Exploring the Reel's Reverse Motion: What Causes It?

[ayoz]

A string is wrapped around a reel. The reel is the set on a horizontal flat surface. This string is set perpendicularly to a surface and pulled. The reel starts to roll in the direction away from the string. However, as the angle between the reel string and the surface decreases, the velocity of the reel decreases, until it reaches an angle where it doesn't roll, but rather just slides. After the angle has been passed the reel starts rolling in the opposite direction, towards the string.

My question is, what causes this phenomenon? Why does the reel start moving in the opposite direction, and not just continue moving in the same direction?

 

[Simon Bridge]

Welcome to PF;
It is a consequence of Newton's laws.
Try sketching out the situations and put the forces on.

 

[A.T.]

See the diagram here:

http://physics.stackexchange.com/qu...ion-of-this-yoyo-under-external-force-will-be