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Energy

Energy. Chapter 5 Section 2. What is Energy?. Energy – A scalar quantity that is often understood as the ability for a physical system to produce changes on a different physical system. Units for energy is a “Joule” The variable for a Joule is a capital “J”. Kinetic Energy.

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Energy

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  1. Energy Chapter 5 Section 2

  2. What is Energy? • Energy – A scalar quantity that is often understood as the ability for a physical system to produce changes on a different physical system. • Units for energy is a “Joule” • The variable for a Joule is a capital “J”

  3. Kinetic Energy • Kinetic Energy – The energy of an object due to it motion. • Kinetic energy depends upon the object’s velocity and mass. • Any mass that is moving has kinetic energy.

  4. Equation for Kinetic Energy KE = ½mv² Kinetic Energy = ½ x mass x (velocity)² • Given the objects velocity and mass, the energy that the object has can be calculated using the formula above.

  5. Example Problem • A car traveling at 50m/s has a mass of 1000kg. Calculate the kinetic energy.

  6. Example Answer • KE = ½mv² = ½ (1000kg) (50m/s) ² =1,250,000 KE = 1,250,000J

  7. Potential Energy • Potential Energy – The energy associated with an object due to its position. • In other words, potential energy is stored energy that has the potential to move because of its position with respect to some other location.

  8. Different Forms of Potential Energy • There are two different forms of potential energy: • Gravitational Potential Energy • Elastic Potential Energy

  9. Gravitational Potential Energy • Gravitational Potential Energy – Potential energy associated with an object due to its position relative to the Earth or some other gravitational source.

  10. Gravitational Potential Energy Equation PEg = mgh Gravitational Potential Energy = mass x free-fall acceleration x height

  11. Converting Gravitational Potential Energy into Kinetic Energy • Imagine a rock falling off a cliff. As it falls, it gains kinetic energy since it picks up velocity as it falls to the ground below. • But where does the kinetic energy come from? • It comes from the gravitational potential energy that is associated with the rock’s initial position on the cliff relative to the ground below.

  12. Origin of Gravitational Potential Energy • Gravitational potential energy is a result of an object’s position, so it must be measured relative to some ZERO level. • Doesn’t matter where you place the zero level (origin), but it must remain consistent through out the problem.

  13. Example Problem • How much potential energy does a rock have that has a mass of 10kg and is 100m off the ground on top of a cliff?

  14. Example Problem Answer • PEg = mgh =(10kg)(9.8m/s²)(100m) =9800J PEg = 9800J

  15. Elastic Potential Energy • Elastic Potential Energy – The potential energy in a stretched or compressed elastic object. • Examples: • Springs • Rubber bands • Shocks

  16. Relaxed Length of a Spring • The length of a spring when no external forces are acting on it is called the relaxed length of a spring. • When an external force compresses or stretches a spring, elastic potential energy is stored in the spring.

  17. Elastic Potential Energy Equation PEelastic = ½kx² Where: • Elastic Potential Energy – PEelastic • k – Spring Constant • x – Distance compressed or stretched

  18. Spring Constant • Spring Constant – A parameter that expresses how resistant a spring is to being compressed or stretched. • For a flexible spring the constant is small and for a stiff spring the constant is large. • The units for the spring constant is “N/m”

  19. Example Problem • A child has a rubber band with a length of 0.10m. The child pulls back on the rubber band to a distance of 0.20m. The spring constant of the rubber band is 5 N/m. What is the potential energy with in the rubber band?

  20. Example Problem Answer PEelastic = ½kx² = ½ (k) (xf – xi)² = ½ (5 N/m) (0.20m – 0.10m)² = ½ (5 N/m) (0.10m)² =0.025J PEelastic = 0.025J

  21. Mechanical Energy • Mechanical Energy – The sum of kinetic energy and all forms of potential energy. • Example: • Any kind of mechanical device • Clocks • Air compressor • Basically anything that has moving pieces… • Doesn’t include electrical, chemical, or heat energy.

  22. Mechanical Energy Equation ME = KE + ΣPE ME = Mechanical Energy KE = Kinetic Energy ΣPE = The sum of all the potential energy

  23. Classification of Energy

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