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FBDs of two stacked boxes?

by InvisibleMan1
Tags: boxes, fbds, stacked
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Jul20-10, 09:14 PM
P: 40
To help me understand how a force is distributed over multiple connected objects, I have been trying to draw the FBDs of two stacked boxes which are sitting on the ground. I haven't been able to solve the problem however, and looking for the solution with Google did not turn up anything useful. I'm aware of Newton's third law, but I still have not been able to solve this... I'm at a loss, which is why I am posting here.

Here is an image of the problem:

One attempt at solving it using Newton's third law ended up as (what appeared to be) an infinite loop of action-and-reaction forces going up and down the stack. I tried thinking of the two objects as a single object, but that just ended in guesswork without anything real concrete.

What would the FBD of A and the FBD of B look like, with all involved forces?
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Jul20-10, 09:38 PM
P: 1,416
So A and B each feel a gravitational force (also resultant in reactionate forces meaning you have to draw two upward vectors in the earth's core, but i think we're ignoring them here). Since A is not moving, it has to experience an equal (in magnitude) force upward: this can only come from B, and this force's reactionate brother is the force B experiences from A, the latter obviously pushing down. This force makes B extra heavy and thus the ground under B must deliver a force not only for B itself, but also the load B is carrying (this is indirectly what allows B to hold A up in its place). This last force from the ground to B also has a reactionate brother pushing from the blocks onto the ground (which is the force that would make for a weighing scale placed under B to react)

I hope this helps?
Jul21-10, 12:30 AM
P: 40
That solved the problem, thanks :)

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