Mechanical Energy

Energy is the ability to do work. In science, work means using a force to move an object. So, bouncing a ball is a type of work because the ball moves, but pushing on a wall is not work because the wall doesn’t move. Mechanical energy allows an object, or person, do to work.

Unlike other types of energy, mechanical energy is made up of both potential and kinetic energy. Mechanical energy is the sum of an object's total potential and kinetic energy.

The Law of Conservation of Mechanical Energy explains that, in a closed system, mechanical energy cannot be created or destroyed. Instead, energy can only be transferred between potential and kinetic energy. Of course, in the real world, some of the energy of the system is always lost to friction and heat.

To better understand mechanical energy at the Law of Conservation of Mechanical Energy, picture a pendulum swinging back and forth. When the pendulum is at its highest point, it is not moving, so it doesn’t have kinetic energy. However, it is not supported by the string, so it has potential energy. 

Remember, mechanical energy is the sum of the potential and kinetic energy of an object. If the pendulum has mechanical energy of 5 Joules at its highest point, it will have a mechanical energy of 5 Joules throughout the swing. We know the pendulum has no kinetic energy, so the potential energy must be 5 Joules.

As the pendulum swings back down, it is moving, so it has kinetic energy. As the pendulum swings closer to the bottom of the string, its potential energy decreases. As the potential energy decreases, its speed increases, so its kinetic energy increases. The potential and kinetic energy of the pendulum are changing, but the mechanical energy is always 5 Joules. 

At the bottom of the swing, the pendulum has no potential energy because it is at the bottom of the string. If the potential energy at this point is zero, the kinetic energy must be 5 Joules.

As the pendulum moves back up, its kinetic energy decreases and its potential energy increases, but the total mechanical energy is always equal to 5 Joules. When the pendulum again reaches its highest point, it will have a potential energy of 5 Joules and no kinetic energy.

It is important to remember that our pendulum was swinging in a perfect closed system. This pendulum would swing forever. In the real world, the pendulum would slow down and eventually stop because of friction.

Every time molecules touch each other they lose some energy. So, the molecules in the pendulum lose energy when they touch the molecules in the air. This decreases the pendulum’s kinetic energy and makes it slow down. Friction is why you need to pump when you are on a swing.

Mechanical energy isn’t just found in pendulums. All moving objects have mechanical energy because it has kinetic energy. For example, cars driving on the highway have mechanical energy.

Objects can also have mechanical energy based on their position. A hammer held above a nail has potential energy, so it has mechanical energy. A compressed spring has potential energy, so it has mechanical energy. A stretched bow has potential energy, so it has mechanical energy. These are all examples of potential energy due to position. Each of these objects will move on their own if they are released.

Mechanical energy is energy due to an object’s motion or position, so not all objects have mechanical energy. For example, an apple sitting on the ground has thermal energy and chemical energy. Thermal energy is a type of kinetic energy, and chemical energy is a type of potential energy, but this apple does not have mechanical energy because it is not moving, and it does not have the energy to move on its own.

You can determine if an object has mechanical energy by asking yourself two questions. Is the object moving? Will the object move if it is released? If the answer is yes, the object has mechanical energy.

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