Unit 4 Astronomy: the science of the stars（Listening, Speaking and Writing）

a. teach the ss how to how to talk about giving instructions b. help the ss to learn how to play different roles in a conversation. teaching methods教學(xué)方法a. pairs work and group work b. discussion and cooperative learning teaching aids教具準(zhǔn)備1 a computer 2 a projector teaching procedures & ways教學(xué)過(guò)程與方式step i revision ask some of the ss to read his / her passage about gravity.

one possible passage: sir isaac newton: the universal law of gravitation
there is a popular story that newton was sitting under an apple tree, an apple fell on his head, and he suddenly thought of the universal law of gravitation. as in all such legends, this is almost certainly not true in its details, but the story contains elements of what actually happened. what really happened with the apple?probably the more correct version of the story is that newton, upon observing an apple fall from a tree, began to think along the following lines: the apple is accelerated, since its velocity changes from zero as it is hanging on the tree and moves toward the ground. thus, by newton's 2nd law there must be a force that acts on the apple to cause this acceleration. let's call this force "gravity", and the associated acceleration the "accleration due to gravity". then imagine the apple tree is twice as high. again, we expect the apple to be accelerated toward the ground, so this suggests that this force that we call gravity reaches to the top of the tallest apple tree. sir isaac's most excellent ideanow came newton's truly brilliant insight: if the force of gravity reaches to the top of the highest tree, might it not reach even further; in particular, might it not reach all the way to the orbit of the moon! then, the orbit of the moon about the earth could be a consequence of the gravitational force, because the acceleration due to gravity could change the velocity of the moon in just such a way that it followed an orbit around the earth. this can be illustrated with the thought experiment shown in the following figure. suppose we fire a cannon horizontally from a high mountain; the projectile will eventually fall to earth, as indicated by the shortest trajectory in the figure, because of the gravitational force directed toward the center of the earth and the associated acceleration. (remember that an acceleration is a change in velocity and that velocity is a vector, so it has both a magnitude and a direction. thus, an acceleration occurs if either or both the magnitude and the direction of the velocity change.)