On block on top of another block connected to a spring on a friction surface!
A block with mass m2 on top of another block with mass m1 on a frictionless surface.
Assume friction coeficient between blocks is mu.
The lower block is attached to a spring with constant k.
Click the play button or you can drag the lower block horizontally, release the mouse and watch how friction force between blocks cause the top mass to move.
Due to the friction force between blocks, the velocity of top block is in the same direction of it's friction force.

Virtual Physics laboratory Original URL

Fu-Kwun Hwang, Dept. Of Physics, National Taiwan Normal University.