1. Gravitational force. This is the force of attraction between any two masses. It is responsible for holding the Earth and other planets in orbit around the Sun, and for keeping the Moon in orbit around the Earth. The greater the mass of an object, the greater its gravitational pull. Gravitational force also causes objects to fall to the ground when they are dropped.

2. Magnetic force. This is the force of attraction or repulsion between any two magnets, or between a magnet and a piece of magnetic material. Magnets have two poles: a north pole and a south pole. Like poles repel each other, and opposite poles attract each other. The strength of the magnetic force between two objects depends on the strength of the magnets and the distance between them. The farther apart the objects are, the weaker the magnetic force.

3. Electric force. This is the force of attraction or repulsion between any two charged particles. Charged particles can be either positive or negative. Positive charges attract negative charges, and negative charges repel negative charges. The strength of the electric force between two particles depends on the strength of the charges and the distance between them. The farther apart the particles are, the weaker the electric force.