PDA

View Full Version : Help With Spin

electromagneticpulse
2004-Dec-16, 01:43 AM
Okay i need help working out centrifugal force. I know the equation F=mv^2/r but that means nothing to me. It comes out with a number in newtons. But i get an answer of something like 2x10^34 which helps me with nothing, is it from the centre of the centrifugal orbit or on the object itself?

It's either my understanding is wrong or i'm working it out wrong. Also does anyone know how to order the formula to work out the velocity?

Thanks for any help :D

Moose
2004-Dec-16, 01:51 AM
Okay i need help working out centrifugal force. I know the equation F=mv^2/r but that means nothing to me. It comes out with a number in newtons. But i get an answer of something like 2x10^34 which helps me with nothing, is it from the centre of the centrifugal orbit or on the object itself?

Both, because of Newton's 3rd.

I can't help you with the velocity, it's been too long for me to remember that far back while exhausted.

swansont
2004-Dec-16, 01:56 AM
Okay i need help working out centrifugal force. I know the equation F=mv^2/r but that means nothing to me. It comes out with a number in newtons. But i get an answer of something like 2x10^34 which helps me with nothing, is it from the centre of the centrifugal orbit or on the object itself?

It's either my understanding is wrong or i'm working it out wrong. Also does anyone know how to order the formula to work out the velocity?

Thanks for any help :D
It would help a little to know the context.

There is no centrifugal force. mv^2/r is the centripetal force (centripetal = "center seeking") For an object to move in a circle, there must be a force directed toward the center. With no force, the object moves in a straight line.

The force in the equation is the net force on the object that is moving in the circular path at speed v.

Tobin Dax
2004-Dec-16, 04:07 AM
To add to this in a way that may help you check your math (without us knowing the context), the centripetal force of on an object in orbit of another should be equal to the gravitational force between the two objects.

Ut
2004-Dec-16, 04:36 AM
It's really futile to bother calculating things if you don't know what the forumlae mean. So, a quick tutorial.

Pretend O = theta, w = omega
O = the anglular displacement; w = the angular velocity

For a circular orbit:

s = rO (= 2*pi*r for O = 360 degrees)
ds/dt = r*dO/dt = r*w = v

a_c = w^2*r = v^2/r where a_c is the centripital acceleration.

F_c = m*a_c = m*v^2/r

So, you have an answer for this in newtons. Where'd you get that from, if you don't have the velocity?

v^2 = F_c*r/m

[v^2] = N*m/kg = (kg*m/s^2)*m/kg = (m/s)^2

Tobin Dax
2004-Dec-16, 07:21 AM
Wait, you don't know how to rewrite that to find the velocity? :o Get thee to thy teacher/TA fast. Basic algebra is very important in basic physics, and if you don't remember it, you really should get tutored in it somehow again.

Were I your TA, I would prefer that you come to me with your questions, especially if your having this much of a problem with your material. Posting to the BABB could work, but the teacher/TA can work with you and show you how to do this in person, which is much better that what you can get here. It's great that your asking for help, but now go ask the right people.

[There is no offense meant to anyone, either emp or other posters, in the above message. However, emp, your teacher/TA is there for a reason.]
[edit: forgot a letter somewhere]

swansont
2004-Dec-16, 11:59 AM
[There is no offense meant to anyone, either emp or other posters, in the above message. However, emp, your teacher/TA is there for a reason.]

Yes, but for all we know he could be learning his physics and astronomy on the streetcorner, with no TA around.

electromagneticpulse
2004-Dec-16, 05:54 PM
[There is no offense meant to anyone, either emp or other posters, in the above message. However, emp, your teacher/TA is there for a reason.]

Yes, but for all we know he could be learning his physics and astronomy on the streetcorner, with no TA around.

Bingo :wink: well not a street corner but close enough. I'm learning in my spare time and I have literally no money and Ive seen physics books costing upward of £20 (\$40). I've found one that only costs £10 but Im currently over £100 in debt with no job because I study health care full time. So Im currently using the free things I can find on the Internet. Be glad your not in my world :lol:

I have the velocity for one equation but it makes zero sense to me. I need the velocity for another object but I could probably work that out if i could understand the formula.

](*,) ](*,) damn brain! :evil:

Tobin Dax
2004-Dec-17, 04:20 PM
[There is no offense meant to anyone, either emp or other posters, in the above message. However, emp, your teacher/TA is there for a reason.]

Yes, but for all we know he could be learning his physics and astronomy on the streetcorner, with no TA around.

