for more info to continued answer go here 

http://forums.udacity.com/ph100/questions/9173/

my-own-gravity-at-10cm-away-from-me-hehehe-and-

now-black-holes-gravity-to-test-if-light-can-e

scape-one-or-not-and-again-some-more-11-questions-

for-help-in-the-black-hole-question

**Q4. Say a white dwarf has a mass of the sun but the radius of earth.

 What is the surface gravity of it? How much will you weigh on it??**

so:

  

mass = 1.98x10^30 kg

radius = 6.4x10^6

gravitational constant = 6.67x10^-11 Nm/kg^2

a = (G x m)/r^2

= (6.67x10^-11 x 1.98x10^30) / 6.4x10^6^2

= 3224267.5 m/s^2

we will wheigh:

m=40kg

a = 3.22x10^6 m/s^2

             

f=ma

=40 x 3.22x10^6

=1.28x10^8 N !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!