# Freight car velocity

Hey guys!

I was wondering if I could get some help with this question:

A railway freight car that is carrying sand is traveling at a uniform velocity on a straight level track. A leak develops in the flat bottom, and the sand begins to pour out of a hole onto the ground. Assuming there aren't any external forces on the car (including friction), does the velocity of the car decrease, increase or remain the same?

According to Newton's 1st law, it says that the velocity of an object will remain constant unless it's acted upon by an external or unbalanced force, which there isn't any in this question. However, I am a bit confused about the sand falling out. Do this affect the velocity of the car? Thanks if you can help me!

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jamesrc
Gold Member
What does Newton's second law say about this situation? Remember that Newton's second law can be written as a relation between the net force and the rate of change of momentum.

Okay, so Newton's 2nd Law in terms of momentum is Δp = F Δt , which is also
F Δt = m Δv.

So in isolating velocity from the formula, as the mass decreases, the velocity would increase?

That is incorrect. Yes the mass of the sand is leaving the car, but the sand itself has a velocity and therefore takes it momentum with it.