What is mechanical energy?

A motor is an example of a system that produces mechanical energy.

Mechanical energy is the sum of energy in a mechanical system, or any group of objects that interact based on basic mechanical principles. This includes kinetic energy, the energy of motion, and potential energy, the stored energy of position. Typically, in a mechanical system, gravity is the only major external force that needs to be considered. In a chemical system, by contrast, the forces between individual molecules and atoms must all be taken into account.

General Background

Mechanical energy exists as kinetic and potential energy in a system. Kinetic energy is present whenever an object is in motion. Potential energy is based on an object’s position; it is stored energy and cannot work on its own. However, it can be converted into other forms of energy, including kinetic energy. A bowling ball suspended 10 feet (3 m) above the ground, for example, has no kinetic energy because it is not moving. However, it has a large amount of potential energy (in this case, gravitational potential energy) that would be converted to kinetic energy if the ball were to fall.

High school physics classes usually begin by instructing students in the basics of mechanical systems and their energy. This is because they are typically easier to visualize and simplify. Basic calculations on these systems can be done without using calculations. In most simple physics problems, the mechanical system remains closed and factors that would normally remove energy from the system, such as friction and air resistance, are ignored.

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How to calculate mechanical energy

The total mechanical energy can be calculated simply by adding the potential and kinetic energy of the system. Potential energy (PE) is a product of the object’s height above the ground (h), its mass (m), and the Earth’s gravitational acceleration (g, which is 9.8 m/s 2 ).

PE = h × m × g

An object’s kinetic energy (KE) is the product of 1/2 of its mass and its velocity squared (v).

KE = 1/2 mv 2

Mass is given in kilograms (kg), height in meters (m), speed in meters per second (m/s) and energy in joules (j).

For example, the potential energy of a 5 kilogram (11 lbs) bowling ball that is 3 meters (10 ft) above the ground is 147 joules (5 kg × 3 m × 9.8 m/s 2 = 147 j) whether the ball is in motion or at rest. If this ball is also falling at a speed of 2 m/s, its kinetic energy is 10 joules (1/2 × 5 kg × 22 m/s = 10 j).

Once the potential and kinetic energy are known, then the total mechanical energy can be found. The two types of energy are simply added together.

Mechanical Energy = PE + KE

In this example, the total mechanical energy of the bowling ball is 157 joules (147 j + 10 j = 157 j).

Mechanical Energy vs. Chemistry and Nuclear

There are many other forms of energy, and it can sometimes be difficult to properly distinguish one from the other. Chemical energy, for example, is the energy stored in the chemical bonds of molecules. Nuclear energy is the energy present in interactions between the particles in the nucleus of an atom. Mechanical energy, by contrast, generally ignores the composition of objects and looks only at the objects in question, without worrying about their molecular composition.

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This focus is designed to simplify calculations of mechanical energy and mechanical systems. Objects in these systems are often treated as individual objects rather than the sum of billions of molecules. Calculating the kinetic and potential energy of a single object is a simple task; calculating these kinds of energy for billions of molecules would be extremely difficult. Without simplifying the parts of a mechanical system, scientists would need to examine individual atoms and all the interactions and forces that exist between them. This is normally reserved for particle physics.

Conversion between types of energy

Mechanical energy can be converted into other types of energy using special equipment. For example, generators are designed to take mechanical work and turn it into electricity. Other types of energy can also be converted to mechanical energy; for example, a car’s internal combustion engine converts the chemical energy of the fuel into the mechanical energy used to make the car move.

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