Newton's Theory of Gravity and impact speed

[Kalie]

A 1.0 kg object is released from rest 500 km above the earth.
A. What is its impact speed as it hits the ground? Ignore air resistance
B. What would the impact speed be if the Earth were flat?
I don't know how to start this.
I said that v=sqrt(2G*M_e*m/(R_e+300))= 1.33*10^9 but that is incorrect...I'm probably being stupid but please help

 

[OlderDan]

A 1.0 kg object is released from rest 500 km above the earth.
A. What is its impact speed as it hits the ground? Ignore air resistance
B. What would the impact speed be if the Earth were flat?
I don't know how to start this.
I said that v=sqrt(2G*M_e*m/(R_e+300))= 1.33*10^9 but that is incorrect...I'm probably being stupid but please help

You seem to recognize that this problem has something to do with energy conservation. With the reference of zero potential at infinity the object has potential energy at altitude, but also when it hits the ground. You have several dimensional inconsistencies in your equation. What is 300? Why are there both M_e and m on the right and no m on the left.?

I have no idea what part C is about. Flat how? A cube? A huge plane of mass? I have no idea.