Download ICNS 132 : Rotational Motion and Equilibrium

ICNS 132 : Equilibrium
Weerachai Siripunvaraporn
Department of Physics, Faculty of Science
Mahidol University
email&msn : [email protected]
Static Equilibrium
•Equilibrium implies that the object moves with both
constant velocity and constant angular velocity
relative to an observer in an inertial reference
frame.
•Will deal now with the special case in which both of
these velocities are equal to zero
– This is called static equilibrium.
•Static equilibrium is a common situation in
engineering.
•The principles involved are of particular interest to
civil engineers, architects, and mechanical
engineers.
Introduction
CH12
F
F
F
F
If object has size, position of force must be considered.
3 factors
-Size of force
-Direction of force
-Position of force
When a force is exerted on a rigid
object pivoted about an axis, the
object tends to rotate about that axis.
The tendency of a force to
rotate an object about some axis
is measured by a vector quantity
called torque 
Unit is N-m but not equivalent to
Joule as in Work.
Torque is the cause of changes in
rotational motion, and is analogous to
force, which cause changes in
translational motion, but should not be
confused with force.
High torque <-> Easy to rotate
High force <-> Easy to move
where r is the distance between the pivot point and the point of application
of F and d is the perpendicular distance from the pivot point to the line of
action of F. (The line of action of a force is an imaginary line extending out
both ends of the vector representing the force.
Direction of rotation is only
“clockwise” or
“counterclockwise”.
Direction of torque is
perpendicular to r and F.
When there are more than
one forces, net force is
used to determine direction
of external force acting on
the object.
Direction of rotation will be
determined by the net torque.