1211 Circular Motion

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12/11 Circular Motion

• Finish Bucket of Water
• HW 12/9 Rotating Drum
• HW 12/9 Rotating DiskThese two are for
  practice (I will show you these today) and they
  also review friction forces.
• Final 8 am Witmer 116

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Bucket of water problem at the top
v
Acceleration points
Fnet WE,W NB,W ma
Draw a FBD of the water.
mw 3 kgr 2 mv 5 m/s(constant v)
Want NB,W, find WE,W and ma
Apply Newtons 2nd law.
WE,W mg 3(9.8) 29.4 N
ma mv2/r 3(52)/2 37.5 N
Fnet 37.5 29.4 NB,W
NB,W 8.1 N
If you swing slow enough the water will come out.
How slow do you have to swing?
Slow enough so that NB,W becomes zero.
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Bucket of water problem at the bottom
Acceleration points
Fnet NB,W - WE,W ma
Draw a FBD of the water.
mw 3 kgr 2 mv 5 m/s(constant v)
Want NB,W, find WE,W and ma
Apply Newtons 2nd law.
WE,W mg 3(9.8) 29.4 N
ma mv2/r 3(52)/2 37.5 N
Fnet NB,W - 29.4 37.5
NB,W 66.9 N
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What about the centrifugal force?

• There is no such force, regardless of what Mr.
  Wizard says.
• I appears to a rotating observer that an object
  released seems to move away and so it seems there
  is a force causing that.
• An outside, stationary observer has no such
  confusion.

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Some Equations and Definitions
The period, T, is the time for one revolution.
The distance for one revolution is 2?r, the
circumference.
The speed, v, is distance time or 2?r/T