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#### donsgal

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I believe that gravitational pull causes things to fall at 16 per second squared. So...I take that to mean that the first second a rock would fall 16 feet and the next second it would fall 32 feet and the next second 64 feet.

Keeping that in mind. I am wondering how long it would take to reach 240 feet. My math is really bad so I really can't figure it out for myself. Can anybody help me out?

donsgal

#### james dilley

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I would think 15 seconds by your theroy, 15 X 16 = 240 so your answer would be 240 divided by 16 = 15 seconds, I don't know if your line of thought on this is on track as I never tried to figure the way you posted!!!!!

#### suburbanite

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x = v*t + g * t^2

v = 0 so

x = g * t ^2

t^2 = x/g

t = squareroot (x/g)

x = 240 feet
g = 32 feet/second^2

t = squareroot (240 feet /32 feet/second^2)
t = squareroot (7.5 second^2)

t = 2.7386 seconds

neglecting air resistance

#### Beltane

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suburbanite said:
x = v*t + g * t^2

v = 0 so

x = g * t ^2

t^2 = x/g

t = squareroot (x/g)

x = 240 feet
g = 32 feet/second^2

t = squareroot (240 feet /32 feet/second^2)
t = squareroot (7.5 second^2)

t = 2.7386 seconds

neglecting air resistance
Oh my! I have REALLY been out of college too long!

Seriously though, I entered as engineering and ended in Anthropology.

#### suburbanite

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Donsgal, just call it an even 3 seconds.

#### sheepish

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Just to add another dimension, if you added a parachute to your rock, or if your well was really deep, the rock would reach terminal velocity. It would not continue to accelerate after that point, but just drop at the same rate for the rest of the way down.

#### OkieDavid

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Um....Physics is a wonderful course of study and serves a very useful conversation starter at dinner parties but why not simply tie said rock on to a spool of twine and lower it until you contact the water then mark and measure the twine? There is also a really neat contraption called a well probe that uses either/and/or flashing lights and buzzers........

#### Deacon Mike

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donsgal said:
I believe that gravitational pull causes things to fall at 16 per second squared. So...I take that to mean that the first second a rock would fall 16 feet and the next second it would fall 32 feet and the next second 64 feet.

Keeping that in mind. I am wondering how long it would take to reach 240 feet. My math is really bad so I really can't figure it out for myself. Can anybody help me out?

donsgal

the correct number is 9.8 m (32 feet) per second squared in a vacuum.

The formula for distance is

d = (1/2)9.8(m/s2)t2

so 240ft = 73.152m
73.152=(1/2)9.8(m/s2)t2

so solving for t, you get 3.863803772 seconds

#### BobK

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If your looking for water depth in the well a string with something that floats works better than filling in your well with rocks.....

#### Deacon Mike

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suburbanite said:
Donsgal, just call it an even 3 seconds.
Actually. closer to 4. Your previous calculation is incorrect. It time zero, the speed is zero, at time 1 speed is 32ft/s Avg speed for first second is 16ft/s.

#### suburbanite

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:doh:

I left out the 1/2 in front of the g* t^2.

That changes the result to 3.6 seconds though, which still disagrees with your result.

#### Deacon Mike

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suburbanite said:
:doh:

I left out the 1/2 in front of the g* t^2.

That changes the result to 3.6 seconds though, which still disagrees with your result.
That's because your using 32ft/s and I'm using 9.8m/s. You'd think they'd be equal but 32 ft equals 9.7536. So I think it's rounding.

#### suburbanite

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Thanks Deacon.

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donsgal,
If you're trying to figure out how far down your water is, forget the math. We tried it and it just didn't work; no matter what we did, the rock bounced off the side of the casing a couple of times on the way down and that kept messing with the math. Instead, what we did was buy one of those alarms people put next to the water heater - it beeps when it gets wet, cost \$12 at Lowes. We tied it to a long line and lowered it down the well. When we heard the beep, we marked the line and pulled it up then measured the line.

#### Cabin Fever

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donsgal,
If you're trying to figure out how far down your water is, forget the math. We tried it and it just didn't work; no matter what we did, the rock bounced off the side of the casing a couple of times on the way down and that kept messing with the math. Instead, what we did was buy one of those alarms people put next to the water heater - it beeps when it gets wet, cost \$12 at Lowes. We tied it to a long line and lowered it down the well. When we heard the beep, we marked the line and pulled it up then measured the line.
No one would use the "math method" to determine the distance to the water level in their well. A logical person would simply tie a washer or bolt to some kite string and drop it down the well casing.

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Cabin Fever said:
No one would use the "math method" to determine the distance to the water level in their well. A logical person would simply tie a washer or bolt to some kite string and drop it down the well casing.

Guess I was misunderstanding the reason for donsgal's question then. :shrug:

#### michiganfarmer

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Guess I was misunderstanding the reason for donsgal's question then. :shrug:
I dont think she gave a reason. She just asked how long it would take for a rock to fall 240 feet

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michiganfarmer said:
I dont think she gave a reason. She just asked how long it would take for a rock to fall 240 feet

I was just assuming. For the life of me, I can't figure any other reason for the question.

#### suburbanite

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Maybe she's planning to murder someone by dropping a brick on them from 240 feet off a building and wants to maximize her getaway plan?

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