Sputnik I gravitational potential energy change from apogee to perigee

[wr1015]

The first artificial satellite to orbit the Earth was Sputnik I, launched October 4, 1957. The mass of Sputnik I was 83.5 kg, and its distances from the center of the Earth at apogee and perigee were approximately 7370 km and 6560 km, respectively. Find the difference in gravitational potential energy for Sputnik I as it moved from apogee to perigee. (Use a positive sign for an increase, negative sign for a decrease in U.)

i've calculated the potential energies at both apogee and perigee and I'm thinking the answer would be negative since you're losing altitude so you're losing potential energy, but the force of gravity only gets stronger the closer to the Earth's surface you get. so would it be negative or positive?

 

[Pengwuino]

Well you need to calculate both. I'm sure you can rather easily find the gravity at both altitudes. Then you can figure out exactly what the potential energies will be.

 

[Astronuc]

http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html

By convention,

the general expression for gravitational potential energy arises from the law of gravity and is equal to the work done against gravity to bring a mass to a given point in space. Because of the inverse square nature of the gravity force, the force approaches zero for large distances, and it makes sense to choose the zero of gravitational potential energy at an infinite distance away. The gravitational potential energy near a planet is then negative, since gravity does positive work as the mass approaches.

 

[wr1015]

i got U @ apogee = -2419913865 J and U @ perigee = -2571509397 J, so then delta U would be Uapg - Uper = 151595532 J, is this right?