Transcript Document

» Normal force: the force that arises in solids
when surfaces come into contact.
» It is an electromagnetic force between
atoms which prevents an object placed on
a solid surface from penetrating into the
space the atoms of the surface already
occupy.
» The normal force is a CONTACT force
exerted by a surface that is always
perpendicular outward from the surface.
» Consider a book placed on a wooden
shelf. The shelf exerts a normal
force on the book which is
perpendicular to the shelf, whether
the shelf is flat or fallen at an angle...
FN
FN
» A 1kg book is resting on a shelf.
Calculate the magnitude of the
normal force that the shelf exerts on
the book.
First, find the net force acting
FN
Fw=mg
on the book.
ΣF=FN-Fw
ΣF=FN-Fw=ma but a=0 why??
FN-Fw=0
FN=Fw=(1kg)(9.8m/s2)=9.8N
» Let the Box sit at rest on a flat Table. If
other forces in addition to the Weight and
Normal forces act in the vertical direction,
then the Normal force and Weight are NO
LONGER EQUAL. For instance, if we stood
on the box, or if we attached a rope and
pulled up slightly. In both these cases, the
normal force must balance all other vertical
forces so that the net vertical force = 0.
» Say a book weighing 2 Newtons is
sitting on an inclined surface. How
much of its weight is parallel to the
surface, and how much is
perpendicular to the surface?
» A 1kg book is resting on a shelf that
is tilted 30°. Calculate the
magnitude of the normal force that
the shelf exerts on the book.
FN
» In the previous example, if there is
no friction on the shelf, the book will
accelerate down the shelf.
» What causes the acceleration?
» Calculate the acceleration of the
book.
FN
» Draw a FBD and label all forces.
» Find the components of each force that
are parallel and perpendicular to your
coordinate system
» For the x components: ΣFx=max
» For the y components: ΣFy=may
» Remember a=0 for constant velocity
(including rest). This could be in the xdirection, or y-direction, or both.