#### What is molecular attraction? How is molecular attraction overcome? Molecular attractive forces, adhesion and cohesion.

We use energy and do work when we overcome another kind of resistance. This kind is caused by the attraction between the molecules in materials. When you tear a piece of paper, you must exert a force to pull the molecules apart along the tear. Force is needed because the molecules of a material attract each other. This attraction, which holds the molecules of a material together, is the force called cohesion.

The force of cohesion differs in different materials. In some materials, it is so slight that only a small force is needed to pull the molecules apart. In other materials, the force of cohesion is so great that a large force is needed to pull the molecules apart. Without exerting much force, you can tear a fairly thick piece of cardboard or push your hand into water. Yet you cannot tear even a thin piece of steel or push your hand into a stone. To overcome the force of cohesion in these materials, more force must be exerted.

There is another molecular force very much like cohesion. The molecules of a material not only attract each other, but they also attract the molecules of other materials. This attraction, which makes one material stick to another, is the force called adhesion. The letters stay on the pages of this book because of the force of adhesion between the molecules of ink and the molecules of paper. The lead in a pencil is really graphite, a form of carbon. When you write with a pencil, friction between the paper and the carbon rubs off a little carbon. The carbon sticks to the paper because of adhesion.

Like cohesion, the force of adhesion differs between different materials. Some materials stick very tightly to other materials, while others will not stick at ali. Steel and wood will not stick to each other, but paint will stick to both of them. Even two pieces of wood will not ordinarily stick to each other. Yet glue can be used to hold them tightly together because of the very great force of adhesion between the molecules of glue and the molecules of wood in each piece.

Without the molecular forces of cohesion and adhesion, molecules would not stick together. There would be no solids and liquids, because their molecules would fly apart and form gases. The molecules of copper, for instance, stick together because of cohesion. Because of adhesion, one piece of copper can be joined with solder to another piece of copper. However, we often want to overcome the attraction between molecules. To do this, we must use energy to exert a force greater than the molecular forces. Whenever we cut, break, grind, saw, or tear a material, we use energy to exert a force that overcomes the resistance of cohesion. Whenever we separate one material from another material, we use energy to exert a force that overcomes the resistance of adhesion.