Question

A(n) 13,300 lb railroad car traveling at 7.4 ft/s couples with a stationary car of 6790 lb. The acceleration of gravity is 32 ft/s^2. What is their velocity after the collision (ft/s)?

And what impulse did the first car receive? Answer in units of lbs.

Any help would be grateful, THANKS

Answer 1

Use conservation of momentum. When the cars are coupled, they have the same velocity

13,300lb*7.4 ft/s = (13,300+6790)lb*Vfinal

It is not necessary to convert the weight unit to mass in this case. g would appear on both sides and cancel out.

Vfinal = 7.4 ft/s*(0.662) = 4.9 ft/s

The impulse received by the first car equals its loss of momentum:
(13,300/32.2)slug *(4.9 - 7.6)ft/s
= -1115 slug ft/s (or lbf-seconds)

Mass units of slugs are needed when calculating momentum.