It's been a week of ups and downs for shareholder activists — powerful investors who buy up stock in companies to shake them up and get the stock price moving. 

Hedge fund manager Carl Icahn announced he has a large position in Apple — more than $1 billion worth of shares.

Meanwhile, Bill Ackman — perhaps the most combative investor on Wall Street — resigned from the board of J.C. Penney after failing to reshape the retailer's fortunes and getting into a very public dispute with other board members.

This week on Money Talking, guest host Heidi Moore of the Guardian speaks with regular contributors Joe Nocera of the New York Times and Rana Foroohar of Time magazine about shareholder activists, their tactics and their impact on the companies whose stock they buy. 

Looking ahead, Foroohar talks about new six-year high schools in Chicago focused on science and technology that offer associate's degrees. 

Nocera discusses the proposed merger of American Airlines and US Airways, which was blocked by the Justice Department this week.