Which forces belong to external forces(). A: spring force B: damper force C: actuator force D: gravity force

Time change and people change.

The internal force of any structure is zero if it is not subjected to external loads.

The magnitude of the internal force of a bar depends on not only the external force but also the shape of its cross section. A: 正确 B: 错误

Which of the following expressions is not correct (不正确)? A: If a system is only acted by conservative forces （保守力）, the mechanical energy（机械能）must be conserved （守恒）. B: If net external force is zero then the linear momentum of the system must be conserved. C: If net external force is zero, then the mechanical energy of a system must be conserved. D: If net external force is zero, then the mechanical energy of a system may not be conserved.