Chapter 11
Rotational Dynamics and Static
Equilibrium (cont.)
Dr. Jie Zou PHY 1151G
Department of Physics
1
Torque and angular
acceleration

Newton’s second law for rotational motion:


 = net = I ,  is the total (net) torque acting,
I is the moment of inertia, and  is the angular
acceleration caused by the torque.
Analogies between rotational and linear
quantities:
Linear Quantity
m
a
F
Angular Quantity
I


Dr. Jie Zou PHY 1151G
Department of Physics
2
Example

A light rope wrapped around a diskshaped pulley is pulled tangentially with
a force of 0.53 N. Find the angular
acceleration of the pulley, given that its
mass is 1.3 kg and its radius is 0.11 m.
(Hint: The moment of inertia of a disk
rotating about the center axis is I =
(1/2)mr2.) Answer: 7.3 rad/s2.
Dr. Jie Zou PHY 1151G
Department of Physics
3
Static equilibrium

Conditions for static
equilibrium:


(1) The net force acting on the
object must be zero: Fx = 0,
Fy = 0, and
(2) The net torque acting on the
object must be zero,  = 0.
Dr. Jie Zou PHY 1151G
Department of Physics
4
Example

A cat walking along a uniform plank that is 4.00 m long and
has a mass of 7.00 kg. The plank is supported by two
sawhorses, one 0.440 m from the left end of the board and
the other 1.50 m from its right end. When the cat reaches
the right end, the plank just begins to tip. What is the mass
of the cat?
Dr. Jie Zou PHY 1151G
Department of Physics
5
Homework


See online homework assignment on
www.masteringphysics.com
Hand-written homework:

Ch. 11, p. 367, Problem #26.
Dr. Jie Zou PHY 1151G
Department of Physics
6