# Does Dumping Coal into a Moving Train Affect Its Velocity?

• Brob
In summary, the question discusses a scenario where a freight car is moving at a constant speed on a frictionless track when a load of coal is suddenly dumped into it. The teacher confirms that the coal is traveling at the same velocity as the train when dumped. The correct answer is that the speed of the train would decrease, even though air drag on the coal is negligible. This may seem unusual, but the reasoning is correct even if the coal was stationary relative to the train. The teacher may have misunderstood the question and thought they were only confirming that the coal would have the same velocity as the train once dumped.
Brob
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.

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.

mfb

## 1. What is meant by "train at constant velocity"?

A train at constant velocity refers to a train that is traveling in a straight line at a constant speed. This means that the train is not accelerating or decelerating, and its velocity remains the same throughout its journey.

## 2. How is constant velocity achieved in a train?

Constant velocity in a train is achieved by maintaining a steady speed and direction. This is usually done by adjusting the throttle and brake systems, as well as the track conditions. Additionally, external factors such as wind resistance and friction can also affect the train's speed and must be accounted for.

## 3. What are the advantages of a train traveling at constant velocity?

There are several advantages of a train traveling at constant velocity, including energy efficiency, reduced wear and tear on the train and tracks, and increased passenger comfort. By maintaining a steady speed, the train requires less energy to operate, resulting in cost savings and reduced emissions. Additionally, constant velocity reduces the strain on the train and tracks, prolonging their lifespan. Finally, passengers are less likely to experience discomfort or motion sickness when the train is traveling at a consistent speed.

## 4. How does constant velocity affect the braking distance of a train?

Constant velocity has a significant impact on the braking distance of a train. Since the train is already traveling at a steady speed, less energy is required to bring it to a complete stop. This results in a shorter braking distance compared to a train that is accelerating or decelerating.

## 5. Are there any limitations to maintaining a train at constant velocity?

While constant velocity is desirable for a train, it is not always possible to maintain it. External factors such as weather conditions, track conditions, and traffic can affect the train's speed and require adjustments to maintain a safe and efficient journey. Additionally, some trains, such as those with multiple stops, may need to accelerate and decelerate to accommodate passenger boarding and alighting.

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