(20 13? One end of the L-shaped long string is fixed at the O point, and the other end is tied with a small ball with the mass of m, so that the string is horizontally straightened from the static sta

(20 13? One end of the L-shaped long string is fixed at the O point, and the other end is tied with a small ball with the mass of m, so that the string is horizontally straightened from the static state. First, according to the kinetic energy theorem:

12mv？ 2-0=mgL

Solution: v=2gL

If the mass of the ball becomes 2m, other conditions being unchanged, and the speed of the ball to the lowest point is still V, then A is wrong;

B, according to the centripetal force formula:

F-mg=mv2r

Solution: F = 3 mg

So the mass of the ball becomes 2m, and other conditions remain unchanged, so the tension of the string becomes 2F and the length of the string becomes 2L when the ball reaches the lowest point. Other conditions being the same, the string tension remains the same when the ball reaches the lowest point, so B is right and C is wrong;

According to centripetal acceleration formula:

A=v2r=2g, the length of the string becomes 2L, other conditions remain unchanged, and centripetal acceleration remains unchanged when the ball reaches the lowest point, so D is correct.

So choose BD.