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wswartzendruber
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Location: Idaho, USA

 Posted: Mon Sep 12, 2011 2:38 am    Post subject: Kinematics: Let Me Confirm How This Works My physics professor filled the board up with derivatives and integrals, and I'm lost, but I think I may have the absolute basics down. I've been dropping random shit in my house and measuring how long it takes to hit the ground. I'm leaving out units in this post. Here's what I figure so far: Acceleration on the earth is 9.8, so: a = 9.8 If I want the velocity, I integrate with regard to time, so: v = 9.8t And if I want vertical (distance), I integrate again, so: y = 4.9t² So, the final equation for determining the height traveled on earth would be... y = 4.9t² + bt + c ...where b is like an initial velocity and c is the offset distance. So do I have this right, or do I need to keep throwing random shit around my house?
richk449
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 Posted: Mon Sep 12, 2011 2:48 am    Post subject: Looks good to me. Have you really been dropping stuff and measuring it? If so, that is totally awesome.
wswartzendruber
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Posted: Mon Sep 12, 2011 2:51 am    Post subject:

 richk449 wrote: Looks good to me. Have you really been dropping stuff and measuring it? If so, that is totally awesome.

I've been using a metal drywall fastener and dropping that on the ground. It takes the predicted amount to time to collide with my hardwood floor.
richk449
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Posted: Mon Sep 12, 2011 2:55 am    Post subject:

wswartzendruber wrote:
 richk449 wrote: Looks good to me. Have you really been dropping stuff and measuring it? If so, that is totally awesome.

I've been using a metal drywall fastener and dropping that on the ground. It takes the predicted amount to time to collide with my hardwood floor.

I have a hard time imagining the ability to measure time to less than 1 second with a stopwatch. This would require dropping from roughly 5 meters.
aCOSwt
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Posted: Mon Sep 12, 2011 7:39 am    Post subject: Re: Kinematics: Let Me Confirm How This Works

 wswartzendruber wrote: So do I have this right, or do I need to keep throwing random shit around my house?

Well well... throwing random shit around your house will never confirm the equations you wrote !
If you really want to know if you are right, only pure maths can do that.
Pure maths + understanding !

In order to know you are actually right are, you must understand :

1/ Why is the equation of the instant speed equal to the derivative of the equation of the instant position
2/ Why is the equation of the instant acceleration equal to the derivative of the equation of the instant speed
3/ Where the initial offset / initial speed come from mathematically.

If you understood each of the three points then you will have the privilege to know that you are right.

=> And then I will ask you to prove it and please correct the equation you gave for v and then justify the last expression you gave for y.

The other solution you get is :

Believe this is right, learn it by heart and then expect it will still be right tomorrow or elsewhere !
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Bones McCracker
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Posted: Mon Sep 12, 2011 9:53 am    Post subject:

I don't see anything wrong with his equation for instantaneous velocity, other than failing to show units of measure (m/s).

One must also eventually understand that this assumes initial velocity to be zero, assumes away all forces other than gravity such as air resistance, and assumes acceleration due to gravity to be constant, which is not actually true over very long distances such as from space.
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juniper
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Posted: Mon Sep 12, 2011 10:16 am    Post subject:

 BoneKracker wrote: I don't see anything wrong with his equation for instantaneous velocity, other than failing to show units of measure (m/s).

don't you nutters use some crazy units, like f/s or inches/s?

 Quote: One must also eventually understand that this assumes initial velocity to be zero, assumes away all forces other than gravity such as air resistance, and assumes acceleration due to gravity to be constant, which is not actually true over very long distances such as from space.

b is the init velocity, not necessarily 0. the assumption is that the down direction is positive, which is fine. assumption on forces is standard in a beginning kinematics course. As for your last point, I know that american houses are big, but you can assume that the acceleration of gravity is constant in his house
richk449
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Posted: Mon Sep 12, 2011 1:46 pm    Post subject:

 juniper wrote: don't you nutters use some crazy units, like f/s or inches/s?

