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Warm up

Warm up. If you shout, your voice travels at the speed of sound (340m/s) to a surface and back , and you hear an echo. If you hear an echo 5.2s after you shout, how far away is the surface from which the sound bounces? . Energy. Chapter 4. Energy. What is energy?

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Warm up

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  1. Warm up • If you shout, your voice travels at the speed of sound (340m/s) to a surface and back, and you hear an echo. If you hear an echo 5.2s after you shout, how far away is the surface from which the sound bounces?

  2. Energy Chapter 4

  3. Energy • What is energy? • energyis an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems. Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.

  4. Energy • What is energy? • The ability of cause change • Energy is transferred and sometimes changed in form. • How do we observe energy or what does it do for us? • Motion • Heat • Electricity - light

  5. Energy • What are some forms of energy? • Electricity • Chemical (gas for the car, food for you) • Light • Heat • Sound

  6. Energy • Kinetic Energy • The energy an object has because of its motion. Depends on mass and speed. Kinetic energy = ½ mass x velocity squared KE = ½ mv2

  7. Energy • A jogger whose mass is 60 kg is moving at a speed of 3 m/s. What is the joggers KE? KE = ½ mv2 KE = ½ (60 kg) (3 m/s)2 KE = ½ (60 kg) (9 m2/s2) KE = 270 kg m2/s2

  8. Energy • The derived unit of energy is kg m2/s2 • This unit is called the Joule (J)

  9. Energy • What is the kinetic energy of a baseball moving at a speed of 40 m/s if the baseball has a mass of 0.15 kg? KE = ½ mv2 KE = ½ (0.15 kg) (40 m/s)2 KE = ½ (0.15 kg) (1600 m2/s2) KE = 120 J

  10. Energy • A car moving at a speed of 20 m/s has a kinetic energy of 300,000 J. What is the car’s mass? KE = ½ mv2 300,000 J = ½ (m) (20 m/s)2 KE = 1500 J

  11. Energy • What is the velocity of a baseball with 150 J of kinetic energy if the baseball has a mass of 0.15 kg? KE = ½ mv2 2KE/m = v2 √2KE/m = v V = 44.7 m/s

  12. Energy • Motionless objects also have energy. • Instead of energy of motion this is energy of position and is stored in an object. • This energy of position is referred to as Potential Energy.

  13. Energy • A motionless object on a shelf has potential energy. If it falls the energy is converted to kinetic energy.

  14. Energy • Potential Energy is the storing of energy. • Elastic PE – a stretched rubber band • Chemical PE – energy stored in chemical bonds • Electrical PE – energy stored due to position related to other charges.

  15. Energy • Gravitational Potential Energy • Anything that can fall has stored energy called GPE. This energy is due to the height above Earth’s surface and the mass of the object.

  16. Energy • GPE = mass (kg) x accel due to gravity (m/s2) x height (m) GPE = mgh

  17. GPE = mass x acceleration due to gravity x height The GPE of an object can be calculated using this equation: How is GPE calculated? • Mass is measured in kilograms (kg). • Acceleration due to gravity is 9.8 m/s2 on Earth • Height is measured in meters (m). • GPE is measured in joules (j).

  18. Factors affecting GPE

  19. Calculating GPE question 1 An eagle with a mass of 2 kg flies at a height of 200 m above the ground. How much gravitational potential energy does the eagle have? GPE = mass x accel due to gravity x height = 2 kg x 9.8 m/s2x 200 m = 3920 J

  20. changein height GPElost = mass x accel due to gravity x Calculating GPE question 2 An apple with a mass of 200 g falls 3 m from its branch to the ground. How much GPE will the apple have lost when it reaches the ground? = 0.2 kg x 9.5 m/s2x 3 m = 5.88 J

  21. GPE, mass and height calculations

  22. This is a web site with a 14 question quiz on ke/pe. Not too hard. Good for a couple minutes of review http://www.cstephenmurray.com/onlinequizes/physics/workandenergy/kineticvspotentialenergy.htm

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