the search for T
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Short exploratory note by Phil dated 1.12.17, part of Appendix F on a dumbbell satellite in a rotating frame. He tries to recover the angular-coordinate estimate T = 3m1 ω² r1 cos²θ from the three Cartesian equations of motion plus the constraint x²+y²+z²=r1². Eliminating terms leads in circles, and substituting the known T back in nearly satisfies the constraint check, leaving a residual -ω²x². The note ends unresolved.
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The Search for T PhL 1.12.17
Question: How can I get an estimate of T as I did in the angular world in (F.6.29)? There I had three equations in θ,φ and I used the first radial one to estimate T. In Cartesians I have this
= - [(ω2b3/r'13) + (T/m1r1)]x
= - [(ω2b3/r'13) + (T/m1r1) - ω2]y + 2ω
= - [(ω2b3/r'13) - ω2] (b+z) - (T/m1r1)z - 2ω
x2+y2+z2 = r12 (F.8.10)
From (F.6.25) I know that
(b/r'1)3 ≈ 1 - 3(r1/b)cosθ ≈ [1 - 3z/b]
so equations become
= - [(ω2[1 - 3z/b]) + (T/m1r1)]x
= - [(ω2[1 - 3z/b]) + (T/m1r1) - ω2]y + 2ω
= - [(ω2[1 - 3z/b]) - ω2] (b+z) - (T/m1r1)z - 2ω
First equation says
( I am looking for a result T = 3m1 ω2r1cos2θ = (3m1 ω2/r1)r12cos2θ = (3m1 ω2/r1)z2
m1r1 = - m1r1[(ω2[1 - 3z/b]) + Tx
m1r1 = - m1r1ω2 + m1r1ω23(z/b) + Tx
I don't see my result coming out of this equation! I do know that
x2+y2+z2 = r12
x +y +z = 0
x + 2 + y +2 + z + 2 = 0
but that does not seem to help much. Suppose I drop the z/b terms in all three equations above:
= - [ω2 + (T/m1r1)]x
= - [ω2 + (T/m1r1) - ω2]y + 2ω
= - (T/m1r1)z - 2ω
again unclear what I can do here. Go back again to
= - [(ω2b3/r'13) + (T/m1r1)]x
= - [(ω2b3/r'13) + (T/m1r1) - ω2]y + 2ω
= - [(ω2b3/r'13) - ω2] (b+z) - (T/m1r1)z - 2ω
x2+y2+z2 = r12 (F.8.10)
What happens if I eliminate the (ω2b3/r'13) terms instead of eliminate T?
1 = - (A + B)x
2 = - (A + B - ω2)y + 2ω
3 = - (A - ω2) (b+z) - Bz - 2ω
4 x2+y2+z2 = r12 (F.8.12)
When I try to eliminate A from 1 and 2, B also goes away and that gives what I already know
y - x = -ω2xy - 2ωx (F.8.13)
Let's now try to eliminate A from 1 and 3
1*(b+z): (b+z) = - (A + B)x(b+z)
3*x: x = - (A - ω2) (b+z)x - Bzx - 2ω x
= -A (b+z)x + ω2 (b+z)x - Bzx - 2ω x
Now subtract to get
(b+z) - x = - (A + B)x(b+z) +A (b+z)x - ω2 (b+z)x + Bzx + 2ω x
= -Ax(b+z) - Bx(b+z) +A (b+z)x - ω2 (b+z)x + Bzx + 2ω x
= - Bx(b+z) - ω2 (b+z)x + Bzx + 2ω x
= - Bxb - Bxz - ω2 (b+z)x + Bzx + 2ω x
= - Bxb - ω2 (b+z)x + 2ω x
and so B manages to survive. I then have
(b+z) - x = - Bxb - ω2 (b+z)x + 2ω x
or
b + z - x = - Bxb - ω2 (b+z)x + 2ω x (*)
But I also know that
z - x = Abx - ω2(b+z)x + 2ωx
so equation (*) becomes
Abx - ω2(b+z)x + 2ωx + b = - Bxb - ω2 (b+z)x + 2ω x
or
Abx + b = - Bxb
or
Ax + = - Bx
or
= - (A+B)x around in a circle of course.
So unlike in spherical coordinates, there seems to be no easy want to extract an approximate expression for T because it seems you need the radial equation to do that!
What happens if I just insert my known result for T into these equations?
= - [(ω2b3/r'13) + (T/m1r1)]x
= - [(ω2b3/r'13) + (T/m1r1) - ω2]y + 2ω
= - [(ω2b3/r'13) - ω2] (b+z) - (T/m1r1)z - 2ω
x2+y2+z2 = r12 (F.8.10)
T = 3m1 ω2r1cos2θ = (3m1 ω2/r1)r12cos2θ = (3m1 ω2/r1)z2 dimT = M T-2L = ma correct
(T/m1r1) = 3ω2(z/r1)2 T-2 = T-2
Go back instead to
= - [(ω2[1 - 3z/b]) + (T/m1r1)]x
= - [(ω2[1 - 3z/b]) + (T/m1r1) - ω2]y + 2ω
= - [(ω2[1 - 3z/b]) - ω2] (b+z) - (T/m1r1)z - 2ω
and insert (T/m1r1) = 3ω2(z/r1)2to get
= - [(ω2[1 - 3z/b]) +3ω2(z/r1)2]x
= - [(ω2[1 - 3z/b]) + 3ω2(z/r1)2 - ω2]y + 2ω
= - [(ω2[1 - 3z/b]) - ω2] (b+z) - 3ω2(z/r1)2z - 2ω
Now drop the z/b terms where appropriate
= - [(ω2[1]) +3ω2(z/r1)2]x
= - [ - 3ω2z/b + 3ω2(z/r1)2]y + 2ω
= - [ - 3ω2z/b] (b+z) - 3ω2(z/r1)2z - 2ω
or
= - [(ω2 +3ω2(z/r1)2]x
= - [ - 3ω2z/b + 3ω2(z/r1)2]y + 2ω
= - [ - 3ω2z/b] b- 3ω2(z/r1)2z - 2ω
or
= - ω2[(1 +3(z/r1)2]x
= - 3ω2[ -z/b + (z/r1)2]y + 2ω
= 3ω2z - 3ω2(z/r1)2z - 2ω
or
= - ω2[(1+3(z/r1)2]x
= - 3ω2(z/r1)2y + 2ω
= 3ω2z[1 - (z/r1)2] - 2ω
We know that
x + 2 + y +2 + z + 2 = 0
but if velocities are small we can say
x + y + z ≈ 0
Insert now from above (with velocities ignored) to check this result
x + y + z
= x{- ω2[(1+3(z/r1)2]x} + y{ - 3ω2(z/r1)2y} + z{3ω2z[1 - (z/r1)2]}
= - ω2[(1+3(z/r1)2]x2 - 3ω2(z/r1)2y2 + 3ω2[1 - (z/r1)2] z2
= -ω2x2 - 3ω2(z/r1)2x2 - 3ω2(z/r1)2y2 + 3ω2z2 - 3ω2 (z/r1)2z2
= -ω2x2 + 3ω2z2 - 3ω2(z/r1)2[ x2+y2+z2]
= -ω2x2 + 3ω2z2 - 3ω2(z/r1)2[r12]
= -ω2x2 + 3ω2z2 - 3ω2z2
= -ω2x2 // same result I got last time.
so it almost works! Very unclear.
x2+y2+z2 = r12
x +y +z = 0
x + 2 + y +2 + z + 2 = 0