Transcript ppt - Physics Rocks!

Journal Entry:
• While watching the following video,
answer the following questions:
– Clearly describe the motion as if you were
talking to someone who had never seen this
object before
– Describe what values you could measure
(and how!)
– Describe what values you might be able to
calculate (and how!) using information from
this video.
Journal Entry: video information
• This video is just over 5 minutes long.
You will have until the video is stopped to
answer each part of the question from the
previous slide. Use the time given 
• Watch:
https://www.youtube.com/watch?v=WrM8r
XnfajM
Break it down!
• On your group’s whiteboard, make 2
columns and record your answers for the
2nd and 3rd questions:
– What can be measured (and how)?
– What could be calculated (and how)?
• You have 3 minutes to complete your
columns and discuss with your group.
Rotational Mechanics
Kinematics…but going in circles!
Blue Book: pp. 223 – 229; 236 – 238
IB Book: pp. 549 - 551
Review: Uniform Circular Motion
• UCM: used to describe objects moving in a
circle with a constant speed
𝒎𝒗𝟐
𝑭=
𝒓
• Centripetal Force:
– The force that pulls an object in toward the
center of a circular path
• Centripetal Acceleration:
– The acceleration directed towards the center of
a circular path experienced by an object in UCM
Angular Velocity
• Angular velocity (w): the rate of change
of angular position. Typically measured in
rad·s-1
• Tangential velocity (speed): the speed of
a single point on a rotating object.
• How do you determine tangential velocity?
• How do you determine angular velocity?
• What is the mathematical relationship
between these velocities?
Angular Velocity and Tangential Speed
• Using dimensional analysis, and knowing
that speed should be in m/s:
•
𝑟𝑎𝑑
𝑠
1 𝑟𝑒𝑣
2𝜋 𝑟𝑎𝑑
2𝜋𝑟 𝑚𝑒𝑡𝑒𝑟𝑠
1 𝑟𝑒𝑣
• Angular velocity *1/(2p)* circumference = speed
• 𝒗 = 𝝎𝒓
Angular Acceleration
• Angular Acceleration: The rate of
change in angular velocity
– Units: rad·s-2
𝚫𝝎
𝜶=
𝚫𝒕
𝝎 = 𝝎𝟎 + 𝜶𝒕
Look familiar?
Rotational Kinematics
• Using what you know, write the 3 main
kinematic equations for rotational motion
• (Hints: I already gave you one of them;
angular position is symbolized with q)
1 2
Δ𝜃 = 𝜔0 𝑡 + 2𝛼𝑡
𝜔 = 𝜔0 = 𝛼𝑡
𝜔2 = 𝜔0 2 + 2𝛼(𝜃 − 𝜃0 )
Practice problem #1
• A CD uniformly accelerates from rest to its
operating speed of 500.0 rpm in 3.50 s.
What is its angular acceleration during this
time?
• What is its angular acceleration if it comes
uniformly to a stop in 4.50 s?
Practice problem #2
• A microwave oven has a 30.0 cm diameter
rotating plate for even cooking. The plate
accelerates from rest at a uniform rate of
0.87 rad·s-2 for 0.50 s before reaching its
operational speed.
– How many revolutions does the plate make
before reaching its operational speed?
– What is the operational speed of the rotating
plate?
Homework and reminders
• Register for WA (those of you who haven’t
yet)
• WAB.1 Angular Mechanics is due on
Friday morning
• Short Quiz covering rotational kinematics
(the stuff in the WA and these notes) on
Friday.