(1)① When studying what factors are related to kinetic energy, the magnitude of kinetic energy is reflected by the pushed distance of the wood block, and the transformation method is adopted;

② When the voltage is constant, study the relationship between current and resistance.

③ Use the formation of water flow to simulate the formation of water flow. The analogy method is adopted, so ① is selected.

(2) Comparing experiments (a) and (b), we know that the probe of the liquid manometer is immersed in water and salt water to a certain depth in order to explore the relationship between liquid pressure and liquid density;

Comparing experiments (b) and (c), it can be seen that the different immersion depths of the liquid pressure gauge probe in salt water are to explore the relationship between liquid pressure and liquid depth.

So, the answer is: ①; Density; Depth.