Download Section 8.3 Equilibrium

Mr. Borosky
Physics Section 8.3 Notes
Page 1 of 3
Section 8.3 Equilibrium
Objectives
Define center of mass.
Explain how the location of the center of mass affects the
stability of an object.
Define the conditions for equilibrium.
Describe how rotating frames of reference give rise to apparent
forces.
Read intro paragraph p. 211
THE CENTER OF MASS
Read Section.
Center of Mass – the point on the object that moves in the same way
that a point particle would move.
To find the center of mass of an object, suspend the object from any
point, when the object stops swinging, the center of mass is along
the Vertical Line drawn from the suspension point. (Figure 8-11b)
Then suspend the object from another point and draw a second
Vertical Line. The Center of Mass is at the Point where the 2 lines
cross (Figure 8-11c).
A human’s center of mass is NOT fixed because we are flexible.
A human’s center of mass is slightly below the navel.
The path of the center of mass is a PARABOLA for a ballet dancer in
a leap.
CENTER OF MASS AND STABILITY
Read Section.
A tall, narrow box tips more easily than a low, broad box.
An object is said to be Stable if an external force is required to
tip it.
The broader the base the more stable the object.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 8.3 Notes
Page 2 of 3
The lower the location of an object’s center of mass, the greater
the stability.
If the center of mass is outside the base of an object, it is
unstable and will roll over without additional torque.
If the center of mass is above the base of the object, then it is
stable.
CONDITIONS FOR EQUILIBRIUM
Read Section.
An object is said to be in Static Equilibrium if both its Velocity
and Angular Velocity are zero or constant.
For an object to be in Static Equilibrium you need 2 things
1. It must be in Translational Equilibrium, that is the net force
exerted on the object must be zero.
2. It must be in Rotational Equilibrium, that is the net Torque
exerted on the object must be zero.
SKIP
Example Problem 5 p. 214-215
Practice Problems p. 215 # 36-39
ROTATING FRAMES OF REFERENCE
Read Section.
Newton’s laws do not apply for accelerated (Rotating) frames of
reference.
CENTRIFUGAL FORCE
Read Section.
Centrifugal Force – the apparent force that seems to pull on a
moving object, but does not exert a physical outward push on it and
is observed only in rotating frames of reference.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 8.3 Notes
Page 3 of 3
Centripetal Acceleration is proportional to the distance from the
axis of rotation and depends on the square of the angular velocity.
Thus if you double the rotational frequency, the acceleration
increases by a factor of 4.
THE CORIOLIS FORCE
Read Section.
Coriolis Force – the apparent force that seems to deflect a moving
object from its path and is observed only in rotating frames of
references.
Because of the Coriolis Force winds rotate counterclockwise around a
Low Pressure systems in the Northern Hemisphere and they rotate
clockwise around Low Pressure systems in the Southern Hemisphere.
SKIP
8.3 Section Review p. 217 # 40-46
Physics Principals and Problems © 2005 Started 2006-2007 School Year