Weight on Mars: Calculating 56lbs from 150lbs

[Afide]

Homework Statement

The problem is to find what 150lbs on Earth = Xlbs on mars.

Homework Equations

F=G(m1)m2/d^2

G = 6.67x10^-11 (obviously)

The Attempt at a Solution

I'm getting incredibly annoyed because he gave us the wrong units etc, and we didn't even go over this in class. The last time I did this was in Highschool Physics...150lbs = 68KG

mars has .108 the mass of Earth and the diameter is 6,787KM
FG (W) = 6.67x10^-11 (68 KG) 6.45x10^23 KG
___________________________ = 8.63 Newtons?...
3393.5^2 KMI know my answer needs to be 249N which = 56lbs (I went onto an online calculator thing)

 

[AEM]

Homework Statement

The problem is to find what 150lbs on Earth = Xlbs on mars.

Homework Equations

F=G(m1)m2/d^2

G = 6.67x10^-11 (obviously)

The Attempt at a Solution

I'm getting incredibly annoyed because he gave us the wrong units etc, and we didn't even go over this in class. The last time I did this was in Highschool Physics...


150lbs = 68KG

mars has .108 the mass of Earth and the diameter is 6,787KM



FG (W) = 6.67x10^-11 (68 KG) 6.45x10^23 KG
___________________________ = 8.63 Newtons?...
3393.5^2 KM


I know my answer needs to be 249N which = 56lbs (I went onto an online calculator thing)

Check your units. Shouldn't you have the units of Mars' radius in meters? Make sure your units for G go with your other units.

 

[Doc Al]

This might be easier to solve using ratios.

$$F_e = \frac{GmM_e}{R_e^2}$$

$$F_m = \frac{GmM_m}{R_m^2}$$

Divide to find F_m/F_e. Doing it this way, several things cancel and you don't have to worry about converting units.