<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>* An object in orbit is constantly accelerating. In the case of a circular orbit, that* acceleration is called the centripetal (or center seeking) acceleration "a" and* takes the formThat's what I was thinking, because the trajectory of a orbiting object is changing constantly in relation to it's motion vector (the object "would want to continue on a straight line" but gravity does not allow it forcing change in trajectory thus creating perpetual acceleration). Changing trajectory (vs. motion vector) means acceleration, hence constant acceleration that you so well explained as the centripetal acceleration.But this also made me think about energy loss, because any acceleration in my mind requires energy.Where does the energy come for centripetal acceleration? <br />Posted by aphh</DIV></p><p>Actually, you might find it clearer if you think of things in the force/acceleration regime, rather than the energy regime.</p><p>The reason I say that, is that for a circular orbit, with drag neglected, energy is a constant. The magnitude of the velocity vector is constant. Now, you might say, "Wayne, if there is an acceleration in the system, the magnitude of the velocity has to change, doesn't it?"</p><p>What happens is this. For the satellite, a central force is acting, that is, the gravitational attraction between the object and the Earth. The force is (for a circular orbit), perpendicular to the velocity vector of the satellite, so it does not change the magnitude of the velocity vector, but it does change its direction. In this case, it changes it in such a way that the trajectory is curved into a circular trajectory.</p><p>The key here to remember is this - an acceleration leads to time rate of change in the velocity vector. That change can be in direction, or magnitude, or both.</p><p> Wayne<br /></p> <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>