Phy 2053 Announcements
Reitze and Chan still away
scientific conferences this week
1.
„
American Physical Society March Meeting
Laser
Interferometer
Gravitational-wave
Observatory Collaboration meeting
„
Office hours cancelled for this week
„
Reitze will begin lecturing next week
Torque, τ = r F^
„
„
http://www.ligo.org/
„
2.
„
http://www.aps.org/meetings/march/index.cfm
„
„
From the last time: Torque and
Equilibrium
attending
„
„
SI unit is N.m
Conditions for equilibrium when rotation is
r
involved
ΣF = 0 or
r
r
ΣFx = 0 and ΣFy = 0
„
Forces: net external force must be zero
„
Torques: net external torque must be zero
r
Στ = 0
rotation counterclockwise Æ the torque is positive
rotation clockwise Æ the torque is negative
Webassign HW set #8 is now available online
Example of a Free Body Diagram--Ladder
Example – P8.21
A uniform plank of length 2.00 m and mass 30
kg is supported by three ropes, as indicated
by the blue vector in the figure below. Find
the tension in each rope when a 700 N person
is 0.500 m away from the left end.
L
w
Θ
free body diagram shows normal force and
force of static friction acting on the ladder at
the ground
ƒThe last diagram shows the lever arms for the forces
from axis of rotation at ground
ƒPick axis and sum torques
„
Torque and Angular Acceleration
Newton’s Second Law for a Rotating Object
Στ = Iα
analogous to
∑F = ma
I = moment of inertia
For Uniform Ring I = Σm r 2 = MR2
i i
moment of inertia depends
on quantity of matter
and its distribution
and location of axis of
rotation
HITT RF Remote Login Procedure:
The radio channel number for this room is “07” (zero, seven).
It is STRONGLY recommended to login your remote for every class just
to be sure it is on the correct radio channel and working before class.
1.
2.
3.
4.
PRESS AND HOLD THE DOWN ARROW KEY until
the GREEN light on the remote turns RED.
PRESS THE “0” KEY and you will see the RED light
flash GREEN.
PRESS THE “7” KEY and you will see the RED light
flash GREEN.
PRESS AND RELEASE THE DOWN ARROW KEY
again and you will see the red light search for the
receiver, if it BLINKS GREEN MULTIPLE TIMES you
are logged in.
1
Newton’s Second Law for Rotation-Example
Draw free body diagrams of
each object
„ Only the cylinder is rotating,
so apply Στ = I α
„ The bucket is falling, but not
rotating, so apply ΣF = ma
„ Remember a = α r and
solve the resulting equations
„
Example #8.11
A solid frictionless cylindrical reel of
mass M = 3 kg and radius R = 0.4
m is used to draw water from a
well. A bucket of mass m = 2 kg
is attached to a cord that is
wrapped around the cylinder. Find
the tension T in the cord and the
acceleration a of the bucket
2