阅读理解

　　Lang Lang is a famous Chinese pianist. He was born in 1982 in Shenyang. He showed his talent(天赋)for playing the piano when he was two. He began to take piano lessons at the age of 3. At the age of 5, he won the Shenyang Piano Competition. When he was 11, he won the first prize at the Fourth International Young Pianists Competition.

    He began his study at Curtis Institute(学院)in America when he was 15. He became the first Chinese student at the institute. During his study there he made great progress and became a top pianist. On August 14th, 1999, the great Andre Watts was ill so Lang Lang played at concert instead of him and he was successful. Then he became famous all over the world.

    In August of 2008, more than 5 billion people watched Lang Lang's performance at the opening ceremony(开幕式)of the Beijing Olympic Games. He also played at the opening ceremony of the Shanghai Expo(世博会).

一个物体只受到两个力的作用，这两个力的三要素完全相同，这两个力（　　）