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Energy

Introduction to Physical Science Monday, Wednesday, Thursday Tom Burbine tomburbine@astro.umass.edu. Energy. Energy is the property of a system that enables it to do work. Work. Work = Force x Distance. To do work. You need to move something. Energy units.

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Energy

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  1. Introduction to Physical ScienceMonday, Wednesday, ThursdayTom Burbinetomburbine@astro.umass.edu

  2. Energy • Energy is the property of a system that enables it to do work

  3. Work • Work = Force x Distance

  4. To do work • You need to move something

  5. Energy units • In English Units, we use calories to measure energy • In science (and in this class), we will use joules to measure energy • 1 Joule = 1 kg*m2/s2

  6. Joule • One joule is defined as the amount of work done by a force of one Newton moving an object through a distance of one meter

  7. Calories • The small calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.2 joules. • The large calorie or food calorie approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1000 small calories or about 4.2 kilojoules.

  8. Potential Energy • Stored Energy that has potential to do work • Compressed Spring • Drawn bow • Coal • Water in an elevated reservoir

  9. Gravitational Potential Energy • PE = weight x height • PE = mgh • where m is the mass, g is the gravittaional acceleration, and h is the height

  10. Kinetic energy • Kinetic energy = ½ mv2 • m is mass in kg • v is velocity in meters/s • Fd = ½ mv2 • Remember: a joule has units of kg*m2/s2

  11. How much kinetic energy does a 2 kg rock have if it is thrown at 20 m/s? • Kinetic energy = ½ mv2 • A) 200 J • B) 400 J • C) 40 J • D) 800 J

  12. Answer • KE = ½ * 2 * (20) *(20) = 400 joules

  13. Work-Energy Theorem • Work is change in kinetic energy • Work = ∆KE

  14. As the kinetic energy of a system increases, its potential energy decreases by the same amount, and vice versa

  15. The change in potential energy is always equal to the change in kinetic energy (assuming there are no other energy losses). • Δmgh = Δ½mv2

  16. Kinetic Energy and Momentum • Kinetic energy and momentum are properties of moving things • Momentum is a vector and capable of being cancelled • Kinetic energy is scalar and can not be cancelled • Momentum depends on the velocity and kinetic energy depends on the velocity squared

  17. Scalars and Vectors

  18. Conservation of Energy • Energy is neither created or destroyed – • it just changes forms • Conservation of Energy • The energy in a closed system may change form, but the total amount of energy does not change as a result of any process.

  19. Types of collisions • Elastic collision – all the momentum is transferred to another object during a collision • Inelastic collision – Colliding objects become entangled

  20. Momentum and Kinetic Energy • Momentum is conserved in elastic and inelastic collisions • Kinetic energy is only conserved in elastic conditions • Energy is always conserved • http://en.wikipedia.org/wiki/Elastic_collision

  21. Inelastic Collisions • Reduction of kinetic energy • Some of the kinetic energy is turned into another form of energy (e.g., thermal energy, sound) • http://www.physicsclassroom.com/mmedia/momentum/cthoi.cfm

  22. Power • Power = work done/time interval • Units are Watts (Joules/seconds)

  23. Any Questions?

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