# Gravitational potential energy question

1. Oct 30, 2005

### 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?

2. Oct 30, 2005

### 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.

3. Oct 30, 2005

### Staff: Mentor

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

By convention,

4. Oct 30, 2005

### 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?