单选题
What is the lecture mainly about?
【正确答案】
B
【答案解析】[听力原文] 12-17
P Well, we're going to be talking about a special property of light. Actually, it's a small part of Einstein's special theory of relativity. Now, don't worry. I'll explain things really, really slowly, and use plenty of examples. OK, first, you have to understand the idea that there are things called frames of reference. Imagine that you are on an elevator. In that box, that's one frame of reference. And as you go from the first floor to the third, you pass by some guy standing on the second floor. That second floor, that's another frame of reference. Frame of reference just means, then, the place where a person is standing.
S1 Is there one perfect frame of reference?
P Good question. Nope. There are no stable frames. All of them move in some way. Now listen to this. Imagine that you're in your living room and you have a block of wood. If you measure the wood sitting on your couch, or if you measure it while riding on a bus, the block of wood will have the same measurements. Let me add here that we have to assume that the bus is going down a nice straight, smooth road and at a constant speed. OK, here's the first part of what Einstein was talking about. The laws of physics are true in all frames of reference.
S1 So, that block of wood is going to be the same size in your living room or on the bus?
P Right. Now you'll really have to pay attention. Imagine that you're driving in a car and the car is going 50 mph. You throw a rock out the front at 20 mph. In your frame of reference, the rock is going 20 mph. What about some guy standing by the side of the road? Remember that he's in a different frame of reference. Right? Well, the speed of the car and the speed of the rock together equal 70 mph, and that's the speed he'll see the rock moving.
S2 What happens if you're in that same car, still going along at 50 mph, and you turn on the headlights? Doesn't light always have the same speed? But, wouldn't it be like the rock, the speed of light plus the speed of the car?
P Well, think about speed. Speed is distance divided by time. OK. Here's another crazy part. Distance and time change when things start getting close to the speed of light. How about distance? Well the faster an object moves, the shorter it gets. Say you buy a really nice limousine, and you want to drive it around and show it off. Well, drive it slowly, because the faster you drive, the shorter the car will appear to the people on the street. You see the car, in your frame of reference, as staying the same length as when you bought it. The people on the street, standing there in a different frame of reference, see it as shorter than you do.
S2 What about time?
P Let's say you have two frames of reference. One is a guy standing on the ground and the other is a guy flying in a super fast airplane. The guy on the ground sees the guy in the super fast plane as moving slower in time. That's crazy, right? Listen one more time. From the point of reference of the guy on the ground, the guy in the plane goes through time slower. So, how does light maintain the same speed no matter what the frame of reference? Why is the light from the headlights not moving at the speed of light plus the 50 mph from the car? Well, it's because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. It's totally crazy, right? OK. Any questions?
P Why is the light from the headlights not moving at the speed of light plus the 50 mph from the car? Well, it's because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. It's totally crazy, right?
Why does the professor say this:
P It's totally crazy, right?
[解析] 主旨题 教授在讲座的开始部分就说要针对“光的特性”进行讨论。因为光的特性属于狭义相对论的一小部分，因此A项是不对的。C项和D项虽然是教授说明的理论中很重要的概念，但不是讲座的主要内容。
P Well, we're going to be talking about a special property of light. Actually, it's a small part of Einstein's special theory of relativity. Now, don't worry. I'll explain things really, really slowly, and use plenty of examples. OK, first, you have to understand the idea that there are things called frames of reference. Imagine that you are on an elevator. In that box, that's one frame of reference. And as you go from the first floor to the third, you pass by some guy standing on the second floor. That second floor, that's another frame of reference. Frame of reference just means, then, the place where a person is standing.
S1 Is there one perfect frame of reference?
P Good question. Nope. There are no stable frames. All of them move in some way. Now listen to this. Imagine that you're in your living room and you have a block of wood. If you measure the wood sitting on your couch, or if you measure it while riding on a bus, the block of wood will have the same measurements. Let me add here that we have to assume that the bus is going down a nice straight, smooth road and at a constant speed. OK, here's the first part of what Einstein was talking about. The laws of physics are true in all frames of reference.
S1 So, that block of wood is going to be the same size in your living room or on the bus?
P Right. Now you'll really have to pay attention. Imagine that you're driving in a car and the car is going 50 mph. You throw a rock out the front at 20 mph. In your frame of reference, the rock is going 20 mph. What about some guy standing by the side of the road? Remember that he's in a different frame of reference. Right? Well, the speed of the car and the speed of the rock together equal 70 mph, and that's the speed he'll see the rock moving.
S2 What happens if you're in that same car, still going along at 50 mph, and you turn on the headlights? Doesn't light always have the same speed? But, wouldn't it be like the rock, the speed of light plus the speed of the car?
P Well, think about speed. Speed is distance divided by time. OK. Here's another crazy part. Distance and time change when things start getting close to the speed of light. How about distance? Well the faster an object moves, the shorter it gets. Say you buy a really nice limousine, and you want to drive it around and show it off. Well, drive it slowly, because the faster you drive, the shorter the car will appear to the people on the street. You see the car, in your frame of reference, as staying the same length as when you bought it. The people on the street, standing there in a different frame of reference, see it as shorter than you do.
S2 What about time?
P Let's say you have two frames of reference. One is a guy standing on the ground and the other is a guy flying in a super fast airplane. The guy on the ground sees the guy in the super fast plane as moving slower in time. That's crazy, right? Listen one more time. From the point of reference of the guy on the ground, the guy in the plane goes through time slower. So, how does light maintain the same speed no matter what the frame of reference? Why is the light from the headlights not moving at the speed of light plus the 50 mph from the car? Well, it's because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. It's totally crazy, right? OK. Any questions?
P Why is the light from the headlights not moving at the speed of light plus the 50 mph from the car? Well, it's because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. It's totally crazy, right?
Why does the professor say this:
P It's totally crazy, right?
[解析] 主旨题 教授在讲座的开始部分就说要针对“光的特性”进行讨论。因为光的特性属于狭义相对论的一小部分，因此A项是不对的。C项和D项虽然是教授说明的理论中很重要的概念，但不是讲座的主要内容。
单选题
According to the professor, why is there no perfect frame of reference?
【正确答案】
C
【答案解析】[解析] 细节题 当学生问及是否存在完美的参照系的时候，教授的回答是：因为所有的参照系都处于运动中，所以不存在完美的参照系。
多选题
According to the professor, what properties does the bus need? Click on 2 answers.
- A. Constant speed
- B. Heavy weight
- C. Large wheels
- D. Smooth road
【正确答案】
【答案解析】[解析] 细节题 讲座指出，道路平坦和匀速是公共汽车行驶的条件。
单选题
Why does the professor mention driving a car while throwing a rock?
【正确答案】
B
【答案解析】[解析] 结构题 教授提到这个例子的原因是为了比较不同的参照系，即说明车内部和公路边两个参照系的不同。A，C，D三项和这个例子完全无关。
单选题
According to the professor, what happens to a clock carried in a fast vehicle?
【正确答案】
D
【答案解析】[解析] 推理题 在讲座的最后一部分可以找到线索。飞机和汽车等的参照系如果以非常快的速度移动的话，时间会过得更慢。因此D项是正确答案。
单选题
Why does the professor say this:
【正确答案】
B
【答案解析】[解析] 判断意图题 这是判断教授意图的问题。教授说这是超越常识的稀有现象。A项看似是正确的，但是这不是担心学生们理解不了而说的话。C项和D项与教授的意图完全无关。