Angular Momentum (print version)
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Transcript Angular Momentum (print version)
Angular Momentum (l) -
l rp
l r mv
l rmv sin
Units:
m
m kg m s kgm2
s
Angular Momentum is a vector whose direction is
perpendicular to the plane containing r and p given by the
right hand rule.
1. A particle P with mass 2.0 kg has a position vector
r r 3.0 m and velocity v v 4.0 m as shown.
s
It is acted upon by a force F F 2.0 N . All three
vectors lie in the xy plane. About the origin, what are
F
v
30.
P
r
a. the angular momentum of the particle,
l r mv
l rmv sin
30.
45
l 3.0 m 2.0 kg 4.0 m sin 150
s
2
kgm
Out of the page
l 12
s
kˆ
150
v
30.
r
1. A particle P with mass 2.0 kg has a position vector
r r 3.0 m and velocity v v 4.0 m as shown.
s
It is acted upon by a force F F 2.0 N . All three
vectors lie in the xy plane. About the origin, what are
F
v
30.
r
b. and the torque acting on the particle.
r F
rF sin
45
3.0 m 2.0 N sin 30.
3.0 Nm
Out of the page
kˆ
F
30.
r
30.
P
2. Two objects are moving as shown.
What is their total angular momentum
about point O?
r
l
since r mv
6.5 kg
1
s
1.5 m
is into the page
O
v
2 .2 m
3 .6 m
2.8 m
L l2 l1
v
l
since r mv
is out of the page
r
2
3.1 kg
s
2. Two objects are moving as shown.
What is their total angular momentum
about point O?
6.5 kg
1
2 .2 m
s
1.5 m
O
3 .6 m
2.8 m
2
3.1 kg
L l2 l1
L r2 m2v2 sin 90. r1m1v1 sin 90.
L 2.8m 3.1kg 3.6 m
2
kgm
L 9.8
s
s 1.5m6.5kg 2.2 m s
Out of the page
kˆ
s
y
3. What is the angular momentum of a
rigid object rotating about a fixed axis?
li ri mi vi
li ri mi vi sin 90.
ri
li ri mi vi
n
L ri mi vi
i 1
But
vi ri
n
L ri mi ri
i 1
z
mi
vi
x
y
3. What is the angular momentum of a
rigid object rotating about a fixed axis?
n
L ri mi ri
L
i 1
But
constant for all mi
L mi ri2
i 1
n
But
ri
z
n
I mi ri2
i 1
L I
Angular Momentum of a rigid
object rotating about a fixed axis
mi
vi
x
3. Three particles, each of mass m, are
fastened to each other and to a rotation
axis by three massless strings, each with
length l. The combination rotates around
the rotational axis at O with angular velocity
ω in such a way that the particles remain in
a straight line. In terms of m, l and ω and
relative to point O, what are
a. the rotational inertia of the combination,
m
l
m
l
O
l
3
I mi ri2
i 1
I ml 2 m2l 2 m3l 2
I ml 2 4ml 2 9ml 2
I 14ml 2
m
3. Three particles, each of mass m, are
fastened to each other and to a rotation
axis by three massless strings, each with
length l. The combination rotates around
the rotational axis at O with angular velocity
ω in such a way that the particles remain in
a straight line. In terms of m, l and ω and
relative to point O, what are
O
b. the angular momentum of the middle
particle,
2 methods
m
l
m
l
l
Treat as a separate object
lm r mv
lm rmv sin
lm 2l mv sin 90
lm 2lmv
m
But
v r
v 2l
lm 2lm2l
lm 4ml 2
3. Three particles, each of mass m, are
fastened to each other and to a rotation
axis by three massless strings, each with
length l. The combination rotates around
the rotational axis at O with angular velocity
ω in such a way that the particles remain in
a straight line. In terms of m, l and ω and
relative to point O, what are
O
b. the angular momentum of the middle
particle,
2 methods
Treat as a rigid object
lm I m
But
I m 4ml 2
lm 4ml 2
m
m
l
m
l
l
3. Three particles, each of mass m, are
fastened to each other and to a rotation
axis by three massless strings, each with
length l. The combination rotates around
the rotational axis at O with angular velocity
ω in such a way that the particles remain in
a straight line. In terms of m, l and ω and
relative to point O, what are
c. the total angular momentum of the
three particles.
m
m
l
m
l
O
L I
L 14ml 2
l