From the chemical energy stored inside my body to the rise in internal energy of both my keyboard and fingers as I am composing this article, I — like everybody — am surrounded by energy. The definition of energy, as given by Physics, is that energy is a physical property possessed by an object, and it is a measure of the ability of this object to act on the physical world, to make things happen.
Energy Is Conserved
The law of conservation of energy states that energy is neither created nor destroyed: contrarily, it is just converted into its various types. So, next time you use the “I’ve run out of energy excuse”, remember that you never run out of energy. You most probably have few supplies of chemical energy inside your body, because you converted them into kinetic, sound or internal energy (to be discussed).
To shed light on this claim, this article is going to present the conversions of energy that take place via a simple and everyday example: a child, naughty Mary, playing with her wind-up toy.
Naughty Mary Demonstrates Physics
Naughty Mary is curious. She picks up a wind-up toy from the floor and places it on the table on her left. She turns the key on the side of the toy, which winds up a metal spring inside the toy. The spring unwinds, causing the toy to move across the table. The toy then slows down because of frictional forces. Just as it is stopping, it reaches the edge of the table and falls to the ground, hitting it with a crash. Naughty Mary gets terrified and runs frantically to her mother. What were the conversions of energy that occurred during this everyday scene?
To begin with, Naughty Mary moves because she possesses chemical energy: as her muscles are contracted in her attempt to pick up the toy, chemical reactions occur and chemical energy is released because of this muscle contraction.
The chemical energy is then converted into kinetic energy in Naughty Mary’s hand when she picks up the toy. Kinetic energy is a notion easy to grasp: it is the energy of motion or the energy an object acquires because of this motion.
When Naughty Mary places the toy on the table, kinetic energy is converted into:
- internal energy. This means that the total energy of the constituent molecules of both the toy and the table increases and this brings about an increase in temperature of both objects.
- gravitational potential energy. This is the type of potential energy (i.e. energy that is stored in some way or “awaits for action” and is usually associated with an object’s position in space) that is related to gravity and which increases the further an object is placed from the center of the Earth. In our example, gravitational potential energy increases due to the change of height between the floor and the table as the toy is raised.
When Naughty Mary turns the key on the side of the toy, energy is stored as strain potential energy in the spring. Strain potential energy is the energy stored within an object when the latter is stretched, squashed or deformed. Here the spring unwinds (it is stretched) and strain energy is converted into kinetic energy, as the toy move across the table. Later, the frictional forces that act on the toy convert some of the kinetic energy into internal energy and the toy slows down.
As the toy is stopping, there is more internal energy and less kinetic energy. At the edge of the table and at the point where the toy pauses before crashing onto the ground (as usual with all Naughty Mary’s toys) internal and kinetic energy are converted into gravitational potential energy, which is then converted into kinetic energy until just before the toy lands (harshly). Kinetic energy is now converted into sound energy (the crash) and internal energy: if you measured the temperature of both the toy and the floor with a highly sensitive thermometer, you would record a slight increase in temperature of both.
And Gets in Trouble
Naughty Mary did it again. The toy was her Christmas present and she managed to crash it three hours after she had received it from her godmother, who still fails to understand that we should keep Naughty Mary away from objects that may break. Anyway, Naughty Mary now converts probably all the chemical energy stored in her body into kinetic energy to run frantically to her mother and…isn’t the world of Physics great?
Further Source
S104 Exploring Science (2008), Energy and Light, Milton Keynes: Open University
Stavroula Dimitropoulos - MSc, BSc
