Chapter 9
description
Transcript of Chapter 9
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Chapter 9
Systems of Particles
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Center of Mass
How do I define the position of the cow?What is h for U = mgh?
Take average position of mass: “Center of mass”Treat extended objects as point particles at CM
Real objects are not point particles
(Easy for spherical objects)
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Center of Mass (Discrete)
To find the center of mass for a bunch of point masses:
m1, m2, m3,… at r1, r2, r3,…
rCM
r1
r2
r3
iii
total
totalcm
rmM
M
rmrmr
1
...2211
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For an extended object, break it into point masses (mass = dm)
r
Center of Mass (Continuous)
dmrM
rtotal
cm 1
Note: This equation assumes that density is constant
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Center of Mass (Demo)
021
2211
mm
rmrmrcm
m1
0.5 kg
4 3
m2
???
To balance, rcm = 0
kgm
kg
r
rmm
rmmr
67.0
3
)4)(5.0(
0
2
2
112
2211
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Center of Mass (Bigger Demo)
m1
14.7 kg
141
m2
???
Again, to balance: rcm = 0
021
2211
mm
rmrmrcm
kgm
kg
r
rmm
05.1
14
)1)(7.14(
2
2
112
As before: Why didn’t it work this time?!
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Center of Mass (Bigger Demo)
m1
14.7 kg
141
m2
???
We forgot about the bar!
mbar=0.745 kg
7
CM in center of bar:
kgkgmbar 695.0)745.0(15
142,
0.5kgkgmbar 050.0)745.0(
15
11,
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Center of Mass (Bigger Demo)
m1
14.7 kg
141
m2
???
021
2,2,1,1,2211
bar
barbarbarbarcm mmm
rmrmrmrmr
kgkgkgkg
m
r
rmrmrmm barbarbarbar
015.114
)7)(695.0()5.0)(050.0()1)(7.14(2
2
2,2,1,1,112
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Center of Mass Example
m1 = 3 kg
m2 = 5 kg
m1 at position (1,1) meterm2 at position (2,0) meter
Mtotal = 8 kgrCM
ji
kg
ikgjikg
M
rmrmr
totalcm
ˆ8
10ˆ8
13
8
)ˆ2)(5()ˆˆ)(3(2211
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Center of Mass for 2D objects
Often don’t need to do integral: Use symmetry
Can also add symmetric objects together
m2 = 5 kg
m1 = 3 kg
rCM
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Another Way to Determine CM
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Why Do We Want Center of Mass?
Can treat extended objects or groups of objects as points
Gravity pulls on CM: h = hCM for Ugrav
aCM = Ftot /Mtot
Brick falls if CM not supported
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Mystery Hill
Gravity pulls down on the CM of the system…
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Example: (Problem 9.9)A cylindrical can with mass M, height H, and uniform density is initially filled with soda/pop of mass m. We then punch a hole in the top and bottom so that the liquid can drain out. Find:
a) The center of mass of the can when full
b) The center of mass of the can when empty
c) What happens to the height of the CM while draining?
d) Find the height of the soda/pop when the CM reaches its lowest point
This one is pretty tough!
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Ftot = dp/dt
Momentum:
New fundamental quantity (like force,energy,..)
For point particle p = mv
For extended object: pCM = mvCM
For group of particles: ptotal=m1v1+m2v2+...
Relation to Force: Ftot = ma
= m dv/dt
= d[mv]/dt
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Conservation of Momentum
If Ftot = 0, then momentum is constant
For an isolated system (no external forces):
Ftot = dp/dt
pinitial = pfinal
True also for groups of particles:
If Fexternal = 0, then pCM = constant
Even if there are internal forces
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Momentum of a Noisy Cricket
Will Smith: 200 lbs ~91 kg
Thrown back ~3m in 1s 3 m/s
smkgsmkg
vmP SmithSmithSmith
/273)/3)(91(
PSmith PPulse
PulseSmith
PulseSmith
PP
PP
0
smkgPPulse /273
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Momentum of a Noisy Cricket
Looking ahead to Phys 214:
J
smsmkg
pcE
10
8
102.8
)/103)(/273(
So how much energy is this?
m
p
m
mvmvK
22
)(
2
1 222
But light is massless…
Recall: A car at 60 mph has ~ 105 J!
