5.Become a CNN Super Fan:　Why　your　country　needs　you!

Two fans getting in the World Cup mood. (Getty Images)

The World Cup - the biggest carnival in global soccer - is about to kick off in Johannesburg; an event that will see the greatest players and the most powerful nations vie across the fields of Africa for the most coveted prize in the game.

The tournament, which was first played among 13 nations in 1930, has grown into a global behemoth that attracted a cumulative global television audience of 26 billion in 2006 and is expected to see 300,000 visitors travel to South Africa to witness the action in 2010 (figures from FIFA).

Few events have the ability to unite people the world over, but the collective interest generated by the drama of eleven men kicking a ball in pursuit of a golden trophy gets somewhere close.

Whether as a passionate fan or merely as an interested spectator, each edition of the World Cup captures the era, creates new legends and makes history in both a communal and personal way.

And it is these personal stories, from every corner of the globe, that we want to be part of our coverage - so could you contribute by becoming a CNN Super Fan?

We want you to send us video of how you are getting involved with World Cup fever - and the criteria for what makes a Super Fan is as wide as your imagination.

Are you a football-mad fan who will be following every game? If so, send us film that captures the atmosphere of where you will be watching the match.

If you struggle to contain your enthusiasm when your country scores a goal then we want to see the celebrations, help your nation emerge with the most passionate support over the course of the tournament.

Maybe you have strong opinions about a player or incident that you want to get off your chest? You may think Lionel Messi is the best player in the world, or that Ivory Coast will be the first African side to lift the cup or that France cheated their way into qualification. Whatever your take, CNN can be your forum for debate and outlet of your views.

It could be that you have some silky, ball-juggling skills to compliment the action from the international players on show. If so, capture your best moves and upload your video.

Even if it is to just sing and shout about your team, however you want to express your interest, we want your World Cup story on to be told on CNN.

If you would like to sign up as a Super Fan add your contact details to the comment box below. Your details will not be published. Alternatively,superfans and follow the instructions.

4.South African police crack down on protesters, illegal ticket holders

Johannesburg, South Africa (CNN) -- Police used stun grenades and rubber bullets to break up a protest by striking World Cup security guards in a Cape Town suburb Thursday, said National South African Police spokesman Vish Naidoo.

He said the crowd was very rough and refused to leave when asked to disperse. Twelve protesters were arrested for illegal gathering and inciting violence, said Naidoo.

The FIFA World Cup Organizing Committee and the South African Police Service agreed Tuesday that police would replace the striking private security at four stadiums for the duration of the World Cup, officials said.

In addition, South Africa has set up a court dedicated to offenses related to the World Cup.

On Wednesday night, a magistrate in that court sentenced a Nigerian national to three years in prison for unlawfully possessing 30 tickets to World Cup matches, according to a statement from South Africa's National Joint Operational and Intelligence Structure (NATJOINTS).

The magistrate convicted Kunle Benjamin on a charge of possessing stolen property after police discovered the tickets were purchased over the Internet and he couldn't give a satisfactory explanation as to how he got them, said NATJOINTS.

Benjamin had been arrested Sunday in Pretoria after he was allegedly driving erratically and police discovered the tickets on him.

2.Obama praises China's move to allow its currency to float

Washington (CNN) -- President Barack Obama welcomed Saturday's news that China's central bank will allow its national currency to float ahead of the G-20 summit in Toronto, Canada, next week.

　In a statement issued Saturday, Obama praised China's decision to increase the flexibility of its exchange rate, which officials hope will help balance China's trade deficit with the United States and Europe.

"China's decision to increase the flexibility ... is a constructive step that can help safeguard the recovery and contribute to a more balanced global economy," the president said in the statement.

"I look forward to discussing these and other issues at the G-20 Summit in Toronto next weekend," he said.

* Will currency move help U.S.-China relations?

