If there is no friction, the world will be very different from what we are now.
We would not be able to walk but slide on the ground.
Usually, we might think that the friction force is in the oppositive direction of the velocity.
However, it is not always the case. There are many cases in which the friction force is in the same direction of the velocity of the moving object. Especailly, when the friction is the driving force which cause the object to change it's motion.
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.
Question for you:
Does friction force always in the oppositive direction of the relative between two objects?