As you begin your studies of the forms of energy, it is important to keep in mind that all energy allows objects to do work on other objects. When we start talking about the energy in a system, we basically isolate an object or objects together so that we can analyze and quantify the energy within that system. It allows for more simplistic problem solving and allows you are more specific explanation of what is happening to one object or a group of objects. Watch this video to see the concepts we will study more in-depth as we move forward.
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Today's lesson is going to be about energy. Energy is the ability to do work. We need energy to accomplish tasks in life. The more work you do the more energy you need. Energy comes in many forms. There's nuclear energy. There's chemical energy. There's internal energy, electrical. But, the type of energy that we will discuss for this class is mechanical energy. Now it's not a special type of energy. It's just an energy. A way to classify the energy that we're going to speak about. There are three forms of mechanical energy that we're going to discuss. The first is gravitational potential energy. And this is energy stored based on an object's height above the Earth's surface. The formula is gravitational potential energy equals the mass times the gravity times the height at which that object is, or how high above the surface it is. The second form is elastic potential energy. And this is energy stored in a stretched or compressed elastic object. So elastic potential energy is equal to 1/2 of kx squared. Now the k is very simply a spring constant, and that will generally be given to you in a problem. And x is how far the spring is stretched or compressed from its equilibrium position. And this is something that we'll discuss in later lessons. The third and final form of chemical energy is kinetic energy. And this is the energy of motion. And kinetic energy is equal to 1/2 the mass times the velocity of the object squared. The law of conservation of energy is a staple in physics. And it states that energy cannot be created or destroyed, only converted from one form to another. What does this mean? It means that total energy is constant in a closed system. So, the total energy at the beginning is equal to the total energy at the end. Total energy can be a combination of the kinetic energy of an object, the gravitational potential energy, and elastic potential energy of that object, as well as the work being done by that object, or being done on that object. A closed system is merely a set of objects that has no outside forces acting on it, otherwise meaning that we don't have friction or air resistance of any sort. The work energy theorem helps us relate energy to work. It tells us to change the kinetic energy of a system or an object. An outside force is required. Work must be done to change an object's kinetic energy. For example, the belt that pulls a roller coaster cart to the top of the first hill does work. The roller coaster will go from zero kinetic energy to a lot of kinetic energy at the top, lifting the bowling ball to give it GPE, requiring doing work on the ball. So, if you were to lift a bowling ball above the ground, that's going to take energy from yourself to lift the bowling ball up. The work energy theorem is basically work equals change in kinetic energy as I've previously stated. So, we have a formula here, which is work is equal to the change in kinetic energy, which is really equal to the kinetic energy at the end minus the kinetic energy initially. |
Question
Why is the phrase of a system so important when you are thinking about the Law of Conservation of Energy or the Work-Energy Theorem?
It is easy to think that the conservation of energy does not apply when you see an object slowing down (i.e. kinetic energy decreases). But, if you define your system to be only the object that's moving, you must take into consideration anything that comes from OUTSIDE the system. The kinetic energy of a moving object may decrease if an outside force does work on it (the surface applies friction force to oppose the motion). How you define your system will help you determine when work is done on the system. The energy is not created or destroyed, it is just transferred from outside or to outside the system. The same is true for the work-energy theorem. For the internal kinetic energy of an object to change, something outside of the object has to do work on that object.