China's official Xinhua news agency said The People's Bank of China -- China's central bank -- announced Saturday it would push further its rate reform to make the the yuan, also known as the renminbi, more flexible. The move could appease international criticism against China's weak exchange rate, which has created trade imbalances between Western nations and China.

"The decision was made in view of the recent economic situation and financial market developments at home and abroad, and the balance of payments (BOP) situation in China," Xinhua quoted the Central Bank's spokesperson.

"The stability of the RMB (renminbi) exchange rate has played an important role in mitigating the crisis' impact, contributing significantly to Asian and global recovery, and demonstrating China's efforts in promoting global rebalancing," he said.

The International Monetary Fund's managing director, Dominique Strauss-Kahn, released a statement on China's move, which he described as a "welcome development."

"A stronger renminbi is in line with findings of the G-20 Mutual Assessment Process, to be presented in Toronto next week, and will help increase Chinese household income and provide the incentives necessary to reorient investment toward industries that serve the Chinese consumer," he said.

China's move comes ahead of the Group of 20 and the Group of 8 meetings in Canada's Province of Ontario scheduled next week, with finance ministers and central bank governors of major trading powers will discuss trade issues and the world financial crisis.

Representatives of several industrialized nations including India, Brazil, the United States and European countries have previously asked China to allow its currency to float.

anyone stop the Williams sisters at Wimbledon?

CNN) -- With a record winner's prize of almost $1.5 million, there's a lot at stake for the 128 women tennis players lining up at the hallowed lawns of Wimbledon in the coming fortnight.

Last year it was American Serena Williams who lifted the coveted Venus Rosewater Dish for the third time in her career, beating her elder sister Venus in the final and taking home a mere $1.25 million.

And it is hard to talk about Wimbledon without mentioning the duo, who over the last decade have dominated play at the All England Club, winning eight of the last 10 titles and playing each other for four of them.

Many thought the pair would not play beyond their early twenties, but at 28 and 30 they are still going strong, currently ranked one and two in the world.

The top-ranked Serena won her 11th Grand Slam singles title in Australia in January, cementing her place as the world's best player, and Venus added to her title tally by winning at Dubai and Acapulco in February.

Why Wimbledon beats the World Cup

The pair's disappointing runs on clay at the French Open -- Venus lost in the fourth round to Nadia Petrova and Serena in the quarterfinals to eventual runner-up Sam Stosur -- will count for little once they step onto the London grass, their favored surface and where history shows that they can never be counted out.

They will also be in with a shot for the doubles title they have won four times before, after also rising to the world number one and two doubles spots.

The highs and lows of the Williams sisters' wardrobe

The only woman still playing on the tour to have squeezed her name onto the Wimbledon roll of honor in the last 10 years alongside the Williams sisters is Maria Sharapova, who beat Serena to take the title in 2004.

Still only 23 years old, the Russian has been plagued with injury problems over the last couple of years, but has been slowly making her way back up the rankings with a win in Memphis and a runner-up spot at the Wimbledon warm-up event in Birmingham.

Despite losing the final of the English grass-court event to China's Li Na -- who must also be considered an outside threat for the big title -- there are signs that Sharapova is returning to the form that saw her become a Grand Slam champion at Wimbledon at the tender age of 17.

Wimbledon is the one title that eludes seven-time Grand Slam champion Justine Henin, who will be playing at the south-west London tournament for the first time since returning from an 18-month retirement at the beginning of this year.

The former world number one won her 42nd career title at Stuttgart in April and has come back from nowhere to reach 17 in the rankings.

However, an early exit to Stosur in the fourth round of the French Open on her favorite clay surface suggests that the 28-year-old is not yet back to her best, and the twice-beaten Wimbledon finalist may have to wait a little longer to be crowned queen on the grass.

Top 10 greatest grand slam finals

Compatriot Kim Clijsters also made a return from retirement recently and will be playing her first Wimbledon since 2006. In fact, it was training to play for the unveiling ceremony of Wimbledon's new Center Court roof in May 2009 that the Belgian decided she wanted to return to professional tennis.

