# Reason for neglecting frictional force in a particular type of Motion

A thin inelastic string is tightly wound on a cylinder. One of the ends of the string is attached to the above surface. Initially the whole system (string +cylinder) is held near the surface. When it is released from that position, find the acceleration of the centre of mass of the cylinder. When we are calculating the sum of the forces on the body we are not taking into account the frictional force between the cylinder and the thread. Why is it so?

• Short answer: because the problem does not specify the friction coefficient. Never make your life more complicated than it is. Long answer: for macroscopic objects in this problem neglecting the friction coefficient is a good approximation. It is very rare when it is significant in real life situations of this sort. Dec 11, 2018 at 21:50
• Short answer #2: because friction is usually complicated, so, if neglecting it doesn't make things too wrong, then we do it. Dec 11, 2018 at 22:02

The $$T$$ in your diagram is the external force exerted on the cylinder by the string and it is the tension in the string at the point where it just not touching the cylinder.