# [Tutor] Order Of Operations Question

Alan Gauld alan.gauld at btinternet.com
Wed Jul 28 19:45:52 CEST 2010

```"David Hutto" <smokefloat at gmail.com> wrote

> To check it states that the answer should be app. 10**18kg However,
> and I've checked to make sure that the math I've layed out matches
> up
> with the texts, I get 5.07360705863e+20

A couple of comments but my Physics is a bit rusty!!

> import math
> def atmosphereWeight():
>    pi = math.pi
>    """Air Pressure (at sea level) P0. This is the long-term average.
>    P0 = 1.01325 × 10**5"""

You can express scientific notaton directly:

>    airPressCLevl = 1.01325*(10**5)

airPressCLevl = 1.01325e5

>    gravity = 9.82

It was 9.81 when I was at school.
Doubt if that accounts for the difference though...

>    """We can use g to get the kg of mass from the force of air
> pressure P0. Apply the acceleration of gravity
>    (in m/sec2) to the air pressure (in kg · m/sec2).

I'd expect pressure to be in Pa or N/m2 or (Kg*m/s2)/m2 = Kg/ms2?

> mass of the atmosphere in kilograms per
>    square meter (kg/m2).
>    Mm2 = P0 × g"""
>    masAtmoInKgPerSqM = airPressCLevl * gravity

So I'd expect P0/g?

>    """Given the mass of air per square meter, we need to know how
> many square meters of surface to apply
>    this mass to. Radius of Earth R in meters, m. This is an average
> radius; our planet isn’t a perfect sphere.
>    R = 6.37 × 10"""
>    avgRadiusEarth = 6.37 * (10**6)

I assume the comment's exponent is wrong?! :-)

>    """The area of a Sphere.
>    A = 4πr2"""
>    areaSphere = 4 * pi * (avgRadiusEarth**2)

>    """Mass of atmosphere (in Kg) is the weight per square meter,
> times the number of square meters
>    Ma = P0 × g × A"""

>    masEarthAtmoInKgPerSqM = airPressCLevl * gravity * areaSphere

Why don't you use the value you calculated above?

Now if you divide by 10 instead of multiplying by 10 you get a
different of 2 in the exponent?

>    print(masEarthAtmoInKgPerSqM)

HTH,

--
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/

```