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The Celebrated Jumping Frog of Calaveras County
A 2 kg frog sits on the end of a motionless, floating 50 kg log (3 meters long).
3 m
She jumps to the other end.
How far does the log move due to the jump?
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Celebrated Jumping Frog (cont.)
Consider frog and log together as a system of objects
Initial:
Final:x
No external forces momentum of system doesn’t change
pinitial = pfinal = 0
Center of mass of system does not move
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xx = 0 x = 1.5m x = 3m
xCM = 1.53 m
1.53 m
initial
final
1m
CM does not move, log moves to keep it in same place!
1.53 m
From geometry, we see that:
Celebrated Jumping Frog (cont.)
mkg
mkgmkg
mm
xmxmx
logfrog
loglogfrogfrogcm
53.151
)5.1)(50()3)(1(
xmx
mxmm
06.0
353.153.1
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Gun and Bullet
Both are at rest when fired, no net external force
Total Momentum: pinitial = pfinal = 0
What does this say about the motion after the gunis fired?
vG vB
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Gun and Bullet (continued)
mB << mG so vG << vB
The gun is heavier so it moves slower!
vG vB
0 finalinitial pp
0 ggbbfinal vmvmp
bg
bg v
m
mv
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Gun and Bullet (Question 1)
Does the Center of Mass of the system move?
No, because pinitial = pfinal = 0 for the system!gg
ggbb
gg
ggbbcmcm
mm
vmvm
mm
rmrm
dt
d
dt
rdv
vG vB
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Gun and Bullet (Question 2)
If the momenta are the same, why do you want to hold the gun rather than catch the bullet?
Consider the kinetic energy:
K = ½mv2 = p2/(2m)
|pG| = |pB| and mB << mG KG << KB
vG vB
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Spaceship Example
A spaceship in outer space, free from any interactions with other objects, has mship=34,000 kg and vship,i=1,000 m/s i. The ship launches a satellite msat=1,000 kg which moves with vsat,f=(1,500m/s)i + (400 m/s)j. What is the velocity of the spaceship after the satellite launch?
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Spaceship (continued)
No external forces, so momentum is conserved:
fsatsatfshipshipishipsatship
fi
vmvmvm
pp
,,,
Solve for vship,f
ship
fsatsatishipsatshipfship m
vmvmv ,,
,
jsmismv
kg
jsmismkgismkgv
fship
fship
/8.11/985
000,34
ˆ)/400(ˆ)/500,1()000,1()ˆ/000,1)(000,35(
,
,
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Spaceship (question)
Is Kinetic Energy conserved?
Kinitial = Kship = ½mtotal(vinitial)2
= ½(3.5 x 104 kg)(1000 m/s)2 = 1.75 x 1010 J
Kfinal = Kship + Ksatellite = ½mshipvship2 + ½msatvsat
2
= ½(3.4 x 104 kg)[(985 m/s)2 + (11.8 m/s)2] + ½(1000 kg)[(1500 m/s)2 + (400 m/s)2]
= 1.77 x 1010 J
NO! Extra energy added when satellite ejected.
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Rockets
A gun shooting a bullet is a discrete process
We can generalize this to continuous processes
Rockets operate by shooting outa continuous stream of gas
Note: Rocket propulsion was
originally thought to be impossible…Nothing to push against!!
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Rockets
= MvPi
mvvM Mv
v m
mvmvvMmv = Mv
v-vmvv+mM- =
P = PP
ex
ex
ex
fuelrocketf
)())((
exmvv=M
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Rockets
exvmv=M
dMvM dv= ex
dmv=dvM ex
Δm 0
Note: dm = -dM
Dividing both sides by dt dt
dMv=
dt
dvM ex
aMRvex Let dM/dt = -R, where R is the rate of fuel consumption
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Rockets
dMM
v dv= ex
f
i
f
i
M
M
ex
v
v M
dM = -vdv
f
iexif M
M+v=vv ln
To find the velocity of the rocket we must integrate
Note: Velocity is a function of mass loss, not time