0
$\begingroup$

Say, a sealed hollow container with 1 atm air pressure inside, is falling through earth's atmosphere.
It is falling steadily with one side always facing the ground (Front Side)
Inside the container, stuck on the backside, is melting ice
Which way would the melted ice droplets translate relative to the container falling?

With drag slowing down the fall of the container, shouldn't the droplets fall(translate from the Back side of the container to the Front) faster than the container(translate from high altitude to the ground)?


Origin: "Apollo 13 (1995)"
Command Module's Re-entry on earth's atmosphere. Water droplets raining down at velocities less than the CM. (Hence, droplets raining from front to rear end of the CM)

$\endgroup$

2 Answers 2

3
$\begingroup$

The container will accelerate to its terminal velocity, then move at a constant speed. When the container is moving at its terminal velocity, it no longer accelerates, so a person (or anything) inside the container will feel gravity exactly as if the container were standing still on the Earth's surface.

Air inside the container is stationary with respect to the container, so if you're inside the container watching water droplets drip from the ceiling, they will fall exactly as if the container were standing still on the Earth's surface. The droplets, too, have a terminal velocity which depends on their size:

"At sea level, a large raindrop about 5 millimeters across (house-fly size) falls at the rate of 9 meters per second (20 miles per hour). Drizzle drops (less than 0.5 mm across, i.e., salt-grain size) fall at 2 meters per second (4.5 mph)." droplet terminal velocity

If your container has not yet reached its terminal velocity, it will still be accelerating at a rate a somewhere between zero and g. Droplets inside the container will experience an acceleration of g - a.

So, if your container is relatively tall, and it has reached terminal velocity, then the water droplets will accelerate initially at g, accelerate more and more slowly until they reach their own terminal velocity with respect to the air in the container.

A person inside the container will feel gravity that seems to be zero initially when the container starts falling, and smoothly increases until the container reaches terminal velocity, at which point he will feel the full gravity of g. A drop that falls at that starting moment will seem to hover briefly, and then fall with an acceleration that changes as the cylinder's acceleration changes.

Note: terminal velocity is proportional to the square root of g (or in the case of this question, to the square root of the effective value of g. Air resistance is proportional to the square of the velocity. If you need an equation to describe the droplet's motion, you'll need to use the formulas presented here.

$\endgroup$
2
  • $\begingroup$ So the relative velocity of the droplet with container would be = relative v of droplet - relative v of container? $\endgroup$
    – Hunkoys
    Commented Mar 13, 2019 at 5:00
  • $\begingroup$ The velocity of the droplet relative to the Earth is the velocity of the droplet relative to the container plus the velocity of the container relative to the Earth. But, as described in my answer, that's just part of the story. $\endgroup$
    – S. McGrew
    Commented Mar 13, 2019 at 14:19
0
$\begingroup$

Yes, and if you can calculate the aceleration of the container you can calculate the aceleration of the droplets. After certain time, the container will move with the terminal velocity, in that case the droplets aceleration relative to the container will be the gravity aceleration.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.