Thermal energy is that energy related to temperature (the higher the temperature, the greater the molecular movement, and the greater the energy). If one object has more thermal energy than an adjacent substance, the substance at the higher temperature will transfer thermal energy (at a molecular level) to the other substance. Note that the energy is moving from one place to another (it is in motion) and is referred to as transient energy or, more commonly in the case of thermal energy, heat.

The only stored energy in a solid material is internal energy. Internal energy is the energy stored in a substance because of the motion and position of the particles of the substance. Heat and internal energy will be covered in the Fundamentals Manual on Heat Transfer, Fluid Flow, and Thermodynamics.

Mechanical Energy

Mechanical energy is energy related to motion or position. Transient mechanical energy is commonly referred to as work. Stored mechanical energy exists in one of two forms: kinetic or potential. Kinetic and potential energy can be found in both fluids and solid objects.

Work

Work is commonly thought of as any activity requiring exertion. However, the definition in physics is much more specific. Work is done by a force acting on a moving object if the object has some component of motion in the direction of the force. Work can be done BY a person, a machine, or an object by applying a force and causing something to move. More specifically, work is done by a force acting on a moving object if the object has some component of motion in the direction of the force. Work can be done ON an object by applying a force that causes it to move. For example, if you push on a box (apply a force) and it moves three feet, work has been performed BY you to the box, while work has been performed ON the box. If you push on the box and it does not move, then work, by our definition, has not been accomplished. Work can be defined mathematically by Equation 5-3.

where:

Example: You push a large box for three minutes. During that time, you exert a constant force of 2001bf to the box, but it does not move. How much work has been accomplished?

W = Fxd

W = 2001bf x 0 ft

W = 0 ft-lbf work done

Remember that if no movement is achieved, no work has been accomplished. Even if you feel fatigued, no work has been done. Work can be thought of as what has been accomplished. If nothing is accomplished, then no work has been done.

Example: You push the same box as mentioned above. You apply a horizontal force of 2001bf to the box, and the box moves five feet horizontally. How much work have you done?

W = Fxd

W = 2001bf x 5 ft

W = 1000 ft-lbf

In this case, work can be described as work done by the person pushing the box or work performed on the box. In either case, the amount of work is the same.