Unit 3 Section 3 Notes

Conservation of Energy

Energy Transformations

• Energy is most noticeable as it transforms from one type to another.

• What are some examples of electrical energy transforming to heat energy?• A light bulb• A hair dryer

Energy Transformations

• An example of transforming chemical energy is a car engine. Chemical potential energy in gasoline is transformed into kinetic energy of the car as it moves.

Energy Transformations

• Imagine you are sitting in the front car of a roller coaster. A conveyor belt pulls the car slowly up the first hill. When you reach the crest of the hill, you are barely moving. Then, you go over the edge and start to race down the hill. You speed faster and faster until you reach the bottom of the hill.

Energy Transformations

• In the course of a roller coaster ride, energy changes forms many times. The conveyor belt is the most important part of the ride; the energy from that initial work is stored as gravitational potential energy at the top of the first hill. After that, most of the energy goes through a series of transformations turning into KE and then back to PE.

Energy Transformations

• In many situations, there is a conversion between kinetic and potential energy.

• The total amount of potential and kinetic energy in a system is called the mechanical energy.

• Mechanical energy = PE + KE• Mechanical energy is due to the position

and motion of the object.

Energy Transformations

• A small quantity of the stored energy is transferred to the wheels as heat and to the air as vibrations that make a roaring sound.

• Energy readily changes from one form to another.

• Whatever forms the energy takes during a transformation, the total amount of energy always remains the same.

Other Examples of PE to KE

Energy Transformations

• What happens to the mechanical energy of an apple as it falls from a tree?

Energy Transformations

• As the apple falls to the ground, its height decreases. Therefore, its GPE decreases.

• The potential energy is NOT lost… it is converted into kinetic energy as the velocity of the apple increases.

• What happens to the mechanical energy?

Energy Transformations

• The mechanical energy does not change because the loss in potential energy is simply transferred into kinetic energy.

• The energy in the system remains constant!!

Energy Transformations

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Energy Transformations

• Think about the changes in energy when you are on a swing…

• At what point do you have the most potential energy?

• At what point do you have the most kinetic energy?

• What happens to the mechanical energy?

Law of Conservation of Energy

• The Law of Conservation of Energy states that energy cannot be created or destroyed.

• The big picture… the total energy in the universe remains constant.

Law of Conservation of Energy

• But how? If I stop pumping while I’m swinging, I stop!! So, where’s the energy?

Law of Conservation of Energy

• You need to remember friction…..

• As you slow down on the swing, the hooks and the chain rub against each other and air pushes against the rider.

Law of Conservation of Energy

• Friction causes some of the mechanical energy of the swing to change to thermal energy and the temperature of the hooks and chain heat up a little.

• The energy is still there, just in a different form!!

Energy Transformations

• Objects lose mechanical energy that can be in the form of sound or heat.

• When energy appears to disappear, it has just changed to a nonmechanical form.

Conservation of Energy

• Energy is transformed… not destroyed!!

• So even though the total amount of energy remains constant, less energy is available for doing useful work.

Energy Transformations

• Energy transformations in a bouncing ball:

Energy in Our Bodies

• Even the energy converted in your body follows the law of conservation of energy.

• Chemical potential energy is transferred to kinetic energy that allows your body to move!!

Energy and Calories

• A Calorie (C) is a unit to measure energy in foods.

• 1 Calorie is equal to about 4,184 Joules.• A person uses about:

• 55 Calories while sleeping for 1 hour• 210 Calories while walking for 1 hour• 850 Calories while running for 1 hour