Types of friction
We usually know about three types of friction: Static friction, limiting friction and sliding friction. But there is one more category to the types of friction known as the rolling friction. In this article we will learn about the difference between rolling and sliding friction. But before that we need to analyze the two basic types of friction sliding and rolling motion. In sliding motion, each point on the body has only translational or linear motion. But in case of rolling motion, different points have a combination of linear as well as rotational motion. Look at the following figure:
If we consider only the rotational motion then point B tends to move backwards suppose with a velocity VR. Similarly we consider only linear motion then point B moves with a velocity VL in the forward direction. Now the resultant motion of the point B will be a superposition of both the above velocities. Depending upon the values of VR and VL the rolling friction can act in the following directions:
if VR > VL then the point B tends to move backwards so as we know friction opposes relative motion between the surfaces in contact hence it will act in the forward direction.
Similarly, if VR < VL then the point B tends to move forward and hence the friction acts in the backward direction.
Also, if VR = VL although the ball moves in the forward direction the frictional force acting on the ball will be zero. This is unlike sliding friction where the friction force will definitely act if the body is moving.
So now we know different types of friction and in which case they will act in which direction. But one thing is for sure they will always oppose the motion between surfaces in contact.
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