Work Step by Step
You might think that, similar to MisConceptual question 2 in reverse, that as the milk drains the tank car’s speed increases. However, the crucial difference here is that when the milk drops away, it has the same forward horizontal velocity that it had when it was inside the tank. No horizontal forces act on the spilled milk, so it exerts no horizontal force on the tank car and the remaining milk. By Newton’s first law, the car’s speed doesn’t change.