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Kinetic and Potential Energy

Introduction

Kinetic and Potential Energy: Introduction

Energy can be categorized into two general types. Potential energy is energy that is not being used, so is often referred to as stored energy. A paratrooper preparing to jump from an airplane possesses gravitational potential energy due to his position because he jumps from somewhere high above the center of the Earth. He has the potential of moving toward the center of the Earth once he jumps. A battery possesses electrical potential energy because its energy is stored between regions possessing high numbers of electrons and regions possessing low numbers of electrons. A liter of gasoline possesses chemical potential energy in the form of the energy of the chemical bonds of the gasoline molecules. When the paratrooper falls, the battery is connected to a circuit, or the gasoline is ignited, potential energy is converted into kinetic energy.

Kinetic energy is the energy of matter in motion. In the examples above, matter is considered to be the molecules of the paratrooper, the electrons stored in the battery, and the molecules making up the gasoline. When the paratrooper jumps, his potential energy is transformed into kinetic energy as he moves closer to the ground. When the battery is connected to a circuit, the electrons from the negative terminal form an electric current that moves through the circuit toward the positive terminal. The potential energy of the battery is converted into kinetic energy in the form of the current of electrons flowing along the wires. When the gasoline is ignited, the potential energy of the chemical bonds of the gasoline is converted into kinetic energy as the gases produced by the explosion move outward as they expand.

Gravitational Potential Energy

Electrical Potential Energy

Chemical Potential Energy

Law of Conservation of Energy

The Law of Conservation of Energy states that energy can neither be created nor destroyed, but it can be converted from one form to another. Each of the previous examples illustrates the Law of Conservation of Energy since each type of potential energy can be converted into kinetic energy. The total energy of an object is the sum of its potential and kinetic energies according to the following equation:

tot energy

The conversion from potential energy to kinetic energy is not an “all-or-none” event. When the paratrooper is still in the airplane, 100% of his energy is potential energy. At the moment he impacts the ground, 100% of his energy is kinetic energy. Between the airplane and the ground, the total energy of the paratrooper is a constant that is the sum of ever-changing amounts of his potential and kinetic energies. As the paratrooper approaches the ground, his potential energy constantly decreases while his kinetic energy constantly increases as its velocity increases. For example, if he has traveled one-third of the distance between the airplane and the ground, about 67% of the energy is in the form of potential energy, and 33% of the energy is in the form of kinetic energy.

 

Newton’s Cradle. The kinetic energy of the ball on the left is transferred to the next ball, which transfers the energy to the next ball, so on down the line. When the last ball is hit, the kinetic energy causes it to kick out from the others. It swings back with kinetic energy, which it thus returns to the next ball and the progression continues.

Energy is often referred to as being “lost”, implying that it is destroyed. However, this idea contradicts the Law of Conservation of Energy. In reality, while energy may no longer be useful in a particular system, it cannot be destroyed but is instead converted to another form or transferred to another object. Take, for example, the impact of a ball dropped to the floor. When it hits the floor, it transfers the kinetic energy of its fall (movement, kinetic energy) to the floor. The matter of the floor undergoes a slight movement unnoticed by an observer since it is so slight. This movement represents an increase of the kinetic energy of the matter of the floor as a result of the transfer of the kinetic energy from the ball.

Gravitational potential energy exists as the gravitational attraction between two bodies that are separated from one another. It is most often thought of in the context of two planetary bodies, such as the Earth and Moon, which each exert gravitational forces on one another. The falling paratrooper is also an example of the effect of gravitational potential energy since each body (the Earth and the paratrooper) exerts a gravitational force on the other. The force of the Earth’s gravity on the paratrooper is obvious since he falls toward the Earth. What is not obvious is that the paratrooper exerts a gravitational force on the Earth since the amount of that force is infinitesimal in size compared to the Earth’s gravitational force on him!

In Investigation 2, you will investigate electrical potential energy, which can be converted into the kinetic energy of moving electrons. Electrons possess a negative charge making them attractive to matter with a positive charge. This attraction can be thought of as electrical potential energy if the electrons are separated from the positive charge by an incomplete circuit. The flow of electrons toward a positive charge results in the movement of electrons with a certain kinetic energy. An electrical current or electricity represents the flow of electrons along a path that consists of a metal wire or other conductor.

The kinetic energy resulting from the movement of electrons can be converted into light energy. As the moving electrons flow through the filament in an incandescent light bulb, the kinetic energy of the electrons is transferred to the atoms of the filament which is usually made of a fine tungsten wire. The kinetic energy of the atoms increases causing the temperature of the filament to increase. The heat increases the kinetic energy of the electrons of the tungsten atoms which release this kinetic energy as photons of light.

