题目内容
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A rigid body is in equilibrium state under the coplanar force system. Which set of equilibrium equations are not independent of each other？
A、SX = 0, SmO(F) = 0, SmA(F) = 0
B、SmO(F) = 0, SmA(F) = 0, SmB(F) = 0
C、SmA(F) = 0, SmC(F) = 0, SY = 0
D、SX = 0, SmA(F) = 0, SmB(F) = 0
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A moving rigid body with mass
has a momentum
and mass center velocity
, then
should be satisfied.
A、If the rigid body is in translation, then ½vA½=½vB½ must be satisfied.
B、If ½vA½=½vB½ is satisfied, then the rigid body must be in translation.
C、If the rigid body is in translation, then ½vA½=½vB½ must be satisfied and the directions of the two velocities could be different.
D、If the rigid body is in translation, then the directions of vA and vB must be the same but probably |vA|¹|vB|.
There is few opinions about torque (1) For a rigid body rotating about a fixed axis, the inner torque will not change the angular acceleration of the rigid body; (2) The torque of a pair of the action force and reaction force to a same axis their sum is definitely zero; (3) When two objects with the same mass, different shape and size are under the same torque, their motion states are definitely the same; For statements above, which one is correct ()
A、Only (2) is correct;
B、(1) and (2) are correct;
C、(2) and (3) are correct;
D、All (1), (2) and (3) are corrct.
A rigid body is in equilibrium state under the coplanar force system. Which set of equilibrium equations are not independent of each other？
A、SX = 0, Sx = 0, SmA(F) = 0
B、SmO(F) = 0, SmA(F) = 0, SmB(F) = 0
C、SmO(F) = 0, SmC(F) = 0, SY = 0
D、SX = 0, SY = 0, SmO(F) = 0
Two forces acting on a rigid body that has a fixed axis: (1) When these two forces are acting parallel to the axis, the combined torque to the axis is definitely zero. (2) When these two forces are acting perpendicular to the axis, the combined torque to the axis may be zero. (3) When the combined forces of these two forces is zero, their combined torque is also zero. (4) When the combined torque to the axis is zero, their combined forces is zero.
A、Only (1) is correct.
B、(1) and (2) are correct, (3) and (4) are wrong.
C、(1), (2) and (3) are correct, and (4) is wrong.
D、All (1), (2), (3) and (4) are correct.
A、忽略地球曲率 Ignore Earth Curvature
B、忽略地球自转 Ignore the Earth Rotation
C、忽略外力 Ignore external forces
D、飞机视为刚体 The vehicle is treated as a single rigid body
A.主动刚体与被动刚体都可以受动力场的影响
B.主动刚体与被动刚体可以产生碰撞
C.主动刚体可以与粒子产生碰撞，而被动刚体都不可以
D.主动刚体与被动刚体之间可以使用约束
Eusocial insect societies have rigid caste systems, each insect's role being defined by its behavior, body shape, and physiology. In naked mole rat societies, on the other hand, differences in behavior. are related primarily to reproductive status (reproduction being limited to the queen and a few males), body size, and perhaps age. Smaller non-breeding members, both male and female, seem to participate primarily in gathering food, transporting nest material, and tunneling, larger non-breeders are active in defending the colony and perhaps in removing dirt from the tunnels. One work has suggested that differences in growth rates may influence the length of time that an individual performs a task, regardless of its age.
Cooperative breeding has evolved many times in vertebrates, but unlike naked mole rats, most cooperatively breeding vertebrates (except the wild dog, Lycaon pictus) are dominated by a pair of breeders rather than by a single breeding female. The division of labor within social groups is less pronounced among other vertebrates than among naked mole rats, colony size is much smaller, and mating by subordinate females may not be totally suppressed, whereas in naked mole rat colonies subordinate females are not sexually active, and many never breed.
Which of the following most accurately states the main idea of the passage？
A．Naked mole rat colonies exhibit social organization based on a rigid caste system.
B．Behavior. in naked mole rat colonies may well be a close vertebrate analogue to behavior. in eusocial insect societies.
C．The mating habits of naked mole rats differ from those of any other vertebrate species.
D．Naked mole rat colonies are the only known examples of cooperatively breeding vertebrate societies.
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