Clijsters missed out on Roland Garros due to a foot injury but is fully fit and raring to go for the London event. The 27-year-old has never been beyond the semifinal stage, but would dearly love to add the title to her two U.S. Open victories from 2005 and 2009 -- the latter coming in her third tournament back on the tour.

The unlikely heroines of the French Open -- Stosur and champion Francesca Schiavone -- cannot be ruled out of doing well at Wimbledon.

Previously known as a doubles a player, Australia's Stosur is a Wimbledon doubles champion, having only recently shot to the top-10 of the singles rankings.

Her failure to overcome Schiavone in Paris will surely spur her on to win her first Grand Slam singles title, but she might struggle on the fast-paced grass having never progressed beyond the third round before.

Schiavone is enjoying her highest ever ranking of sixth in the world after becoming the first Italian woman to lift a Grand Slam singles trophy.

Should the French Open leave Roland Garros?

She reached the Wimbledon quarterfinals in 2009, proving she can play on grass, but a first-round defeat in the first round of the Eastbourne warm-up event suggests her run to the Paris final may have taken too much out of her.

It is hard to see past another Williams-Williams final, with perhaps some disruption from the Belgians, but if the surprises of Roland Garros are anything to go by, Wimbledon could yet see some upsets.

Former world number one Ana Ivanovic of Serbia will be a dangerous floater in the draw if she can recapture some of her pre-injury form.

Marion Bartoli of France, the losing finalist in 2007, will also be one to look out for, still ranked within the top 20.

Serbia's Jelena Jankovic, another former No. 1, will provide the main outside threat from the top-10 along with Denmark's third-ranked Caroline Wozniacki. However, their inconsistent displays at Wimbledon in the past mean it is hard to predict how they will do.

British tennis fans will be hoping for much from their number one player Elena Baltacha, who has twice reached the third round of the competition and is now ranked a career-high 55 in the world.

1.Voluntourism: The benefits and pitfalls you need to know

　(CNN) -- Two members of the voluntourism or volunteer holiday industry go head-to-head over the oft-debated benefits and pitfalls of a holiday choice that continues to grow in popularity.

Christopher Hill is the founder and managing director of Hands Up Holidays, a UK-based eco-luxury voluntourism travel company that organizes tailor-made vacations that typically have a 25 to 30 percent volunteering component. Here, he argues why the growing voluntourism industry is so good for the world.

A huge proportion of people who experience volunteering while on vacation are inspired to volunteer long-term, passionate advocates or donors for their own community.

This is a huge benefit of the booming voluntourism industry, one that is so often overlooked.

A voluntourism experience offers a bit of volunteering within a normal holiday, and that's why there is a cost involved -- because people are paying for a holiday, but there is also a donation to a project involved too.

The industry is growing because more and more people are seeking meaning, fulfillment and a sense of purpose in life. If this is not met in their careers and daily lives, volunteering whilst traveling is a powerful way to meet this desire.

It is important to remember voluntourism and aid work are two distinct markets, appealing to two completely different groups.

Aid programs perform an essential role, but people involved with it are long-term participants, volunteers or otherwise. Voluntourism is for people who are going on vacation who do not have the time to be involved with traditional aid programs.

Benefits of voluntourism to the host communities depend on the skills of the volunteer. For example, doctors provide a specific need within a community while volunteers can help build libraries, schools or houses that the community could not otherwise afford.

There also need to be benefits to the individual to ensure they have a well-rounded travel experience. A responsible voluntourism provider needs to identify the best available accommodation, which has strong environmental and social responsibility credentials and guides who are trained in responsible travel principles.

Voluntourists should look for an organization that consults with the local community, and assurance that the needs of the project has been sourced by the community rather than imposed on it externally.

Their skills should be matched to a project and be put in touch with previous clients who have volunteered there before. Voluntourism can and does make a huge difference.

