Simple Machines by Tinybop allows students to experience how to create simple machines. It would be great for cause and effect. The app is currently on special for $1.29 (this is for a limited time). Another great app to add to your list of problem-solving apps.
The Tinybop website also has a Simple Machines downloadable PDF, which has additional information about how levels, axels, pully’s, inclined planes etc work. Each section has discussion questions which correspond with the app. Additional experiments in each section are also suggested.