# Vector Equivalent

## Recommended Posts

neilski_2003    142
Hi guys i have calculated the velocity of an object orbitting earth (which has a radius of 63678km + 240 km)to be the following w = 2Pi/86400 = 7.27 * 10^-5 v = r*w = 6618000 * (7.27 * 10^-5) = 481.13 m/s What i am trying to do is calculate the vector equivalent so i can use it in the relative velocity equation. Otherwise what i will have is Va = Vb + (w X r) Va = 481.13 + 0.00481j I'd guess thats not a correct answer as its a mixture of a scalar and a vector quantity so i need to get the Vb in its vector form and then add them together. Any help would be much appreciated. Cheers all Neil

##### Share on other sites
Bob Janova    769
An object in a circular orbit travels at right angles to the exis of orbit (i.e. the normal to the plane of the orbit) and also at right angles to the difference of locations between the object and the centre of the orbit (a fundamental property of a circle). So with those two vectors you can say the vector of the object's velocity at any time is

v = v(r×a)

v: vector velocity
v: scalar speed
r: normalised direction of object relative to centre of orbit
a: normalised axis of orbit

##### Share on other sites
neilski_2003    142
Ok, thanks very much.

In this case the object i am calculating is directly overhead. Therefore the normal would point straight down the y axis - which would be r, correct?

As for the other one (a) - do i need to calculate that seperately or should it be staring me in the face? (now that i have said that is the normal at that point not simply straight along the x axis?)

Neil