Solve problems with knowledge: (1) Why do the bubbles rising at the bottom of the pot gradually become bigger when boiling water? A: The bubble is rising.

Solution: (1) In the rising process, the water pressure on the bubble gradually decreases, and the air pressure inside the bubble is greater than the external water pressure, so the bubble will gradually become larger, and the water pressure will become zero when it reaches the water surface, and the bubble will burst under the action of the internal air pressure;

(2)① Bricks are subjected to gravity and tension, which are a pair of balancing forces;

② As shown in the figure, tension and gravity are in balance with each other;

(3) Because the mass and speed of bricks are unchanged, the kinetic energy is unchanged; Because the height is increasing, the gravitational potential energy is increasing; Kinetic energy plus potential energy equals mechanical energy, so mechanical energy increases; In this process, electric energy is consumed and converted into mechanical energy;

(4) The power of the crane P = WT = FST = MGST = 200kg×10n/kg×10m60s = 333.3W;

(3) Experimental steps:

(1) Tie the stone with a string, and measure the gravity of the stone with a spring dynamometer in g;

(2) Lift the stone with a spring dynamometer and immerse it in the water, and read the indication of the spring dynamometer as f1;

(3) Then use a spring dynamometer to hang the stone and immerse it in milk, and read that the pointer of the spring dynamometer is F2 at this time.

Expression for calculating milk density:

∫F float =ρgV liquid discharge.

∴G-F 1=ρ water company gV row

G-F2=ρ milk gV line

∴ρmilk =G-F 1G-F2？ Rho water;

(4) the mass of the cylinder m = gg = fg = PSG

The rope winding method in Figure 2 is as shown in the figure, with three sections of rope to bear the load;

Proof: ∫η = w with w total = w with w with+w amount = g object Hg object h+G0h = g object g object+G0.

When the weight of the object is the same as the gravity of the moving pulley, the mechanical efficiency is independent of the number of rope segments, that is, the same.

So the answer is: (1) decreasing, greater than 0;

② ① Heavy, pull; ② As shown in the figure; (3) Constant, increasing, increasing and electrifying; ④333.3 w；

(3) the same as above;

4 ditto.