Rotational Motion of an Object
Physics Matters: Chapter 7
Rotational Motion
• All Spinning Objects
• Axis of Rotation
• The line about which everything rotates.
• Speed of Rotation
• Period of rotation
• The time of a single complete rotation (T)
• Frequency of rotation
• The number of cycles completed in a given time (f = hertz)
• Period = 1/Frequency or Frequency = 1/Period
• T= 1/f
f=1/T
Spinning
• Angular speed
Rotational angle

 
Time
t
•  = s/r ( is measured in radians)
• For 360 degreed, s = 2r
• 3600 = 2 radians
• Angular speed = 2f
Torque
• Tangential force being applied times the
perpendicular distance from the axis of
rotation
•  = Ftr
Angular Momentum
• Moment of inertia
• In general, the farther away a mass is from the
axis the greater its moment of inertia is.
• I = kmr2
Angular Momentum (Cont.)
• Momentum of inertia times angular speed
• L=I
• Conservation of Angular momentum
• Direction of Rotation
• The right hand rule
Center of Gravity
Center of Mass
"All of science is nothing more than
the refinement of everyday thinking."
-- Albert Einstein
Center of Gravity
• Point of an object located at the average
position of weight.
Center of Mass
• The Average position of matter
Toppling
• Toppling occurs when the center of gravity
extends beyond the support base.
Stability
• Unstable – CG is lowered with
displacement
• Stable – work must be done to raise the CG
• Neutral – displacement neither raises or
lowers the CG
Objects rotate around their center
of gravity.
Let’s try some activities!!