Bingo :wink: well not a street corner but close enough. I'm learning in my spare time and I have literally no money and Ive seen physics books costing upward of £20 (\$40). I've found one that only costs £10 but Im currently over £100 in debt with no job because I study health care full time. So Im currently using the free things I can find on the Internet. Be glad your not in my world :lol:

I have the velocity for one equation but it makes zero sense to me. I need the velocity for another object but I could probably work that out if i could understand the formula.

](*,) ](*,) damn brain! :evil:

Ah, gotcha. Please excuse my post, then. Had to release a little steam at the end of the semester, as I am currently a TA. :)

Good luck with your studies, and I'll be glad to answer more questions (more nicely, even :wink: ) after I finish my final on Saturday morning. [Yes, you saw that right. Be glad you're not me. :( :wink: ]

A Thousand Pardons
2004-Dec-17, 04:40 PM
Okay i need help working out centrifugal force. I know the equation F=mv^2/r but that means nothing to me. It comes out with a number in newtons. But i get an answer of something like 2x10^34
That's a very big number. Can you fill in the details of how you arrived at that? What was m, v, r?

electromagneticpulse
2004-Dec-17, 09:41 PM
Okay i need help working out centrifugal force. I know the equation F=mv^2/r but that means nothing to me. It comes out with a number in newtons. But i get an answer of something like 2x10^34
That's a very big number. Can you fill in the details of how you arrived at that? What was m, v, r?

There was a small calculation error but it's still large, which it needs to be but i doubt that large.

m=5.9742x10^24 kg (had it at 27)
v=30,000 m/s
r=152,887,680,000 m (about the earths orbit IIRC)

G=260m/s^2
M=1.98892^10^30

As you might guess its the size of earth. When i tryed to work out the velocity, which i just attempted again, came out at 937,768,758.4 km/s if you hadn't noticed the earth is traveling at over 1,000 times the speed of light. Oh well got to keep working on it :roll:

swansont
2004-Dec-18, 01:22 AM
Okay i need help working out centrifugal force. I know the equation F=mv^2/r but that means nothing to me. It comes out with a number in newtons. But i get an answer of something like 2x10^34
That's a very big number. Can you fill in the details of how you arrived at that? What was m, v, r?

There was a small calculation error but it's still large, which it needs to be but i doubt that large.

m=5.9742x10^24 kg (had it at 27)
v=30,000 m/s
r=152,887,680,000 m (about the earths orbit IIRC)

G=260m/s^2
M=1.98892^10^30

As you might guess its the size of earth. When i tryed to work out the velocity, which i just attempted again, came out at 937,768,758.4 km/s if you hadn't noticed the earth is traveling at over 1,000 times the speed of light. Oh well got to keep working on it :roll:

If you mean G to be Newton's gravitation constant (as in GMm/r^2), then it's 6.67 x 10^-11 Nm^2/kg^2

The centripetal force will be equal to the gravitational force, so
mv^2/r = GMm/r^2, or v= sqrt(GM/r). I get roughly 30 km/s

A Thousand Pardons
2004-Dec-18, 12:11 PM
v=30,000 m/s

That agrees with swansont :)

r=152,887,680,000 m (about the earths orbit IIRC)

G=260m/s^2

What is G?

M=1.98892^10^30

As you might guess its the size of earth. When i tryed to work out the velocity, which i just attempted again, came out at 937,768,758.4 km/s if you hadn't noticed the earth is traveling at over 1,000 times the speed of light. Oh well got to keep working on it :roll:
You are calculating the centripetal force on the Earth?

Kizarvexis
2004-Dec-18, 10:57 PM
As you might guess its the size of earth. When i tryed to work out the velocity, which i just attempted again, came out at 937,768,758.4 km/s if you hadn't noticed the earth is traveling at over 1,000 times the speed of light. Oh well got to keep working on it :roll:

My wife did something similar in her college calculus class. She had to figure the speed of a satellite in orbit and made a simple math error in the calculations. She ended up with the satellite moving at warp 8! :) So don't worry about those simple kinds of errors, everyone does them at one time or another. Just ask NASA about the Mars Climate Orbiter . :)

Kizarvexis

electromagneticpulse
2004-Dec-19, 04:46 PM
I never got taught calculus, British school system too shoddy. We need scientists but they don't teach us science #-o

So it looks like Im learning calculus... lucky Im good with math :)

Kizarvexis
2004-Dec-19, 04:52 PM
I never got taught calculus, British school system too shoddy. We need scientists but they don't teach us science #-o

So it looks like Im learning calculus... lucky Im good with math :)

I don't know if what you are doing is the same as hers. She had to calculate the orbit and had all these strange terms I had never heard before. I just followed the formula and pointed out the math errors. I never did understand what calc formulas she was using, just that the end result was how fast the satellite was traveling in orbit.

Kizarvexis