and the direciton of the velocity of the cars after the collision?

Answer 1

This is a momentum problem, and normally momentum is measured in kg * m/s which are metric units. I'll give an answer in both customary & metric units. 

 

Weight * velocity of train 1 + weight * velocity of train 2 = weight on both trains * new velocity

 

20,000 * 5 + 40,000 * 7.81 = 100,000 + 312,400 = 412,400 = 60,000 * new velocity 

 

New velocity = 412,400/60,000 = 6.873 feet per second = 82.48 inches per second = 2.09 meters/sec