5.1. Normal Force and Friction Force http://www.ck12.org
FIGURE 5.6
Therefore∑
x
F=Fcosθ−fk= 0 ,60 cos 30◦=fk;fk= 51. 96 = 52 N.(c) We assume the block has no vertical motion, therefore,∑
y
F= 0
∑
yF=FN+Fsinθ−mg=0;FN=mg−Fsinθ= 100 −60 sin 30◦= 70 N(d) Sincefk=μkFN;μk=FfkN= 7052 = 0 .74;μk= 0. 74
Static Friction
The original meaning of “static” is “not moving”. Static friction,fs, exists when the contact surfaces do not slide
relative to one another. Two examples would be: (1) a coin on an inclined surface that remains stationary and (2)
exerting a force on a heavy couch that refuses to move.
For a given pair of surfaces, the coefficient of static frictionμsis larger than the coefficient of kinetic friction,μk.
Put simply, there is less friction when objects are in motion. We mentioned earlier in the section that regardless of
how smooth the surfaces of two objects appear, at the microscopic level they are very rough. Atoms actually interact
along the irregular contact areas between the surfaces forming electrical bonds. As soon as there is relative motion
between the surfaces, the bonds begin to break. Heat due to friction continues to aid in breaking the bonds, making
it easier to maintain motion. Kinetic friction is smaller than static friction; andμk<μs.
ConsiderFigure5.7.