college physics
posted by eric on .
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

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 lbfseconds)
Mass units of slugs are needed when calculating momentum.