Q: When a block attached to the string is placed on a ramp (an inclined surface). You said in class that if the block is staic there was no friction because there was tension force from the string. Let us say the block was slowly moving downward by releasing the string
slowly, would there be a tension force and friction? Or can there only be one of the 2 acting on the object at once?
A: Let us look at each of the 2 cases.
#1. Static: The block remains rest on the inclined surface. Refer to the Free Body diagram, Since tension T cancels out wx (x component of weight) for zero net force in x-axis, there is NO motion tendency in x-axis. As a result, there is NO static friction (fs = 0).
#2. Dynamic (in motion): The block is moving downward on the inclined surface. Refer to the Free Body diagram. Since there is a motion, we have kinetic friction fk between the block and the surface which is opposite to the direction of motion. Depending on the acceleration in the motion (then Fnet in x-axis), we may have kinetic friction alone or both kinetic friction fk and tension force T at the same time.
