Datasheet

Page 28· Applied Robotics with the SumoBot
WN
0WN
0Fy
=
=−
=
∑
N = W indicates that the normal force, N, is pushing back just as hard as the object's
weight is pushing down on the surface. This has to be true. If it wasn't, the block would
sink through the table, or maybe the table would start sinking into the earth.
Newton's Third Law can be summarized like this: if two bodies (such as the block and the
surface it's on) interact, each body exerts an equal and opposite force on the other.
The drawings of the forces acting on the block and setting the sum of all the forces equal to
zero is dictated by this law.
Coefficients of Friction
Different pairs of surfaces tend to exert different kinetic and maximum static frictional
forces. For example, if you try to slip your shoe along concrete on a sidewalk, it'll
probably resist pretty strongly. However, if there's ice on the sidewalk, your shoe will
slip right along it with barely any frictional force.
Since each pair of materials resists the applied force with different levels of f
S
and f
K
, a
term called the coefficient of friction is used to predict how much force it will take to
make one material slide along another. In the case of static friction, this coefficient, µ
s
is
the maximum force you can apply before the object starts sliding divided by the normal
force. In the case of kinetic friction, µ
k
is the force required to keep the object sliding,
divided by the normal force.
N
f
and
N
f
K
K
Max,S
S
== µµ
The Greek letter mu - µ - is commonly used for to denote coefficients of friction. µ is also a
coefficient for units such as seconds, amps, and meters (µs, µA, µm). In those cases, µ is a
different coefficient, with a value of one-one-millionth (
1
/
1,000,000
). If you are referring to the
coefficient of static friction, µ
S
, always remember to subscript the capital S. When referring
to microseconds, it's just µs, with a lower-case 's'.