# Centripetal forces

1. May 15, 2007

### Michael123

Hi, I'm currently studying Physics for my GCSEs and I'm revising for them but the text books only explain what happens but not why.

I know that centripetal force is the force requirement for an object to move round in a circle. But I don't see why this is the case for satellites orbitting the earth. The gravity is pulling the object inwards yet the object moves in a circle... why is this? The satellite is moving at a steady speed which keeps it following the orbit path but is there anything that caused this speed initiate? Because when someone jumps off a building they'll fall straight to the ground rather than orbit the earth at high speed.

2. May 15, 2007

### Hootenanny

Staff Emeritus
Welcome to the forums,

When the satellite was launched, it was done so such that it had some tangential velocity. This tangential velocity was calculated to allow the satellite to obtain the required orbit at a certain height above the earth. Now, it is the gravitational attraction between the earth and the satellite which provides the centripetal force which is required for circular motion. So the satellite, although it experiences a force toward the centre of the earth and is constantly accelerating toward earth, remains in a circular orbit.

Note that the circular orbit is just an approximation; generally the satellite orbits are elliptical like those of the planets. I hope that made sense

3. May 15, 2007

### Michael123

Great explanation, thanks!