Expressing equation in terms of variables

[dori...@berkeley.edu]

Hi all,

I am new to SymPy and I am trying to rearrange equations in order to express one variable in terms of another in SymPy.

For example, I have two physics equations F=ma and rho=m/v which I defined in the beginning. I want to re-express ``a`` in terms of f,rho, and v.

However, the ``solve`` function sets the right hand side of the equation so that it is equal to zero so then when I try to get ``a`` in terms of `f` it returns zero.

Is there a way  I could do this SymPy, if so, which function should I be looking at instead?

Thank you.

In [6]:

from sympy import *

In [11]:

f = Symbol('f')

m = Symbol('m')

a = Symbol('a')

rho = Symbol('rho')

v = Symbol('v')

In [37]:

f=m*a

rho=m*v

In [38]:

solve(a,[f])

Out[38]:

[]

In [39]:

solve(rho,[v])

Out[39]:

[0]

In [42]:

solve_linear(f-m*a,f)

Out[42]:

(m, 0)

[Amit Saha]

On Mon, May 25, 2015 at 5:18 AM, <dori...@berkeley.edu> wrote:
> Hi all,
>
> I am new to SymPy and I am trying to rearrange equations in order to express
> one variable in terms of another in SymPy.
> For example, I have two physics equations F=ma and rho=m/v which I defined
> in the beginning. I want to re-express ``a`` in terms of f,rho, and v.
> However, the ``solve`` function sets the right hand side of the equation so
> that it is equal to zero so then when I try to get ``a`` in terms of `f` it
> returns zero.
> Is there a way I could do this SymPy, if so, which function should I be
> looking at instead?

Is this what you want?

First find m in terms of F and a:

>>> m1 = solve(F-m*a, m, dict=True)[0][m]

Now, feed in above to the second equation:

>>> solve(rho - m1/v, a)
[F/(rho*v)]



Best,
Amit.

>
> Thank you.
>
>
>
> In [6]:
>
> from sympy import *
>
> In [11]:
>
> f = Symbol('f')
>
> m = Symbol('m')
>
> a = Symbol('a')
>
> rho = Symbol('rho')
>
> v = Symbol('v')
>
> In [37]:
>
> f=m*a
>
> rho=m*v
>
> In [38]:
>
> solve(a,[f])
>
> Out[38]:
>
> []
>
> In [39]:
>
> solve(rho,[v])
>
> Out[39]:
>
> [0]
>
> In [42]:
>
> solve_linear(f-m*a,f)
>
> Out[42]:
>
> (m, 0)
>

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[Chris Smith]

On Monday, May 25, 2015 at 12:07:22 AM UTC-5, dori...@berkeley.edu wrote:

Hi all,

I am new to SymPy and I am trying to rearrange equations in order to express one variable in terms of another in SymPy.

For example, I have two physics equations F=ma and rho=m/v which I defined in the beginning. I want to re-express ``a`` in terms of f,rho, and v.

However, the ``solve`` function sets the right hand side of the equation so that it is equal to zero so then when I try to get ``a`` in terms of `f` it returns zero.

Is there a way  I could do this SymPy, if so, which function should I be looking at instead?

Instead of defining a Python symbol, e.g. f = m*a, define an equation containing the symbols of interest, e.g. e1 = Eq(f, ma). Then, when solving, tell solve what you *don't* want to solve for and it will attempt to solve for the remaining symbols:

>>> e1=Eq(f,m*a)

>>> e2=Eq(rho,m*v)

>>> solve((e1,e2),exclude=[f,rho,v])  # i.e. solve for m and a

[{m: rho/v, a: f*v/rho}]

>>> 

/c