# Thread: Powder "Free Falling" in Vacuum

1. ## Powder "Free Falling" in Vacuum

I made experiment on "Free Fall". I made a conical hopper of 15 liter volume and a vacuum chamber of , installed hopper upon vacuum chamber of 65 liter volume. Fixed a 2.5" gate valve to discharge powder in vacuum chamber. Connected both, hopper and vacuum chamber to vacuum pump (63 cum/ hr) and evacuated air to till pressure was <-759 mm HG, from both hopper and chamber. By opening the valve of the hopper dropped the powder in vacuum chamber.

As per Newtonian physics, free fall is any motion of a body where gravity is the only acceleration acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on it.

But the powder did not fall straight instead it scattered all over the vacuum chamber.

Anybody can tell me the reason

Regards.

Ravindra

2. Dear Ravindra,

In a vacuum, the only force on a particle is the gravity, that is correct.

However, if the particle starts falling, as in your experiment, it is also getting horizontal impulses from the hopper while flowing out and thereby acquiring horizontal velocities.

That is what you see when the particles scatter around. The horizontal velocity components move the particle away from a perfect vertical fall.

I do not think that we need Einstein’s general relativity here. Newton is sufficient.

Remind, if you see particles moving in different directions, it is always caused by forces in different directions at any moment in time.

3. ## Powder "Free Falling" in Vacuum

Originally Posted by Teus Tuinenburg
Dear Ravindra,

In a vacuum, the only force on a particle is the gravity, that is correct.

However, if the particle starts falling, as in your experiment, it is also getting horizontal impulses from the hopper while flowing out and thereby acquiring horizontal velocities.

That is what you see when the particles scatter around. The horizontal velocity components move the particle away from a perfect vertical fall.

I do not think that we need Einstein’s general relativity here. Newton is sufficient.

Remind, if you see particles moving in different directions, it is always caused by forces in different directions at any moment in time.
Dear Teus Tuinenburg

you for advice. But what is the cause of this horizontal impulses and how to eliminate these impulses.

4. Dear Ravindra,

The horizontal impulses could come from collisions from the particles against a slope hopper wall, a valve restriction, asymmetric inter particle collisions.

The vertical acceleration gravity) causes a vertical velocity, but the collisions can convert the vertical velocity energy into horizontal velocity energy.

If you observe horizontal movement of the particles, where you expect only vertical movement, the first thing is to find out how that horizontal movement is acquired.

5. ## Behaviour in vcuum

If there is any pressure differential between the two chambers the particles will be dispersed by the expansion of the residual gas.

Lyn

6. Good day Lyn,

You are quite right.

If you observe any behavior in an experiment, you cannot rule out the causes of that observation nor the accuracy of the setup of that experiment.

Even the flow pattern of the out-flowing particles can cause this.

F.i. the particles exiting along the edge of the outlet will have a lower velocity than the center of the flow, due to friction.
This will cause inter particle collisions, which can cause the horizontal impulses.

7. ## Bulk Behavior

The fineness of the particles may also have something to do with this behavior. The feed hopper sketch seems sufficiently steep to create mass flow and sliding contact with the hopper may develop static that attracts the particles to the the chamber walls. The close packing of the particle could also inhibit deaeration of the mass and provide the residual air to disperse the particles on exposure to a higher vacuum. Alternately, inter-particle collisions due to differential flow rates will also cause lateral displacement. The experimental scale is not quite at the level of quantum effects so there is a reason for the effects experienced. If of practical interest the equipment should stand for an extended period prior to discharge so that the two vessel pressures are fully balanced, check the powder potential for static, then fit a short stand pipe under the slide valve to develop uniform exit velocity. Practical trials allows an understanding of behavior to be developed.

Lyn

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