Kinetic Energy in reference frame

1. Apr 12, 2007

waterfaire

One way to attack a satellite in Earth orbit is to launch a swarm of pellets in the same orbit but in the opposite direction. Suppose a satellite in a circular orbit 500km above Earth's surface colides with a pellet having a mass of 4g.

What is the KE of the pellet in the reference frame of the sallite

R = 6.87x10^6m
G = 6.67x10^-11
Mearth = 5.98x10^24
M of pellet = .004kg

For this...I tried finding the velocity of the Satellite...V = (squareroot (GMearth/R)), which turned out to be 7620m/s

I know that the pellet is going in the oppsite direction..so that would make the speed of the pellet -7620m/s

then I used the KE equation. K = .5mv^2 and got the answer to be 116118J, which isnt the right answr...

HELP!

2. Apr 12, 2007

D H

Staff Emeritus
Your derived the velocity of the pellet relative to the center of the Earth. What is the velocity of the pellet relative to the satellite?

3. Apr 12, 2007

waterfaire

V pellet to satellite = V pellet to earth + V earth to satellite

V pe = -7620...
V es = 7620...

so wouldn't it just be 0????

4. Apr 12, 2007

D H

Staff Emeritus
Does that answer make any sense whatsoever?

Bringing this problem down to Earth, imagine you are driving 60 mph on a freeway. A mile or so down the road in front of you, you see a car driving toward you in your lane (i.e., its driving the wrong way down the freeway). Let's say that vehicle is also going 60 mph. How fast is it going relative to you?

5. Nov 20, 2010

auk411

So....

Anyone want to answer this person's question. I'm having the same problems as the original asker. The responses have been less than helpful.