For example, five years after Hurricane Katrina, New Orleans is a growing destination for people who want to help out in a variety of areas: environmental conservation, wildlife preservation, and assisting people affected by this calamity.

There is still much to be done to help renovate houses damaged by that storm or help with skills training of long-term unemployed. This is an excellent example of all the good things voluntourism can do.

Mark Jacobs is the managing director of Azafady, a small London-based charity working in the south-east of the African island of Madagascar on sustainable development, community and conservation projects with volunteering placements. Here, Jacobs talks about what's wrong with the ever-growing voluntourism industry.

If you're volunteering with our organization, you'll be required to fund raise a direct charitable donation, not pay a fee to a profit-making company. The communities we work with have requested all of the projects we work on.

The projects are ongoing, so that even if you are on our two-week placements -- which are our shortest -- you will leave knowing that you have made a real contribution and the benefits of your work will continue long into the future.

But it's almost impossible for the discerning volunteer to figure out which volunteering organizations are doing good work and which are not.

They can cost a fortune, but local communities see little of the benefits -- and all volunteers are left with is a dose of cynicism. It's a minefield out there.

The small organizations genuinely doing good on the ground are overlooked because they can't afford to engage in mass marketing.

Most volunteers who work with us find us, rather than the other way around. And it's not easy for them to find us -- there's so much "noise" out there in the form of big travel companies charging exorbitant amounts for volunteering holidays set up with only profit and entertainment in mind.

The profits involved have meant massive expansion in the industry. The basis of my cynicism is that many organizations produce little good and can even lead to skilled, well-motivated people turning away from international charity work altogether.

We have just three staff in the UK, keeping most paid positions for local staff in Madagascar. We are proud that 90 percent of our income is spent on our charitable aims.

In the past five years, with volunteers' support, Azafady has managed to build 11 schools, plant 200,000 trees, and increase access to clean drinking water by 50 percent in our main area of activity.

We don't spend large amounts of money on marketing or PR, but this means that we struggle to compete with big organizations that spend a fortune on glossy advertising. We often don't fill the capacity we have to take volunteers on our projects.

A lot of big travel companies now create volunteering opportunities, but it just doesn't fit with what's really needed overseas.

There are key questions that individuals need to ask these organizations before they choose a volunteering project: Are the projects needed? Are you a charity or a profit-making company? Can I see a copy of your accounts to see where the money actually goes?

There needs to be a regulatory body that assesses the validity of different organizations and keeps the industry honest.

But the large travel organizations that dominate the industry are powerful and going to be dead against the idea of a regulatory body because it will make them look as ridiculous as they are behind the gloss.

But that's what is needed -- easy access to information so individuals know their time and money will be put to good use.

21、如图9所示，上图为一列横波中距离波源x=2m处的某点的振动图象，下图是该波在t=2s时刻波动图象。求该波的频率、

波长、波速，并指出它的传播方向。

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⑴电容器两板间电势差为多少？

⑵两板间的电场强度为多少？

⑶油滴从A点运动到C点共用多少时间？

19、在370JRB22 彩色显象管中，电子从阴极至阳极通过22.5KV电势差加速，试求：

①　　 电场力做的功W=？

②　　 电子的电势能变化了多少？

18、如图7所示的电场中，有A、B、C三点，则负电荷在　 点所受电场力最大，负电荷在

　 　　点具有的电势能最大。

17、在一点电荷Q电场中有一点P，今将另一点电荷q= － 2.0×10c从无穷远处移到P点，电场力做了2.0×10J的功。设无穷远处电势为零，则P点的电势为　　 V。Q点电性为　　 电荷。

16、在一个高为40cm的圆柱形容器，装有20cm高水银柱后密封容器，此时容器内气体压强为75cmHg，如图6所示，后把容器底部钻一小孔使水银从孔中流出，那么最后剩余的水银高度是　　　 cm .