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Rotational Motion
Emily Burns
Molly McGeady
Hung Nguyen
Definition
 Rotational motion- the measure of an
object that is making a circular or
spinning motion.
Angular
 Angular displacement
 Angular displacement—change in the angle as the object
rotates.
 2pi(rad)
 Distance given by d=r(angle)
 Angular velocity
 Angular velocity—angular displacement divided by the time
taken to make the displacement.
 W=(change in angle)/(change in time)
 Angular acceleration
 Angular acceleration—defined as the change in angular
velocity divided by the time required to make the change.
 A=(change in w)/(change in time)
Rotational Dynamics
 Lever arm—
perpendicular distance
from the axis of rotation
to the point where the
force is exerted.
 L=r(sin*angle)
 R—distance from the axis
of rotation.
 Change—angle between
the force and the radius
from the axis of rotation to
the point where the force
is applied.
 Torque—measure of
how effectively a force
causes rotation.
 T=Fr(sin*change)
 Torque is equal to the
force times the lever arm.
The Moment of Inertia
 Moment of inertia—resistance to rotation.
 I=m(r^2)
 The moment of inertia of a point mass is
equal to the mass of the object times the
square of the object’s distance from the axis
of rotation.
Rotating Frames of
Reference
 Motion is important to us because the
Earth rotates. (DUH!)
 The effects of the rotation of Earth is too
small to be notice in a classroom or lab.
 Significant influences on the motion of
the atmosphere, and therefore on climate
and weather.
Centrifugal and Centripetal
Force
 Centrifugal force is
used for two different
concepts.
 Centrifugal force is one
of the fictitious forces
that appears to act on an
object when its motion is
viewed from a rotating
frame of reference.
 Magnitude of centripetal
force is F=mv2/r.
 Force that maintains
circular motion is
centripetal force.
Coriolis Force
 Coriolis effect is an
apparent deflection
of a moving object in
a rotating frame of
reference.
 Centrifugal and
Coriolis force are not
real forces.
Newton’s Second Law for
Rotational Motion
 States that the angular acceleration is
directly proportional to the net torque and
inversely proportional to the moment of
inertia.
 A= T(sub net)/I
 The angular acceleration of objects is equal
to the net force torque on the object, divided
by the moment of inertia.
The End
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