http://www.bbc.co.uk/schools/scienceclips/ages/8_9/friction.shtml

( Paste this URL on to a freshly opened Google  Bar if it does not display as a link)

Play the game on this site and answer the questions in the quiz that follows. You will have to drag the surface from the box below to the surface of the track for it to be displayed on he track. It is a good way to understand a force.

What is the force we are talking about here?
What is your understanding about the force we work with in the game?
How does it apply to our Central Idea of the present Inquiry?