But what if you have 3000 dollar capital and buy the 3000 shares or you have 300 dollar capital with 1:10 leverage and then buy the 3000 shares. Isnt this the same? Cant you just replace capital with leverage as long as you dont make to much risk?
Leverage is limited depending on your account size, broker, and even country.

Assuming you are in the U.S.:

1) $2,000 required for a margin account to gain 2 to 1 leverage at U.S. sanctioned brokers.
2) $25,000 required for 4 to 1 leverage for day-trading at most brokers to protect smaller accounts. There are exceptions to this rule though depending on the broker. Example: MB Trading offers 4 to 1 leverage to any margin account for day-trading but you have to follow the other SEC rules (and all margin accounts):
3) If under $25,000 in account, limited to 3 day-trades on a rolling 5 day period.
4) Only allowed 2 to 1 leverage overnight no matter how big your account is.

Read more http://www.mbtrading.com/stocksmarginrequirements.aspx

Be sure to check with your broker before trying to load up 4 to 1 intra-day, and keep count of your round-trip day-trades if under $25,000.

Also there is in the news that a plain crashed, which is offcourse a very terrible event for a lot of people.. However this also effects the stock markets and traders?
This is the million dollar question..... Your guess is as good as the gurus on CNBC. Seriously. No one knows. But if your right, it doesn't make you genius nor if your wrong it doesn't make you stupid. It's just one of those things, dang if you do, dang if you don't.

Trading is emotion based, so of course some people will freak out, scream 'world war 3' type shit, but depending on your strategy you can analyze your positions logically to take action. If you invested based on fundamentals, ask yourself if has affected the fundamentals of your stocks? If you own a Russian, airline or defense stock, maybe it will. If you own McDonalds or Walmart, maybe not. If you invested based on technicals, then stick to the chart read and your stop. If you invested based on both or even the potential to have potential (story dream) then you have to analyze in the same fashion that lead you to buy and determine if you should sell. Or you could hedge and play against what you own with some put option strategies or general market bear or gold plays.

Good Luck.