11_28_SHM - 11/28 Simple Harmonic Motion Presentation...

11/28 Simple Harmonic Motion
/
Presentation
“Angular Momentum Problems”
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HW “Simple Harmonic Motion” Due Friday
11/30
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Exam IV Thursday Eve 12/6 5-7 Wit 116
Angular Momentum
Simple Harmonic Motion

Assorted angular momentum problems.
Drop a ring on a disk that is rotating. Both have mass m.
ϖ = 0
ϖ
i
ϖ
f
= ? (want a
fraction of ϖ
i
)
rotate
together
L = I ϖ L
i
= L
f
?
Yes, no external torque
I
disk
=
1
/
2
mr
2
I
ring
= mr
2
I
sys
=
3
/
2
mr
2
I
disk
ϖ
i
= I
sys
ϖ
f
1
/
2
mr
2
ϖ
i
=
3
/
2
mr
2
ϖ
f
ϖ
i
= 3 ϖ
f
ϖ
f
= ϖ
i
/3
Drop a bag of sand on a disk that is rotating. Both have mass m.
ϖ
f
= ? (want a
fraction of ϖ
i
)
rotate
together
ϖ
i
From rest I
bag o’ sand
= mr
2
Why same as a ring?
All the mass at the edge.
ϖ
f
= ϖ
i
/3 It’s the same
problem!!!

Two spinning wheel deal. Two spinning rings with same mass, radius, and ϖ
Method 1--Hold the axles Method 2--Free axles
Both
stop
Is L conserved?
L = ? L = ? L
sys
=
L
sys
= 0
NO!!!
There are external torques
because we are holding the axles.