Plastic Balls Supplier introduces the main functions of throwing plastic balls:

(1) The foam plastic ball is bounced by the sounding fork, indicating that the tuning fork is vibrating. This phenomenon indicates that the sound is produced by the vibration of the object;

(2) Because the vibration of the tuning fork is often difficult to observe, it is necessary to "amplify" the experimental effect. The vibration of the tuning fork is presented by the vibration of the foam plastic ball, which can amplify the tiny vibration for easy observation;

(3) Increase the intensity of knocking the tuning fork, the amplitude of the tuning fork will become larger, and the loudness of the sound will also become larger;

(4) When the tuning fork on the right is hit, the sound is transmitted to the tuning fork on the left through the air, causing the tuning fork on the left to vibrate, and the tuning fork on the left bounces the foam plastic ball.

So the answers are: (1) Sound is produced by the vibration of objects; (2) A; (3) The greater the amplitude, the louder the sound; (4) The air can transmit sound.