本篇talk 主要给大家上了一堂“物理”课，讲解了有关“重力”的话题。人们会想当然地以为重力是物体的自然属性，是不变的衡量。文章指出，这是错误的观点。物体的重力和地球对其的引力有关，当物体离开地球，比如去了月球，它的重量也会改变。这是因为月球比地球小，它对物体的吸引力也比地球对物体的吸引力小。文章虽然是学术类话题，但此物理原理大家一般都比较熟悉，相信理科还不错的同学应该没有大问题吧。

We are so used to our life on the surface of the earth that it is not easy for our minds to break free of all the ideas that we take for granted. For example, we can feel things are heavy, so we think of weight as being a fixed quality in an object. But actually, it is not fixed at all. If you could take a one-pound packet of butter four thousand miles out from the earth, it would weigh only a quarter of a pound. The reason for such a change of weight is this: all objects have a natural attraction for all other objects. This is called gravitational attraction, but this power of attraction between two objects gets weaker as they get farther apart. So the weight of an object on the surface of the earth is not the same as its weight 4000 miles out in space. What about the weight of a one-pound of butter on the surface of the moon? There won’t be attraction between the butter and the moon, but the butter will weigh only about 1/6 as much as it does on the earth. This is because the moon is so much smaller than the earth. The amount of a gravitational pull that a body produces depends on the amount of material in it. A packet of butter has a gravitational pull of its own, but it is very small in relation to the pull of something as large as the moon or the earth.

Q23: According to the speaker, what quality of an object is taken for granted by people?

Q24: How much will a pound of butter weigh if it is brought 4000 miles into space?

Q25: What is the gravitational attraction?

Q26: Why does an object weigh much less on the moon than it does on the earth?