I think all American schools use MKS units. The antiquated units are used in industry, and in the media. I bet that 99.9% of engineers in the US are more comfortable working in MKS units.
ichbinsisyphos
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 Posted: Mon Sep 12, 2011 1:58 pm    Post subject: You should refer to it as SI - "Système international d'unités" instead of MKS, or FIF - "freedom international freedom" in American.
richk449
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Posted: Mon Sep 12, 2011 2:41 pm    Post subject:

 ichbinsisyphos wrote: You should refer to it as SI - "Système international d'unités" instead of MKS, or FIF - "freedom international freedom" in American.

Okay. In electromagnetics, we make a distinction between MKS and CGS, and I thought that both were part of SI.
ichbinsisyphos
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 Posted: Mon Sep 12, 2011 2:53 pm    Post subject: Nope, both are metric systems though. I just read on Wikipedia that MKS isn't identical with SI, SI replaced and extended MKS, but CGS is something entirely different.
juniper
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Posted: Mon Sep 12, 2011 7:10 pm    Post subject:

richk449 wrote:
 ichbinsisyphos wrote: You should refer to it as SI - "Système international d'unités" instead of MKS, or FIF - "freedom international freedom" in American.

Okay. In electromagnetics, we make a distinction between MKS and CGS, and I thought that both were part of SI.

never heard it called cgs (presumably 1/100m, 1/1000 kg, s). I call it SI.
Bones McCracker
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Posted: Mon Sep 12, 2011 11:35 pm    Post subject:

 BoneKracker wrote: I don't see anything wrong with his equation for instantaneous velocity, other than failing to show units of measure (m/s). One must also eventually understand that this assumes initial velocity to be zero,"

 juniper wrote: b is the init velocity, not necessarily 0.

There is no 'b' in his equation of instantaneous velocity.
 Quote: If I want the velocity, I integrate with regard to time, so: v = 9.8t

Which should really be the following, if you want to do away with the assumption of zero initial velocity.
 Code: v(t) = -9.8(t) -b m/s

Maybe that's what aCOSwt was getting at when he asked wswartzendruber to "correct" his equation for velocity.
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juniper
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Posted: Tue Sep 13, 2011 10:28 am    Post subject:

 BoneKracker wrote: There is no 'b' in his equation of instantaneous velocity.

I was looking at his final equation. You are right about the other one.
rtomek
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Posted: Tue Sep 13, 2011 7:16 pm    Post subject: Re: Kinematics: Let Me Confirm How This Works

 wswartzendruber wrote: Acceleration on the earth is 9.8, so: a = 9.8
Are you at sea level with no mountains nearby? Also, what time of day is it, because the moon may have an effect too.

My older brother's physics class used 10 for a. Those assholes had it so easy, could just do most of the equations in their heads.
wildhorse
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Posted: Tue Sep 13, 2011 8:34 pm    Post subject:

BoneKracker wrote:
Which should really be the following, if you want to do away with the assumption of zero initial velocity.
 Code: v(t) = -9.8(t) -b m/s

Maybe that's what aCOSwt was getting at when he asked wswartzendruber to "correct" his equation for velocity.
Arrrgh! Help! Let me out!

patrix_neo
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 Posted: Tue Sep 13, 2011 8:44 pm    Post subject: I don't know shize but if you drop some random stuff in your house, you have to take in account of two things at least. * The earth spinning, and the body decelerating it by some vector*km/second. * Fraction of the surounding atmosphere..might be spelled as friction Those are the force vectors besides 9.8 m/s-2 ...I know of. But as usual, I think you guys are nuts. Last edited by patrix_neo on Tue Sep 13, 2011 8:51 pm; edited 1 time in total
szczerb
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 Posted: Tue Sep 13, 2011 8:48 pm    Post subject: You meant friction of the atmosphere? Unless he lives at CERN or some other fun place with nice toys, it's not observable at his place ;]
patrix_neo
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Posted: Tue Sep 13, 2011 8:48 pm    Post subject: Re: Kinematics: Let Me Confirm How This Works

 rtomek wrote: ...because the moon may have an effect too. ..

The tidal forces, then is the time for me to measure my length...I feel a bit longer then.

 Friction...ofcourse, nothing else to read here. Move along.
patrix_neo
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Posted: Tue Sep 13, 2011 8:56 pm    Post subject:

 szczerb wrote: You meant friction of the atmosphere? Unless he lives at CERN or some other fun place with nice toys, it's not observable at his place ;]

I like vaccum cleaners...
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