# physics

posted by .

take a satalite going around the earth...

what allows us to say

net force (radial) = Fg = Fr = ma

were the Fg is the force of gravity exerted on the satalite

and ma is the mass and acceleration of the satalite

ok I understand newtons second law but what I don't understand is what allows us to say Fg = Fr

I thought in problems were the force were along the same direction... you added them together to obtain the net force...

i thought the net force would be
Fg + Fr

sense the Fg is points downwards towards the earth and the radial force (sometimes called centripetal) is also pointing along this direction...

this lead me to believe that the net force was

Fg + Fr...

Thanks!

• physics -

Fg (the gravity force) and Fr (the raqdial force) are the same thing. You don't add them.

Assuming the satellite is in a circular orbit, the gravity force is perpendicular to the direction of motion, which is tangent to the crcular orbit.

The radial (gravity) force equals mass times centripetal acceleration, m V^2/R

• Question -

ok why are they the same thing???

• physics -

Because the gravity force acts radially