Download Summary: Friction Friction is a Force that opposes the relative

Summary: Friction
Friction is a Force that opposes the relative motion of two surfaces in contact. Its magnitude is
determined by two factors:
1. The “roughness” of the surfaces together. This is quantified using the “coefficient of
friction”, μ.
a. this number is always > 0
b. this number has no units
c. this number is specific to the two surfaces in contact
2. The Normal Force that acts on the object.
So, in general, we say that f = μFN.
Static friction acts on an object when it is not in motion relative to the surface, and acts in
a direction to counteract any applied force F. We must overcome static friction to cause an
object to move relative to the surface it is on. The magnitude of static friction varies depending
on the applied force F and the maximum amount of static friction fs,max:
1. If F ≤ fs,max, then the object will not move, and then fs = F.
2. If F > fs,max, then the object will move, fs briefly = fs,max, but when the object moves, we
no longer have Static friction.
3. ** Note that, when F = fs,max, this is the “tipping point”; at this stage, the object is just
barely ready to start moving.
** Note that fs,max = μsFN.
Kinetic friction acts on the object once it is moving relative to the surface it is in contact
with. The direction of this force is always directly opposed to the velocity of the object. The
magnitude of this force is a constant value: fk = μkFN.
Lastly, note that, in all cases, fs > fk. This is primarily because μs > μk. An interesting fact: the
force of friction does not depend on the surface area of contact.