Calculating Force for Moving Railroad Car

[queenspublic]

Homework Statement

A railroad car moves under a grain elevator at a constant speed of 4.50 m/s. Grain drops into the car at the rate of 680 kg/min. What is the magnitude of the force needed to keep the car moving at constant speed if friction is negligible?

Homework Equations

p = mv
F = dp/dt

The Attempt at a Solution

?

 

[LowlyPion]

OK. So what is the Δmv?