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

but as we increase the muzzle velocity for our imaginary cannon, the projectile will travel further and further before returning to earth. finally, newton reasoned that if the cannon projected the cannon ball with exactly the right velocity, the projectile would travel completely around the earth, always falling in the gravitational field but never reaching the earth, which is curving away at the same rate that the projectile falls. that is, the cannon ball would have been put into orbit around the earth. newton concluded that the orbit of the moon was of exactly the same nature: the moon continuously "fell" in its path around the earth because of the acceleration due to gravity, thus producing its orbit. by such reasoning, newton came to the conclusion that any two objects in the universe exert gravitational attraction on each other, with the force having a universal form:

the constant of proportionality g is known as the universal gravitational constant. it is termed a "universal constant" because it is thought to be the same at all places and all times, and thus universally characterizes the intrinsic strength of the gravitational force.step ii presentationt: do you remember the conversation between li yanping and professor wallis?s1: yes.t: we all know that space walk is difficult and dangerous. what should an astronaut take with him?s2: oxygen can.s3: spacesuit.s4: gravity boots.s5: water system.s6: special food.s7: special ropes. t: great! do you know the usage of them? now discuss them with your partner according to the form on page31. you can refer to our listening material on page65.when ss discuss. teacher walk around the classroom to give them some help. step iii speaking (page31)(after a few minutes.) t: have you finished? now speak one or two sentences for each equipment you should take. sa: i will take water with me in my spacesuit in order to warm or cool me if it’s too cold or too hot.sb: i will take oxygen can on my back in order to help me breathe oxygen in space. sc: i’ll wear spacesuit. t can protect me from many dangers. it can carry many necessary things in or on it.sd: i’ll take space buggy in order to travel on the moon.se: i’ll take a rope to tie me to the spaceship, or i’ll fly away.sf: i’ll take special food so that i can eat easily, because the food will float in space.sg: i’ll wear special sunglasses in order to protect my eyes.sh: i’ll take space camera in order to take photos.t: wonderful! anything else?si: i’ll wear space boots in order to walk there.