Ball A moving at 12 m/s collides elastically with ball B, initially at rest as shown. If both balls have the same mass, then what is the final velocity of ball A? 

A block of mass \( m \) is in contact with the cart C as shown in the figure. The coefficient of static friction between the block and the cart is \( \mu \). The acceleration \( a \) of the cart that will prevent the block from falling satisfies:
