In the case of the experiments that you describe pith is used because it is a very low density material and again a conductor because of its moisture content. A hanging pith ball would be insulated from the ground because the silk or plastic thread which is a good insulator.

Pith balls are little styrofoam (or similar material) balls coated in conductive paint. When a charged rod is brought close to the pith balls, their charges polarize. If they come in contact with the charged rod, they can inherit the same sign charge. Since like charges repel.

Touch the pith ball with your hand to neutralize it. Rub the ebonite rod with the fur or wool and bring it close to the pith ball WITHOUT touching it.

Description

1. Hang pith balls side by side, touching.
2. Rub down rubber rod with fur, rod will acquire a negative charge.
3. Carefully slide charged rod over one of the pith balls to transfer over as much negative charge as possible.

The pith-ball electroscope, invented by British schoolmaster and physicist John Canton in 1754, consists of one or two small balls of a lightweight nonconductive substance, originally a spongy plant material called pith, suspended by silk or linen thread from the hook of an insulated stand.

In The Info Box You Will Find The Value Of Each Pith Ball’s Mass (m,-0.050 Grams), Charge #-0 And Charge On Each Ball-0 C (zero Coulomb)-the Conducting Pith Balls Are Neutral Start Charging The Ball On The Rod By Rubbing The Ball …

The gravitational force (Fg) pulls the pith balls downward. The tension of the string (T) opposes gravity and pulls the pith balls up.

A charged pith ball works well to show the Coulomb force between two charged objects. If a glass rod is rubbed with silk, the silk rubs some electrons off the rod. The rod thus becomes positively charged. Touching the positively-charged rod to two pith ball gives the balls each a small positive charge.

According to Coulomb, the electric force for charges at rest has the following properties: Like charges repel each other; unlike charges attract. Thus, two negative charges repel one another, while a positive charge attracts a negative charge. The attraction or repulsion acts along the line between the two charges.

Gravity or gravitational forces are forces of attraction. It’s like the Earth pulling on you and keeping you on the ground. That pull is gravity at work. Every object in the universe that has mass exerts a gravitational pull, or force, on every other mass.

A trampoline is basically an elastic disc connected to several springs. When a person lands on the trampoline they stretch both the springs and the fabric surface. Hooke’s law states that stretched springs will always try to return to their original shape.