Abaqus 帮助文档例子没有设置重力加速度也可以进行跌落？

在Abaqus帮助文档里面，有一个 circuit board drop test 的例子，为什么里面没有设置重力加速度也可以进行跌落？只有初速度？

如果想继续分析它二次跌落，是不是 一定 要加重力加速度？

circuit.inp

*HEADING
Circuit board drop test -- 1.0 meter drop
SI units (kg, m, s, N)
*SYSTEM
0., 0., 0., 0.5, 0.707, 0.25
-0.5, 0.707, -0.5
*NODE
1,     0.005, -0.010, 0.012
11,    0.005, -0.010, 0.162
501,   0.105, -0.010, 0.012
511,   0.105, -0.010, 0.162
1001,  0.,     0.,    0.
1004,  0.,    -0.009, 0.
1005,  0.,    -0.011, 0.
1008,  0.,    -0.02,  0.
1061,  0.,     0.,    0.024
1064,  0.,    -0.009, 0.024
1065,  0.,    -0.011, 0.024
1068,  0.,    -0.02,  0.024
16001, 0.110,  0.,    0.
16004, 0.110, -0.009, 0.
16005, 0.110, -0.011, 0.
16008, 0.110, -0.02,  0.
16061, 0.110,  0.,    0.024
16064, 0.110, -0.009, 0.024
16065, 0.110, -0.011, 0.024
16068, 0.110, -0.02,  0.024
*NGEN, NSET=BOARDA
1,501,50
*NGEN, NSET=BOARDB
11,511,50
*NFILL,NSET=BOARD
BOARDA, BOARDB, 10, 1
**
*NGEN, NSET=PACKAA
1001,1004,1
1005,1008,1
*NGEN, NSET=PACKAB
1061,1064,1
1065,1068,1
*NFILL,NSET=PACKA
PACKAA,PACKAB,6,10
*NGEN, NSET=PACKBA
16001,16008,1
*NGEN, NSET=PACKBB
16061,16068,1
*NFILL,NSET=PACKB
PACKBA,PACKBB,6,10
*NFILL,NSET=PACK
PACKA,PACKB,15,1000
**
** Reset coordinate system
*SYSTEM
*NODE
50000, -0.10, -0.10, -0.0001
50001, -0.10,  0.10, -0.0001
50002,  0.15,  0.10, -0.0001
50003,  0.15, -0.10, -0.0001
99999,  0.,    0.,   -0.001
**
** Element definitions
*ELEMENT,TYPE=S4R,ELSET=BOARD
1,1,51,52,2
*ELGEN,ELSET=BOARD
1,10,50,1,10,1,10
*ELEMENT,TYPE=C3D8R,ELSET=PACK
500, 1001,1002,2002,2001, 1011,1012,2012,2011
530, 1031,1032,2032,2031, 1041,1042,2042,2041
534, 1035,1036,2036,2035, 1045,1046,2046,2045
*ELGEN,ELSET=PACK
500, 7,1,1, 3,10,10, 15,1000,100
530, 3,1,1, 3,10,10, 15,1000,100
534, 3,1,1, 3,10,10, 15,1000,100
*ELEMENT,TYPE=MASS,ELSET=CHIPS
6001, 60
6002, 357
6003, 403
*ELEMENT,TYPE=R3D4,ELSET=FLOOR
50000, 50000, 50003, 50002, 50001
**
** Section properties
*SOLID SECTION,ELSET=PACK,MATERIAL=FOAM, CONTROLS=HGLASS
*SECTION CONTROLS, NAME=HGLASS, HOURGLASS=ENHANCED
*SHELL SECTION,ELSET=BOARD,MATERIAL=PCB,ORIENTATION=OR1
0.002,
*ORIENTATION,NAME=OR1,SYSTEM=RECTANGULAR,DEFINITION=COORDINATES
0.5, 0.707, 0.25, -0.5, 0.707, -0.5
2, 90.0
*MASS,ELSET=CHIPS
0.005,
*RIGID BODY,ELSET=FLOOR,REFNODE=99999
*NSET,NSET=REF
99999,
**
** Materials
*MATERIAL,NAME=PCB
*ELASTIC
45.E9, 0.3
*DENSITY
500.,
*MATERIAL,NAME=FOAM
*ELASTIC
3.E6,0.0
*DENSITY
100.,
* CRUSHABLE FOAM,HARDENING=VOLUMETRIC
1.1, 0.1
*CRUSHABLE FOAM HARDENING
0.22000E6, 0.0
0.24651E6, 0.1
0.27294E6, 0.2
0.29902E6, 0.3
0.32455E6, 0.4
0.34935E6, 0.5
0.37326E6, 0.6
0.39617E6, 0.7
0.41801E6, 0.8
0.43872E6, 0.9
0.45827E6, 1.0
0.49384E6, 1.2
0.52484E6, 1.4
0.55153E6, 1.6
0.57431E6, 1.8
0.59359E6, 2.0
0.62936E6, 2.5
0.65199E6, 3.0
0.68334E6, 5.0
0.68833E6, 10.0
**
** Element Faces for Contact
*ELSET,GENERATE,ELSET=LOWBOARD
1,10,1
**
** Define output locations and variables
*NSET,NSET=CHIPS
60,357,403
*NSET,NSET=QA_TEST_NSET
BOARD,
*NSET, NSET=BotChip-all
403
*NSET, NSET=BotChip-largeInc
403
*ELSET,ELSET=BOTPART
18,
*ELSET,ELSET=PARTS
18,57,81
**
*BOUNDARY
REF, ENCASTRE
*INITIAL CONDITIONS,TYPE=VELOCITY
BOARD, 3, -4.43
PACK,  3, -4.43
**
*SURFACE INTERACTION, NAME=FRIC
*FRICTION
0.3,
*CONTACT
*CONTACT INCLUSIONS, ALL EXTERIOR
*CONTACT PROPERTY ASSIGNMENT
,  , FRIC
**
*STEP
*DYNAMIC,EXPLICIT
,0.02
*BULK VISCOSITY
0.06, 1.2
**
*OUTPUT, FIELD, VARIABLE=PRESELECT
*OUTPUT, HISTORY, TIME INTERVAL=0.07E-3
*NODE OUTPUT, NSET=CHIPS
U3, V3, A3
*ENERGY OUTPUT
ALLIE, ALLKE, ALLPD, ALLAE, ALLSE
**
*OUTPUT, HISTORY, TIME INTERVAL=0.07E-3, FILTER=ANTIALIASING
*NODE OUTPUT, NSET=CHIPS
U3, V3, A3
*ELEMENT OUTPUT, ELSET=BOTPART
5,
LE11, LE22, LE12, LEP
*OUTPUT, HISTORY, FREQUENCY=1
*NODE OUTPUT, NSET=BotChip-all
U3, V3, A3
*OUTPUT, HISTORY, TIME INTERVAL=0.7E-3, FILTER=ANTIALIASING
*NODE OUTPUT, NSET=BotChip-largeInc
U3, V3, A3
**
** OUTPUT FOR ABAQUS QA PURPOSES
*FILE OUTPUT,NUMBER INTERVAL=3,TIMEMARKS=YES
*NODE FILE,NSET=CHIPS
U,
*OUTPUT,FIELD,NUM=1
*NODE OUTPUT,NSET=QA_TEST_NSET
U,
*END STEP