Class Notes
(811,176)
Canada
(494,539)
McGill University
(26,679)
Physics
(299)
PHYS 101
(41)
Kenneth Ragan
(15)
Lecture
October 21.doc
Unlock Document
McGill University
Physics
PHYS 101
Kenneth Ragan
Fall
Description
Phys 101 Alanna Houston
October 23, 2007
Rotational Motion
 A point at a distance r moves through an arc of length L with: θ =
L/r
 Dealing mostly with radians!
 Objects with the same angular velocity do not always move at the
same speed.
 In , theta is in radians. Radians are useful when angles
are very small since we then have: sin(θ) = x/r = L/r = θ, sin(θ) =
θ, tan(θ) = θ
 The human eye can distinguish an object that subtends an angle
of ~ 5x10^4 rads. What is the minimum spatial extent that can
be distinguished at 1 km distance? (To subtend an angle means to
have that angular size)

 small theta therefore theta is the ratio of x/r, x = 5x10^4 * 1km
= 5m
 The angle theta is the equivalent of displacement x; the
equivalent of the velocity v is then the angular velocity ω.
 ,
 All points on the rigid body rotate at the same rate ω or frequency
 Similarly, angular acceleration will be

 Example: A hard disk spins at 15000 rpm. If the disk has a
diameter of 3”, what is the linear velocity at the edge of the disk?
o 1 inch = 2.54 cm, therefore r = 7.62/2 = 0.038 m
o v = ωr = (15000*2π/60) * 0.038 m = 60 m/s = 200 km/h Phys 101 Alanna Houston
 atan= αr


 If we have constant angular acceleration (α), the kinematics are
exactly analogous to those for constant linear acceleration but
More
Less
Related notes for PHYS 101