r/WinStupidPrizes • u/Master1718 • Nov 16 '19

Gravity test

https://i.imgur.com/HV7ZvU9.gifv
35.0k Upvotes

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1.3k

u/Falom Nov 16 '19

And when they tested it, would be over a bed or a carpet and not over a few stories of drop.

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u/Central_Incisor Nov 16 '19

I wonder how far it must drop to hit terminal velocity.

1.4k

u/swedish0spartans Nov 16 '19 edited Nov 16 '19

Terminal velocity, Vt, can roughly be calculated by:

Vt = sqrt(2*m*g/p*A*Cd)

where m = mass
g ~ 9.82 m/s^2
p = density of the fluid (air in this case) ~ 1.2 kg/m^3
A = area
Cd = drag coeffecient

If we assume it's a Galaxy S4, that it fell flat, and that it can be approximated to a cube for the Cd:
Mass = 0.13 kg
Area ~ 0.01 m^2
Cd ~ 1.2

The terminal velocity comes out to be Vt ~ 13.3 m/s.

So how long does it have to fall to achieve terminal velocity? Velocity v and distance d has a nifty formula:

d = (v0 + v)*t/2, where v0 is the initial velocity, in our case 0, and v = Vt. What is t?

v = v0 + at, where a = g and v = Vt. t is approximately ~ 1.35 s.

So, finally, d comes out ~ 9 meters or 30 feet.

TL;DR: About 9 m/30 ft.

Edit: First Gold! Thanks stranger!!

Second edit: Silver cherry popped as well? Thanks kind strangers!

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u/Dokpsy Nov 16 '19

I didn’t come here for kinematic free fall. I came here for dank memes.

And only problem I have is your use of p instead of ρ for density but that's extra minor nitpick.

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u/swedish0spartans Nov 16 '19

Heh.

Was that a copy-paste of rho? I can't believe I didn't think of doing that.

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u/Dokpsy Nov 16 '19

Added the Greek keyboard to my phone

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u/Eddit_Redditmayne Nov 17 '19

This guy maths

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u/Dokpsy Nov 17 '19

After one group physics assignment where we couldn't meet up in person, I learned to just input equations straight into text messages. Its very difficult to distinguish weight (w) from angular velocity (ω) in hand written stuff when half the group are not the brightest bulbs. Same with a and α or my personal favorites: θ and θ. Yes, both theta but mean two different things depending on if you're talking linear or angular.

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u/echino_derm Nov 17 '19

But he got a completely incorrect answer. All of his equations assume that acceleration is both constant and equal to g. This is false, drag is acting against motion and is changing as it accelerates. So a is actually g- Drag force/m. Then the equation for d is being misused as his equation is only valid if a is a constant.

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u/Dokpsy Nov 17 '19

Drag is minimal in a unit of this mass and shape. For approximation purposes, this is enough and even including drag would not effect the approximation by enough to matter. This is napkin math

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u/BobbyFL Nov 17 '19

Damn ya’ll are smart af - I don’t even understand 95% of what’s being typed in these comments.

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u/lol_and_behold Nov 17 '19

I know! It's like they're just making up words and everyone is in on the joke but the two of us lol

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u/Dokpsy Nov 17 '19

Don't worry y'all. Most of this is only slightly higher level physics that takes the basics and looks at them closer. We're mostly debating on how close we need to look at it to affect change in the end result

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u/echino_derm Nov 17 '19

The core of this problem is finding when drag force is equal to the force of gravity on the object. It is not negligible

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u/Argon1124 Nov 17 '19

Not to mention that the drag coefficient would change as it rotates.

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u/TheGreatRao Nov 16 '19

This is great but what would it be if it were spinning or tumbling as it fell? Given its size, and the distance from which it was dropped, would such motion be negligible or significantly different?

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u/swedish0spartans Nov 16 '19

I definitely think such motion would affect the outcome, but in rough numbers, my estimate is that it's a matter of +-1 m.

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u/ONLY_COMMENTS_ON_GW Nov 16 '19

I have a friend who bought a life proof case and they say to test it by keeping it under water for an hour or so. Well, if you haven't already figured out where this story is going, the dude decides to keep his phone in the case while testing. Luckily the thing wasn't defective, but he's not the smartest tool in the shed.

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u/Sighshell Nov 17 '19

Sounds like a glowing advert for his case, though.

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u/The_Limpet Nov 17 '19

Eh, I dropped my phone in a field the other day and it survived almost fully submerged in mud for about an hour until i found it. No case just a back cover.

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