Energy January 2015 Arsenie

Two groups of Energy Potential Energy (PE) Potential Energy (PE) It isn’t shown through movement It isn’t shown through movement Energy is stored and ready to do work Energy is stored and ready to do work Elastic PE – a spring, a rubberband, a bow Elastic PE – a spring, a rubberband, a bow Chemical PE – batteries, gasoline Chemical PE – batteries, gasoline Nuclear PE – the sun, bombs Nuclear PE – the sun, bombs Gravitational PE – any object elevated Gravitational PE – any object elevated Kinetic Energy (KE) Kinetic Energy (KE) Object is in motion Object is in motion ENERGY of MOTION ENERGY of MOTION

What is “ Mechanical ” Energy? Mechanical energy refers to objects in motion or that have the potential to move. It is the sum of potential and kinetic energy. Mechanical energy refers to objects in motion or that have the potential to move. It is the sum of potential and kinetic energy. Mechanical Energy is the property of an object that enables it to do ‘ work ’. Mechanical Energy is the property of an object that enables it to do ‘ work ’.

Work is the product of the force (F) on an object and the distance (d) that object moved (in the same direction as the force). Work is done to either: to change the speed of an object or against another existing force, such as lifting against gravity. Work is the Energy of a Force

SI Metric Units of Energy Energy is measured in Joules (J). This includes Work, Potential Energy, or Kinetic Energy. 1 Joule = 1 Newton x 1 meter 1 Newton x 1 meter is also called a “ Newton- meter ”.

think! You lift a 100-N boulder 2m. Q. How much work is done on the boulder? Answer: W = Fd = 100 N x 2 m = 200 N·m = 200 J Example problem

Gravity exerts a force on this boulder. In order to move it, work must be done (against the existing force of gravity). Which example requires more work? The boulder is lifted with 100 N of force. The boulder is lifted with 100 N of force. The boulder is pushed up the 4-m incline with 50 N of force. The boulder is pushed up the 4-m incline with 50 N of force. The boulder is lifted with 100 N of force up each 0.5-m stair. The boulder is lifted with 100 N of force up each 0.5-m stair. Potential Energy: Trick question! Each example requires the same amount of work.

Potential Energy But, it ’ s also equal and opposite to Work!!!!!! But, it ’ s also equal and opposite to Work!!!!!! Because we would have to do work against gravity to lift something to get it elevated to store PE Because we would have to do work against gravity to lift something to get it elevated to store PE Therefore, PE converts to Work! Therefore, PE converts to Work! So, what is the gravitational potential energy of the boulder in the lifted position? Answer: it is still 200 J! Why? Because PE is equal and opposite to work!

So, Then, and So, Therefore, W is equal and opposite to PE

Kinetic Energy But it ’ s ALSO equal to the work required to bring something to its final motion or to rest because it is a conversion of potential energy. But it ’ s ALSO equal to the work required to bring something to its final motion or to rest because it is a conversion of potential energy. …and potential energy is also equal to work and measured in Joules, and work is equal to force multiplied by distance. Therefore:

The Law of Conservation of Energy We have discovered a natural law of the universe that all energy is conserved. We have discovered a natural law of the universe that all energy is conserved. Energy is not created or destroyed. Energy is not created or destroyed. All the energy that was here in the beginning is still here, and will be here until the end. All the energy that was here in the beginning is still here, and will be here until the end. Energy simply converts from one form to another. For example, PE converts to KE. Energy simply converts from one form to another. For example, PE converts to KE.

Forms of Energy Thermal Thermal Chemical Chemical Electric Electric Radiant (electromagnetic) Radiant (electromagnetic) Nuclear Nuclear Magnetic Magnetic Elastic Elastic Sound Sound Mechanical Mechanical Luminous Luminous Mass (E=mc 2 ) Mass (E=mc 2 ) Dark (theoretical) Dark (theoretical) Nutritional/food Nutritional/food Can you categorize each of these as Potential or Kinetic?

Forms of Energy Thermal (KE) Thermal (KE) Chemical (PE) Chemical (PE) Electric (KE) Electric (KE) Radiant (KE) (electromagnetic) Radiant (KE) (electromagnetic) Nuclear (PE) Nuclear (PE) Magnetic (KE) Magnetic (KE) Nutritional/food (PE) Nutritional/food (PE) Elastic (KE) Elastic (KE) Sound (KE) Sound (KE) Mechanical (PE) Mechanical (PE) Luminous (KE) Luminous (KE) Mass (E=mc 2 ) (KE) Mass (E=mc 2 ) (KE) Gravitational (KE) Gravitational (KE) Dark (theoretical) Dark (theoretical)