Table of Contents

**Mechanical Energy Definition- Mechanical Energy Examples **

## Mechanical Energy Definition

*The definition of mechanical energy is power that an object gets from its position and motion.*

*or** **Mechanical Energy is Defined as The sum of the potential energy and kinetic energy of a body or system. *

In the physical sciences, mechanical energy is the sum of potential energy and kinetic energy. It is the energy associated with the motion and position of an object. The principle of conservation of mechanical energy states that in an isolated system that is only subject to conservative forces, the mechanical energy is constant. If an object is moved in the opposite direction of a conservative net force, the potential energy will increase and if the speed (not the velocity) of the object is changed, the kinetic energy of the object is changed as well. In all real systems, however, nonconservative forces, like frictional forces, will be present, but often they are of negligible values and the mechanical energy’s being constant can therefore be a useful approximation. In elastic collisions, the mechanical energy is conserved but in inelastic collisions, some mechanical energy is converted into heat.

**Mechanical energy can be classified as potential energy or kinetic energy.**

### Potential energy is energy that is stored in an object. The different types of mechanical potential energy are related to the position of the object.

### Gravitational potential energy is related to the object’s height above the ground. The higher an object is above the ground, the higher the potential energy of the object.

### Elastic potential energy is related to how far an object is stretched. For example, rubber bands have elastic potential energy.

### Kinetic energy is energy of motion. The amount of kinetic energy that an object has depends on the mass and velocity (or speed) of the object. For example, a car moving at 60 miles per hour has more kinetic energy than the same car moving at 30 miles per hour.

**The Total Mechanical Energy**

Mechanical energy of an object can be the result of its motion (i.e., kinetic energy) and/or the result of its stored energy of position (i.e., potential energy). The total amount of **mechanical energy is merely the sum of the potential energy and the kinetic energy**. This sum is simply referred to as the total mechanical energy (abbreviated TME).

**TME= PE+ KE**

## Mechanical Energy Examples :

Today, many technological devices convert mechanical energy into other forms of energy or vice versa. These devices can be placed in these categories:

- An electric motor converts electrical energy into mechanical energy.
- A generator converts mechanical energy into electrical energy.
- A hydroelectric powerplant converts the mechanical energy of water in a storage dam into electrical energy.
- An internal combustion engine is a heat engine that obtains mechanical energy from chemical energy by burning fuel. From this mechanical energy, the internal combustion engine often generates electricity.
- A steam engine converts the heat energy of steam into mechanical energy.
- A turbine converts the kinetic energy of a stream of gas or liquid into mechanical energy.

*Mechanical Energy Example : *

Let’s work through an example of calculating mechanical energy using a pendulum.

The formula to calculate the potential energy is:

PE = mgh

Substitute the values into the formula and you get:

PE = 10 kg x 0.2 m x 9.8 m/s2 = 19.6 J

Whenever the ball is at the ends, the Potential Energy (PE) = 19.6 J. Since the speed is 0 at the end (remember the ball stops at each end), the Kinetic Energy (KE) = 0 J.

The total mechanical energy, ME, of the pendulum at either end would be:

**ME = KE + PE = 0 J + 19.6 J = 19.6 J**

As the ball approaches the middle position, the PE is decreasing while the KE is increasing. At exactly halfway, the height of the ball will drop to zero. This would mean that PE would also equal zero. At this point, theoretically, all PE has transformed (turned) into KE. This would make the KE = 19.6 J while the PE = 0 J. The mechanical energy would be similar to the ends equaling 19.6 J:

**ME = KE + PE = 19.6 J + 0 J = 19.6 J**

The key word in the last paragraph was “theoretically.” In a perfect closed system, as the PE decreases, it all turns into KE. As KE decreases, it all turns back into PE. A closed system is very similar to the closed system we discussed in chemical and physical changes. In this case, a closed system refers to a group of objects that transfer energy only to each other.