Circular Motion
Look at Fig 9.1 pg 122
Why do the
occupants of this
carnival ride not fall
out when it is tipped
almost vertical?
 Axis- the straight line
around which rotation
takes place.
 Rotation-when an object
turns about an internal
axis (located within the
body of the object) Also
called spin.
 Revolution- when an
object turns about an
external axis. Also
referred as revolve about
its axis
 Axis- the straight line around which
rotation takes place.
 Rotation-when an object turns
about an internal axis (located
within the body of the object) Also
called spin.
 Revolution- when an object turns
about an external axis. Also
referred as revolve about its axis
 Does a ball whirled
overhead at the end
of a string rotate or
revolve?
Does a tossed
football rotate or
revolve?
 Linear speed- distance
moved per unit of time
(remember Ch.2)
 Tangential speed- the speed
of something moving on a
circular path.
 For circular motion, we can
use the above terms
interchangeably.
 Rotational speed- number of
rotations per unit of time.
(sometimes called angular
speed)
 All parts of the RIGID merry-goround rotate about their axis in
the same amount of time, thus
all parts have the same rate of
rotation.
Have you heard of
RPM’s??
 Linear speed varies with the distance
from the axis of rotation.
 Therefore…if in a circle linear speed
is interchangeable with tangential
speed we can say the following…
 Linear speed is the distance per unit
of time while rotational speed if the
number of rotations per unit of time.
Period, Frequency, and Speed
Period
Frequency
Speed (while traveling in a circle)
speed = 2
Centripetal force-any force that causes an
object to follow a circular path.
Means ‘center-seeking’
This is not a new force; it is ANY force
that is directed at a right angle to the
path of a moving object and tends to
produce a circular motion.
Examples of centripetal force
Gravitational force directed toward
the center of the Earth holds the
moon in an almost circular orbit
Electrons revolving around the
nucleus of an atom.
A Centrifuge…what is a washing
machine?
Simulated Gravity
• True Gravitational force is always an
interaction between one mass and
another. The gravity we feel is due to
the interaction of our mass and the
mass of Earth.
• Gravity is simulated by centrifugal
force.
• In a rotating reference frame the
centrifugal force has no agent such as
mass……there is no interaction
therefore it cannot be a TRUE force.
• Centrifugal force is an effect of
rotation. That’s why it is referred as
fictitious.