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Consider a drone, or any lifting vehicle, enclosed in a container. Will it lift along with the container? Suppose I place a small drone in a large container of negligible weight and place them in space. Will drone move forward pushing container?

I know it doesn't work, but don't know the reason.


marked as duplicate by Red Act, John Rennie, Jon Custer, ZeroTheHero, Aaron Stevens Jan 3 at 21:55

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drone in boxThe reason why it will not work is because the air that the drone is pushing down will push the bottom of the container down too.

Imagine yourself in a box half your height. Will you be able to get up straight? No, because you will be pushing the bottom of the box down with the same force that you are using to push yourself up :)

Per Aaron Stevens' comment, it is worth mentioning the fact that in a case of a very tall box it is possible that the drone could momentarily (briefly) lift the box as it takes time for the air pressure to build onto the bottom of the box.
It is also possible to momentarily lift the box if the drone speeds up really fast and hits the top of the box. The drone's inertia would move the box.

F1 = F2 because: The air pushed down by the drone's blades has to transfer its momentum to something in order for it to be able to change course and return to the blades, and the bottom part of the box is the only thing around. If the box (container) is completely sealed, it will take the full impact (100%) of the air pushed down by the drone.

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    $\begingroup$ So I agree that the force that the drone pushes on the air is the same as the force the the air pushes on the drone by Newton's third law, but I don't see how $F_1=F_2$ as in your diagram. I'm not saying you are wrong, I just think it needs more explanation. $\endgroup$ – Aaron Stevens Jan 3 at 5:21
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    $\begingroup$ Also, keep in mind that it takes time for disturbances to propagate through air, so what if the container is very very tall? $\endgroup$ – Aaron Stevens Jan 3 at 5:29

The question can be explained by Force Equilibrium equations as stated by @Alex Doe . Therefore we are actually trying to conclude two variables which are Force and Work. You are trying to question if there is a force exerted internally in system, is there any chance to cause work done by the body. To make it more clear I again made a similar Force Equilibrium diagrams which I am giving here.Force Equilibrium

In the case 1, The forces net force that needs to be exerted is "Force =Mass * g"

In case 1; According to newton's third law, Every force has equal and opposite reaction. Here in order to achieve this force the two drones need to exert the same force in downward direction and push the container.

In case 2: It is similar but you've to notice that the force exerted on the container thus increasing the total mass to 2Mg and the more the force the more the mass. And finally it creates a deformation.

This earlier discussion says that, according to Newtonian mechanics the netforce exerted by an internal system is equal to zero. And the net work done by internal forces is zero provided that they are not doing any deformation in the rigid body.

On simple analogy you cannot stop a bus from pushing the back of a bus from inside.


Suppose I place a small drone in a large container of negligible weight and place them in space. Will drone move forward pushing container?

In space? What do you mean by that? Out of Earth's atmosphere? Out of Earth's gravitational field?

I note that all comments and answers didn't address your question, but another, somewhat related: would the container lift?

A second note is that the first tag is erroneous, since aerodynamics has nothing to do with your question.

It's only necessary to observe that a drone within a container can only apply a force internal to the system container+air+drone, which will automatically give rise to an opposite reaction from air or from container, or both.

COM of the whole system cannot move because of that. However if the drone lifts the container will drop so to keep the COM steady. If the drone moves right, the container will move left, etc.


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