# Why is friction force is not considered as centripetal force?

This is a solved example in my text book"Engineering mechanics dynamics. R.C.Hibbeler" :

The 3 kg disk is attached to the end of a cord. The other end of the cord is attached to a ball and socket joint located at the center of the platform. If the platform rotates rapidly, and the disk is placed on it and released from rest, determine the time it takes for the disk to reach a spead great enough to break the cord. The maximum tension the cord can sustain is 100 N.. And the coefficient of the kinetic friction between the disk and the platform is 0.1

when the free body diagram of the particle was drawn, the friction force was considered in the tangential direction,, but it wasn't considered in the normal direction(tension was only considered) why? I was expecting that there is kinetic friction in the normal direction, since the disk is moving relative to the platform.

Because friction resists either a centrifugal force or a centripetal force - ie friction opposes any motion.

• This is in static analysis but in the dynamic analysis, the friction force will be in the same direction of the motion. Jun 27, 2017 at 11:47
• @MuhammadAbdulrasool Since when? I can't think of any scenarios where that is true.
– JMac
Jun 27, 2017 at 12:43
• @JMac The kinetic friction forces (plural!) always act in the same direction as the relative motion of the bodies. By Newton's third law, the two forces (one on each body) have opposite signs, of course. In complicated problems, it's usually easier to work with a consistent sign convention and some negative magnitudes, rather than drawing free body diagrams that look like "Custer's last stand" with arrows pointing every which way. Jun 27, 2017 at 13:26
• @alephzero It acts in the opposite direction of the motion of the body. The force on one body is in the same direction as the movement of the other body.
– JMac
Jun 27, 2017 at 13:29
• Do friction forces on a car act to accelerate it or slow it down? Jun 27, 2017 at 14:27