# Homework Help: Reference frames

1. Feb 12, 2006

### SarahM

Is the comparison of the kinetic energy of a satellite in orbit around the earth to that of a truck travelling down a road realistic? Are they in the same reference frame?

2. Feb 12, 2006

### Andrew Mason

It depends on how fast the truck is moving.

They are not in the same reference frame. They are both in an accelerating reference frame since they are both moving in a circle around the earth.

AM

3. Feb 12, 2006

### Tide

That's actually an interesting question! Consider the situation when the truck is travelling down the road at 60 mph while the satellite is in geostationary orbit and you're observing from some comfortable perch that is fixed with respect to the Earth's surface. The satellite has no kinetic energy while the truck has quite a bit.

Clearly, there is something amiss with the comparison. To compare their kinetic energies properly you should use the same reference frame but the problem here is that the obvious reference frame (connected to the Earth's surface) is a rotating frame. Going to a nonrotating reference frame gives a very different picture.

4. Feb 12, 2006

### SarahM

Well the truck has a velocity of 0.0289km/s and a K.E of 0.844 kgkm^2/s^2 and the satellite a tangential velocity of 7.473 km/s and a K.E of 279249 kgkm^2/s^2.
I was trying to compare the two K.E values but wasnt sure if it was realistic to do so.

5. Feb 12, 2006

### Tide

It is realistic only in the sense of giving you some appreciation of the magnitudes.

Incidentally, energy is usually expressed in Joules = kg m/s^2 so the truck has 844,000 Joules of KE while the satellite has 2.79 x 10^11 Joules of KE.