If you've ever pulled a rolling suitcase through the airport, chances are that you've also been annoyed by this common occurrence: the suitcase starts rocking back and forth on each wheel and finally tips over.
"It's a very classic phenomenon in physics," said Sylvain Courrech du Pont, a physicist at the University of Paris Diderot. "So if you disturb the system it goes very easily unstable. This is shared by many, many phenomena in nature."
Courrech du Pont and his students wanted to get to the bottom of why luggage tips. So they built what looks like the structure of a wheeled suitcase, and then filmed it rolling on a treadmill (a machine for running). Then they modeled what they saw mathematically.
And here's what they found. Under normal rolling conditions, the forward motion of the suitcase is perpendicular(垂直的) to the wheels' axis of rotation(旋转轴). All is good. But when you hit a bump or drag the handle fiercely and abruptly, one wheel lifts up. All of a sudden the pulling motion is no longer perpendicular to the still-rolling wheel's rotation axis, and the wheel corrects that by accelerating towards the center. But by that time the other wheel's coming down, the same thing happens, and now the suitcase is really rocking.
"And the first reaction you will have maybe is to slow down. But actually if you slow down you'll experience very big rocking. Therefore, the way back to a smooth ride is to keep up your speed," he says, "or move even faster, once rocking begins.
There are other solutions to suitcase instability—like doing a better packing job. "If you put heavy contents close to the axis of symmetry, then it will be stable, too."
And Courrech du Pont has one more piece of advice to avoid rocking in the first place: "If you go slow enough, it will always be stable." Keep that in mind, next time you're running to your gate.
【小题1】What can we infer from the first two paragraphs?A.Everyone has been upset by a rolling suitcase at the airport. |
B.Physical phenomenon can be found in our daily life. |
C.The disturbance of the system results from the instability of the suitcase. |
D.The phenomenon of suitcase rolling is rare at the airport. |
A.It hits a bump |
B.One wheel lifts up. |
C.One wheel falls down. |
D.It is not perpendicular to the axis. |
A.tip it over |
B.accelerate its moving. |
C.slow down its moving |
D.drag its handle with great strength. |
A.one | B.two |
C.three | D.four |
Did you know that something as simple as the wallpaper on your computer can tell people a lot about you? According to researcher Donna Dawson,our computers say something about what we're like because they are part of our “personal space.”
A picture of a child or family on the screen often means the person cares deeply about their loved ones.A photo of the person surrounded by friends shows they value their popularity and probably work well with other people.On the other hand,pictures showing examples of past successes,like winning a prize may mean a person is only interested in themselves.
Pictures of nature and natural scenery tell people that you are driven to succeed and use the pictures to relax and remove the pressure(压力)in your life.Cityscapes,pictures of cities or towns,show that you enjoy travel and adventure.Most people with cityscapes as their wallpaper are optimistic(乐观的),happy people who feel even better thinking about the places they’ve visited or want to visit.
Pictures of animals show that you are optimistic and kind-hearted.You are often happy and you enjoy life and act young.Cartoons(漫画)or drawings show that you are a multitasker,someone who does many things at once.You have many responsibilities,but you deal with them well.That's one reason you like having something to smile about when you look at your computer wallpaper.
Then,what does plain blue wallpaper tell us about a person?According to Dawson,a person with plain blue wallpaper probably values their privacy and keeps work from becoming too personal,
【小题1】A prize photo as wallpaper suggests someone_______.A.has a strong wish to succeed | B.is confident about himself or herself |
C.cares only about himself or herself | D.works very well with others |
A.pictures of nature and natural scenery | B.city views or pictures of animals |
C.pictures showing their past successes | D.photos of staying with friends |
A.Where to Look for Your Computer Wallpaper |
B.Computer Wallpaper Shows Your Personality |
C.How to Place Wallpaper on Your Computer |
D.Computer Wallpaper Interests Everyone |
“Yes, he should be praised and we must write a letter to the company,” said a second passenger. “That's right,” another lady said, “I wish a newspaper reporter were here so that more people could learn from this conductor.” Just then a gentleman who looked like a teacher turned to the conductor and said , “Excuse me, but can I know your name, please? Your excellent service must be praised.”
Before he could open his mouth, the three-year-old child sitting on the young woman's lap interrupted, “I know his name. I call him Dad.”
【小题1】The passengers were ______ to see the conductor's kindness to the woman and the child.
A.excited | B.pleased | C.interested | D.surprised |
A.make a demand for more buses |
B.thank the conductor for his good service |
C.criticize the conductor for his rude behavior |
D.invite a newspaper reporter to write about the conductor |
A.has changed his attitude towards his work |
B.has now been kind and polite to all passengers |
C.has not changed his rude behavior to passengers |
D.has now been kind and polite to women with children |
What’s better than a present?
A homemade gift is much more personal, and it shows that a lot of effort and thought has gone into making it.
I’ve given my friends cakes in the past as a present. Cakes are usually a very good gift because there is a huge variety of cakes to make. You could make lots of small cakes and decorate them. You could make a fruit cake with cream in the middle.
Homemade doesn’t necessarily mean something you have made in the kitchen.
A.So what could I make? |
B.So why not have a go? |
C.A homemade present of course! |
D.It can mean other things as well. |
E.They have made the present themselves. |
F.Or my personal favourite, a chocolate cake! |
G.Also, if something is homemade, it is unique! |
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