(1) It is necessary to have a time measuring tool and a stopwatch to explore what factors are related to the time t taken for a simple pendulum to swing back and forth once.

(2) According to the experimental data in the table, when the swing amplitude and mass change, the swing length and swing time remain unchanged; Only when the swing length changes, the time of swinging back and forth changes; Therefore, the swing time of a simple pendulum is only related to the swing length, and has nothing to do with the ball quality and swing amplitude.

When studying the relationship between any one factor and the swing time back and forth, other factors must remain unchanged. This query method is the control variable method.

So, the answer is: (1) stopwatch; (2) Swing length; Control variable method.