Design of a cell phone stand - force balance question

[rudy]

TL;DR Summary

I am trying to determine the minimum length for the base, but I am struggling with the sum of the moments equation to find the length.

Hello all-

I am trying to design a simple cell phone holder that will be printed using a 3D printer. For this phone holder I would like the base to be the minimum length possible to avoid the thing tipping over backwards. I drew a simplified slope and base which represents the phone on the stand, but I am pretty rusty on my mechanics and I think I am making some incorrect assumptions in my problem.

For the problem I am assuming the holder is rigid, the thickness of the phone is negligible, and the mass of the holder is negligible. I did not show it in the drawing but there will be a little ledge at the bottom to prevent sliding down.

I have attached my attempt to solve for the length of the base, but as you can see my solution is nonsense. Is anyone able to see what I am doing wrong?

Thank you for your help. In exchange I'd be happy to print someone a cell phone holder once it is complete. (Or any other printable CAD file you provide. Then maybe I will feel like slightly less of a mooch in these forums).

 

[Baluncore]

If the holder has low mass, the combination will have a high centre of mass.
It will then only sit on an absolutely flat surface that is steady.

If you minimise the area of the base it will fall over.
It needs a large area, heavy base, and a full set of design specifications.

Is the challenge to find how close to the edge of the cliff you can walk before you fall off ?

 

[rudy]

Thanks for the feedback - but isn't that the challenge of every engineering question? Once I find the exact location of the cliff, I will decide how far away I want to stand.

I concede that the design is far from perfect, it's really just a couple of sketches at the moment.

 

[anorlunda]

but isn't that the challenge of every engineering question?

No. Usually the challenge is to optimize things. Close to disaster is seldom the optimum.

 

[rudy]

Neither of you are helping me here. Furthermore, you are twisting my words! I said "decide how far away I want to stand" (think Factor of Safety), not "stand with half of my body mass over the edge".

If you would like to correct my approach then I am open to suggestion, I came here for help. But even though I may not have the background of many of the moderators (or members) here but that does not mean that I should be treated like an idiot.

I know you have contributed a lot to this forum and helped many people (possibly even me in the past) - so thank you for that. However, I have zero thanks in this instance. Not a single word of those responses has been relevant to the question or something I didn't already know.

Looking forward to more of my words being twisted following this.

 

[Baluncore]

Neither of you are helping me here. Furthermore, you are twisting my words! I said "decide how far away I want to stand" (think Factor of Safety), not "stand with half of my body mass over the edge".

You twisted my words from the second post. I said walk, not stand.
Walking the cliff edge is a metaphor for the product surviving in life. That is the difference between continued use, or being thrown in the trash.

It needs a large area, heavy base, and a full set of design specifications.

Design specifications?
What are the dimensions of the phone?
What force must the phone oppose without falling? Directional?
What is the mass of the phone? Where is the centre of mass?
How is the phone attached to the holder?

Why make a bent flat sheet when you can introduce a corrugation or an edge lip that will provide greater rigidity?
Why provide a large thin sheet when you only need a clip-on framework?

 

[rudy]

Thanks for your help (sarcasm), goodbye physics forums.

PS "a", "b" etc are labeled in the drawing as your "specs". I'm sure you're familiar with solving such equations so that numbers can be plugged in later. I'm solving for "b".

Mass is "m", as labeled in the drawing. It's just a preliminary calculation I was trying to get help with.

 

[anorlunda]

Sorry if we were harsh. You want to design a "good" cell phone holder. Why is the minimum length base good? The shorter and narrower the base, the more likely it is to tip?

 

[Tom.G]

@rudy, if you are still around, some further simplification to the problem may get you started.

Try the worst case that someone will push down on the top edge of the phone (maybe it has a touchscreen, or a charging jack up there). That indicates the base needs to reach under the top edge of the phone.

If the phone buttons are pushed horizontally (left to right in your sketch) with enough force, the stand will either slide on its resting surface or tip over from the torque. The torque of course will be the product of the horizontal force and its distance above the resting surface.

If the phone buttons are pushed at some angle between horizontal and vertical, the resultant horiz. & vert. forces can be determined using SIN and COS of the angle and the applied force.

(sorry, too lazy to write the eqs. and solve the problem)

Cheers,
Tom

 

[bobob]

This seems like a complicated way to do something simple. If I were designing a cellphone stand, my first concern would be aesthetics and not what I could get away with in the dimensions.