If you ask a physicist how long it would take for a marble to fall from the top of aten-story building, she will answer the question by assuming that the marble fallsin a vacuum.

• 如果你问物理学家一个弹球从十层高的建筑物的顶部掉到地上所用的时间是多长，她的回答将会是假定弹球是在真空中下落的结果。