- #1

- 14

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and i want to find 'M' on its own, (i never have been good at this) but here is my attempt:

M = 4pi^2/G(T^2/R^3)

is this correct?

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- Thread starter Dark_Dragon
- Start date

- #1

- 14

- 0

and i want to find 'M' on its own, (i never have been good at this) but here is my attempt:

M = 4pi^2/G(T^2/R^3)

is this correct?

- #2

Filip Larsen

Gold Member

- 1,354

- 271

No (not as I read your notation). Note that when you "move" something like a/b to the other side of an equation it becomes b/a.

It may help to transform your equation only one step at a time. If you are in doubt about the transformation rules, then remember that you are transforming the two sides by doing the same thing on both sides, i.e. to "move" the term [itex]a[/itex] in [itex]a+b[/itex] you actually add [itex]-a[/itex] to both sides so that the original side becomes [itex]a-a+b = 0+b = b[/itex]; to "move" the factor [itex]a[/itex] in [itex]a \cdot b[/itex] you actually multiply with [itex]1/a[/itex] on both sides that on the original side becomes [itex](a/a) \cdot b = 1 \cdot b = b[/itex].

It may help to transform your equation only one step at a time. If you are in doubt about the transformation rules, then remember that you are transforming the two sides by doing the same thing on both sides, i.e. to "move" the term [itex]a[/itex] in [itex]a+b[/itex] you actually add [itex]-a[/itex] to both sides so that the original side becomes [itex]a-a+b = 0+b = b[/itex]; to "move" the factor [itex]a[/itex] in [itex]a \cdot b[/itex] you actually multiply with [itex]1/a[/itex] on both sides that on the original side becomes [itex](a/a) \cdot b = 1 \cdot b = b[/itex].

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- #3

- 14

- 0

after having a look at it for a while, i tried one step at a time and i came out with:

M = (4*pi^2)(T^2/R^3)/G

is this any better? please dont take offence if i didnt understand your post.

M = (4*pi^2)(T^2/R^3)/G

is this any better? please dont take offence if i didnt understand your post.

Last edited:

- #4

Filip Larsen

Gold Member

- 1,354

- 271

The problem is with [itex]T^2/R^3[/itex]. Originally that factor is on one side of the equation and must at some transformation step be moved to the other side. You seem to move it verbatim to the other side so it ends up being the same [itex]T^2/R^3[/itex] which is not correct.

Note that the factor is in the form of a fraction (i.e. a/b) and you should carefully consider what happens when you move such a fractional factor to the other side of an equation. If you have a text book you may want to look up reciprocal.

- #5

- 14

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then would the equation be:

M = (4pi^2)(R^3/T^2) / G ?

am i any closer?

- #6

- 17

- 0

- #7

Filip Larsen

Gold Member

- 1,354

- 271

then would the equation be:

M = (4pi^2)(R^3/T^2) / G ?

am i any closer?

Now it looks right, yes.

- #8

- 14

- 0

i have a fair bit of trouble with rearrangements,

so thanks for the help! =)

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