Gravity!

This app simulates the effects of gravity. You can fling particles at each other and watch them interact. Create solar systems and galaxies! Make a planet and fling asteroids into it! Add a black hole and watch everything get sucked in! You can even adjust the size, mass, and color of the particles! The desktop version contains most of the features of the Gravity! Extended Edition for iOS, with two new features that are not in the iOS app. You can keep planets from flying off screen by checking the "keep centered" option. Also, you can manually enter coordinates for planets for more precise simulations. The iOS version is available for free with an in-app purchase to unlock additional content.