How I first got to know about this was through an article from EduMatters. When I downloaded and used the app, i thought it was not only cute because of the local flavour, it was also rather useful for my P5 son to answer MCQs in the form of flashcards to earn badges.
I leave you to read the review from the original article and you decide if this app is something you will pay $3.98 for.
Experiment #7 - Forces (Frictional Force)
Experiment #6 - Forces (Frictional Force)