Does Dumping Coal into a Moving Train Affect Its Velocity?

AI Thread Summary
When coal is dumped into a moving freight car on a frictionless track, the initial velocity of the coal is crucial to understanding the outcome. If the coal is traveling at the same speed as the train when dumped, it theoretically would not require additional force to match the train's velocity. However, the consensus is that the train's speed would still decrease due to the added mass of the coal, as momentum conservation principles apply. The discussion also highlights that if the coal were stationary relative to the ground, it would indeed slow the train down more significantly. Overall, the complexities of real-world physics, including air drag and momentum, play a critical role in this scenario.
Brob
Messages
3
Reaction score
0
Ok so the question is a freight car moves along a frictionless level railroad track at constant speed. The car is open on top. A large load of coal is suddenly dumped into the car. What happens to the velocity of the car?

And I asked my teacher if the coal dumped in was stationary relative to the ground and he said it was traveling at the same velocity as the train when dumped. So I was thinking that the train speed would remain the same because the coal wouldn't need to be accelerated to the trains velocity because it is traveling at the same velocity as the train. Meaning it would not require any extra force to get it to the same velocity as the train. The force of normal would increase but since the train track is frictionless the force of normal would be irrelevant. However the correct answer was the speed of the train would decrease. Thoughts? I do understand that if friction of air was counted the coal would slow down before it fell into the train so it would need to be accelerated and would slow down the train. But, I doubt that that is the reason.
 
Physics news on Phys.org
I agree with your answer. Air drag on the coal before it reaches the train should be negligible (as all other effects of air), otherwise the problem statement would be misleading.
Brob said:
and he said it was traveling at the same velocity as the train when dumped.
That is an unusual setup. Maybe he didn't mean that. Usually the coal is stationary relative to the ground.
 
mfb said:
I agree with your answer. Air drag on the coal before it reaches the train should be negligible (as all other effects of air), otherwise the problem statement would be misleading.
That is an unusual setup. Maybe he didn't mean that. Usually the coal is stationary relative to the ground.

I agree but would my reasoning have been correct if the coal was stationary reltive to the train?
 
Yes, that's why I agree with your answer.
 
mfb said:
Yes, that's why I agree with your answer.
Thanks for the help.
 
Brob said:
and he said it was traveling at the same velocity as the train when dumped.
Very likely he believed he was simply confirming that the coal would have the same velocity as the train once dumped.
 
  • Like
Likes mfb
Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .
Thread 'Voltmeter readings for this circuit with switches'
TL;DR Summary: I would like to know the voltmeter readings on the two resistors separately in the picture in the following cases , When one of the keys is closed When both of them are opened (Knowing that the battery has negligible internal resistance) My thoughts for the first case , one of them must be 12 volt while the other is 0 The second case we'll I think both voltmeter readings should be 12 volt since they are both parallel to the battery and they involve the key within what the...
Back
Top