Hi everyone! I'm really sorry if I'm using the wrong forum. It's my first time at PF. I'm pretty new to physics, as I began studying it just two years ago, but I'm really interested in the subject of astrophysics so I bought myself and started reading 'Introduction to rocket science and engineering' by Travis S. Taylor. So far I'm really enjoying it, but at the beginning of orbital mechanics, where the author also shows the basic dynamic equations related to gravity. And the third one looks weird. It says: F = -mg And as far as I know, it should look like this: F = mg Is this a mistake in my book, or there's a specific case in which the F = -mg can be used? Ps. If I'm using the wrong forum, tell me and I'll copy and delete the thread here and post it on the right one.