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Lets Physics That started in 2012 as the basis of my masters degree final thesis. It was designed to help increase intrinsic motivation in students, which it did to a certain extent (Click to read my thesis!). Since 2012 it has expanded enormously, and was rebranded in 2015 as 'Lets Physics That'. It is now used by students as a homework tool, a practise tool, and an opportunity to extend their learning. It even forms the basis of one of the courses I teach in my general physics class.  Try it out for yourself here! School: Example School, Password: hello (opens new window)
Lets Physics That is now used as the basis for all homework I set.
Extension apps are fully customisable within a topic. You can make them really easy so you can learn the basics, or you can make them really hard to really ensure you know the material well. Students can either free practise or they can complete the tasks based upon extensions to earn points for their efforts.
The best part about extension apps is that generate new questions every single time. No question is ever the same as the last, meaning there is always continual practise for the students... the extensions even randomise the information students are given in some cases to make them think more deeply about how to solve problems.