机械能是守恒的，因为重力是守恒力。
Mechanical energy is conserved, because gravity is a conservative force.
“守恒”这个词来自于物理学;,如果说能量守恒。
Well, the word conservative comes from the idea in physics; if the conservation of energy.
动量守恒，这就叫作动量守恒。
Momentum is conserved, and that's called the conservation of momentum.
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