# [Tutor] Order Of Operations Question

David Hutto smokefloat at gmail.com
Thu Jul 29 01:10:16 CEST 2010

```On Wed, Jul 28, 2010 at 6:19 AM, Hugo Arts <hugo.yoshi at gmail.com> wrote:
> On Wed, Jul 28, 2010 at 11:41 AM, David Hutto <smokefloat at gmail.com> wrote:
>> From a practice exercise in Building Skills In Python page 64 I'm
>> working on How Much Does The Atmosphere Weigh? Part 1:
>> 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
>>
>
> Either there is an error in the texts, or you have not checked
> throughly enough. It goes wrong here:
>
>>    """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). This result is
>> mass of the atmosphere in kilograms per
>>    square meter (kg/m2).
>>    Mm2 = P0 × g"""
>>    masAtmoInKgPerSqM = airPressCLevl * gravity
>
> The Air pressure is in Pascal, which is kg / (m*s^2), not (kg * m)/s^2
> as you state. That is the Newton. Pascal can also be written as
> Newton/m^2, which is (M*g)/m^2. So to get mass per square meter, you
> should divide by the acceleration g, not multiply.

In the initial email I did forget this part:
Pressure is measured in Newtons, N, kg m/sec2. Air Pressure is is
measured in Newtons of force per square meter, N/m2.

Which might  have been a more informed question. I was trying not to
add in the whole question, and place the pieces in, but apparently
forgot one.
>
> With that modification I get about 5e18, which seems correct.

I get this with yours and Evert's modification from mult to div, but
if you're right, then the 10e18 the book states as a checker is a
typo/error.

Thanks, for your replies, I'll redo the example again, with a little
more sleep this time. Don't really need to know atmospheric pressure
right now, but might endup at wikipedia/google later.
>
> Hugo
>
```