Momentum and Energy
Does the total energy of a system ever change?
No. Think about a block of wood on a table. You push it, doing work on the block and transferring energy from your body to the block. The block starts moving, but quickly slows and comes to a stop. Where did its kinetic energy go? What was the effect of the friction between the block and table?
To explore friction, rub a pencil eraser on the palm of your hand. Then quickly put the eraser against your cheek. You probably found that both the eraser and your hand became warmer. The friction between the block and table had the same effect, but the temperature change was probably too small to detect. If the temperature increases, then the thermal energy in the object has increased. Thus the decrease in kinetic energy of the block was accompanied by increased thermal energy in both the block and the table. The energy just changed forms.
Scientists have made careful measurements of energy in a variety of forms and have always found that energy is neither created nor destroyed. In other words, the energy put into a system always equals the energy change in the system plus the energy leaving the system.
These measurements have led to a law: the Conservation of Energy. As long as no objects are added to or removed from a system, and as long as there are no interactions between the system and the rest of the world, then the energy of the system does not change.