How Do You Calculate the Gravitational Field Strength on Mars?

[ahrog]

Homework Statement

A 250-kg space probe weighs 935 N on the surface of Mars. What is the gravitational field strength on the surface of Mars?

Homework Equations

W=mg
g=Gm1/r^2
Fg=Gm1m2/r^2

The Attempt at a Solution

Okay, so at first I thought it would be:
g=(6.67x10^-11)(6.42x10^23)/(3.38x10^6)^2
g=3.75 m/s^2

BUT

Then I remembered the weight of the probe...What do I do with that?

Oh wait, brainflash.

If I went W=mg and changed it to g=W/m
g=935N/250kg
=3.74 m/s^2

...So which answer is right?

 

[Hootenanny]

In the first case you have calculated the gravitational acceleration of the probe, that is the acceleration at which a unit mass will experience if it placed a distance r from the probe in the absence of any other fields.

The latter answer is correct.

Do you now understand why?

 

[ahrog]

Yeah, thanks! That helps! :)

 

[Redbelly98]

...So which answer is right?

Since both answers were the same, where was the problem?

I would go with the second solution method, since that is simpler and it makes direct use of the given information about mass and weight.

 

[Hootenanny]

Since both answers were the same, where was the problem

How very true. I hadn't noticed that the OP had used the mass of Mars!