Non-conservative force

Figure 1. A baseball hitting a bat is deformed by the normal force. The ball bounces off the bat and returns to its normal shape. In that process the atoms of the bat, the ball and the air will start to move, losing some of its energy.

A force is a push or a pull, therefore, forces influence motion. Some forces visibly change the speed or direction of motion. Other types of forces convert large scale motion that's big enough to see with the naked eye or optical microscope (macroscopic motion) to motion at the atomic scale (microscopic motion). These forces are called non-conservative forces. A moving car, bouncing ball, or crawling insect would all exhibit macroscopic motion. Sound, thermal, and light energy are all examples of microscopic motion. Therefore, a non-conservative force converts macroscopic motion into microscopic motion.

An example of non-conservative forces in a baseball game:

• Normal force: When a baseball bat hits a baseball (macroscopic motion), the hit will make a sound (microscopic motion), see figure 1.
• Air drag: After a baseball player hits the baseball, the ball moves through the air (macroscopic motion). The ball will make the molecules in the air vibrate faster—creating heat (microscopic motion). This is the same as the mechanical equivalent of heat, which converts motion of a fluid to heat. The more air drag there is, the faster the ball will dissipate energy.
• Friction: When the player slides to the base (macroscopic motion), friction will cause the atoms in the ground to shake more, the player's pants move more, and make sound (microscopic motion).

All real systems have some non-conservative forces associated with them. For example, when the moon rotates around the Earth it creates tidal forces, which will warm the oceans slightly. However, it's a small effect compared to the energy in the system.

All systems lose some mechanical energy over time; this is part of the second law of thermodynamics. It's important to note that non-conservative forces don't destroy energy they just change it into a less useful (less ordered) form.

PhET: Friction force

The University of Colorado has graciously allowed us to use the following PhET simulation. To get a physical intuition of how friction turns macroscopic motion into microscopic.

To learn more about conservative and non-conservative forces, please check out hyperphysics.

For Further Reading

• Force
• Conservative force
• Mechanical energy
• Thermal energy
• Tidal force
• Law of conservation of energy
• Or explore a random page