If motion starts and stops at the same place is this distance zero ?

Displacement is zero. Displacement is in reference to the starting and ending positions, so starting and ending in the same place would cause a net displacement of zero. Distance, however, is not zero. It is a measurement of the total amount moved, so if the object left the starting position, the distance is automatically not zero.

Abbreviated as mg-25.

For cyclohexane compounds drawn in a chair conformation:   -axial bonds are vertical  -equatorial bonds are parallel to two bonds within the ring.  thus, as i assume the chair conformation is drawn:   -the x group is in an axial position, as are the five other vertical bonds.  -three axial bonds are on the top face of the ring  -the other three axial bonds are on the bottom face  the position of the second substituent on each ring carbon is equatorial.  gauche groups must be on an adjacent carbon such as carbon 6 and carbon 2, and also gauche groups are rotated by 60 degrees relative to each other.  drawing the newman projections for c1–c2 and c1–c6 aids in visualizing the groups that are gauche to the y group on c1.  click this link and watch some academic videos that may you more understand the topic