0

This is not a question about how already rolling objects or stationary objects with torque continue rolling. I understand that this happens because of static friction.

I know that round objects sliding or spinning in place on a surface with friction will roll.

When the object is sliding, my hypothesis was that friction exerts a torque on the object, causing it to spin. However, when the object is spinning in place, this explanation implies that the torque caused by friction opposes the rotation of the object and there is no force causing the object to roll. In this case, it seems that friction acts as a linear force.

My current hypothesis is that friction acts as partially a torque and partially a linear force. However, I have no way to test nor quantify this.

My question is: What is the correct explanation for the two phenomena above?

nonhuman
  • 3

1 Answers1

1

Yes, friction is a force on the object but because it is applied at the point of contact, it also applies a torque. There is no need for the word "partially". Any force not applied at the center of mass will be both a force and a torque. If you start the object spinning in place, friction will cause it to move forward and, at the same time, decrease its angular velocity. More generally, whenever slip is occuring, i.e. $v\neq r\omega$, friction will attempt to "close the gap" and restore no-slip. Provided that there is no other force applied, one of the quantities increases and the other decreases until they are equal again.

Vincent Thacker
  • 15,416