In Investigation 3, students will explore how changing the amount of a chemical reactant changes the amount of chemical potential energy in a reaction. A chemical reaction occurs when two or more chemical compounds or reactants interact to form new chemical compounds called a product(s). In a chemical compound, the chemical potential energy is stored in the chemical bonds between the atoms that constitute the compound. When a chemical reaction occurs, the chemical bonds of the reactants are broken and new chemical bonds are formed.

In some reactions, the reactants contain a greater amount of chemical potential energy than the products resulting from the reaction. This excess chemical potential energy is released from the reaction. The released energy is often converted to heat and light energy as occurs when gasoline is ignited or wood is burned. In the case of wood (mainly composed of cellulose), products of its ignition are carbon dioxide gas, water vapor, and heat. Since heat is released, this type of chemical reaction is known as an exothermic reaction.

On the other hand, in some chemical reactions, the reactants contain a lower amount of chemical potential energy than the products resulting from the reaction. The energy required to make such reactions occur needs to be added by absorbing heat from the immediate environment. As a result, this type of reaction becomes cooler and is known as an endothermic chemical reaction. The endothermic reaction shown below is the reaction that occurs in commercial cold packs. Inside of the cold pack bag is another bag that contains solid ammonium nitrate. Outside this inner bag, the cold pack is filled with water. When you “crush” the cold pack, the inner bag breaks, and the solid ammonium nitrate mixes with the water and dissolves. As you can see below, heat is absorbed by the reaction and, as a result, the cold pack gets cold.

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In summary, this CELL begins by experimenting with kinetic energy, the energy of motion, and observing the effects of velocity on kinetic energy. Potential energy will be introduced, along with the equation for gravitational potential energy. You will examine the effect of height on potential energy and observe the conversion of potential energy to kinetic energy.

The conversion of potential energy to kinetic energy will be further examined by looking at the conversion of electrical potential energy to kinetic energy in the form of light. You will also observe the transfer of energy from one form of kinetic energy (light) to another form of kinetic energy (thermal energy or heat). Finally, you will explore chemical potential energy and its conversion into kinetic energy.

CONTENT

  • Fun Facts
  • Learn the Lingo
  • Get Focused

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FUN FACTS

Playground Science

There is a simple and fun way to understand the conversion between potential energy. It’s also a way to learn science on the playground! Swing on a swing. At the very top of your swing cycle, either on the forward or backward swing, you momentarily stop and have no motion (kinetic energy) at all. Therefore, at the two high points, you have 100% potential energy. Next, at the very bottom of the swing cycle, either in the forward or backward direction, you have 100% kinetic energy and are moving as fast as possible. Exactly halfway up or down, you have 50% kinetic energy and 50% potential energy. Keep thinking about kinetic and potential energy conversion as you swing during a single recess and you will never forget! 

 

Since we are on the playground, consider the seesaw (teeter-totter). Notice that as one person goes up, the other goes down. There is no other way it can operate. They can neither both go up or both go down at the same time. In a similar manner, as potential energy decreases, kinetic energy increases by the same amount.

The Law of Conservation of Matter

These observations concerning the relationship between potential and kinetic energy can be expressed by a simple formula:

Total Energy = Kinetic + Potential

This is simply a mathematical way of stating exactly what we observe on the playground. That is, the total amount of energy is either in the form of kinetic energy or potential energy. Since an increase in one of the forms of energy is always accompanied by a proportional decrease in the other (like a seesaw), kinetic energy and potential energy added together always equals the amount of total energy. This is yet another way of thinking about the Law of Conservation of Energy. That is, energy cannot be destroyed or created, it can only change from one form to another. In this case, total kinetic energy plus total potential always equals the total amount of energy in the system.

LEARN THE LABLEARNER LINGO

The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.

 

Investigation 1:
  • Law of Conservation of Energy: a principle that states that energy is neither created nor destroyed, it simply changes form
  • Kinetic Energy: the energy of motion
  • Potential Energy: The energy stored in an object or substance.
  • Gravitational Potential Energy: the potential energy that exists between two objects that exert a gravitational pull on each other
Investigation 2:
  • Electrical Potential Energy: the potential energy that exists when comparing a region of high electrical charge to one of low electrical charge
Investigation 3:
  • Chemical Potential Energy: the potential energy stored in the chemical bonds of chemical compounds

GET FOCUSED

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

 

Investigation 1:
  • How does the transfer of potential energy to kinetic energy relate to the Law of Conservation of Energy? 
Investigation 2:
  • Can one form of energy be converted to another? Support your answer with data from the experiments.
  • How does converting energy from one form to another relate to the conservation of energy? 
Investigation 3:
  • Can one form of energy be converted to another? Support your answer with data from the experiments. 
  • How does converting energy from one form to another relate to the Law of Conservation of Energy?