Square dancing is the majority of middle-aged and elderly people love a kind of sports and entertainment, as shown in Figure A is often used by square dancing mothers of a portable audio. If the sound

(1) lift the lever force F for power, the object of gravity for resistance, by the lever's equilibrium conditions: F dynamic × L dynamic = F resistance × L resistance, can be obtained F dynamic =

L block

L motion

F block=< tr>

×200N=40N;

(2) The electrical energy consumed during normal operation is obtained as acoustic energy;

(3) The volume knob used in the portable stereo is essentially a sliding rheostat (or potentiometer), and the sliding rheostat relies on the change of the length of the resistor wires connected to the circuit to change the resistance, and hence the current in the circuit. Change the current in the circuit to control the volume.

The answer is: 40; sound; varistor (or potentiometer).