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What is Energy?

What is Energy?. Section 8.3. Objectives. Explain relationship between Energy and Work Define potential energy and kinetic energy Calculate kinetic energy and gravitational potential energy. Work  Energy. Energy is required to do work Doing work can (sometimes) store potential energy.

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What is Energy?

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  1. What is Energy? Section 8.3

  2. Objectives • Explain relationship between Energy and Work • Define potential energy and kinetic energy • Calculate kinetic energy and gravitational potential energy

  3. Work  Energy • Energy is required to do work • Doing work can (sometimes) store potential energy. • Work and energy have the same units (what are they?) • Example: You push with a force of 50 N on a crate for a distance of 2 meters. How much work did you do and how much energy did it require? • W = F * d = (50 N) * (2 m) = 100 J • Energy required = Work = 100 J

  4. Potential Energy • Several types, but most important: • Gravitational Potential Energy • What it’s for: • Energy can be stored in an object by virtue of its height above the ground.

  5. Demo: Doing work to store energy

  6. How Gravitational Potential Energy is used • Stores energy by virtue of an object’s height above ground. • Equation • PE = mgh (mass * acceleration due to gravity * height) • Similar to weight * height • Compare to force * distance • Measured in units of Joules (J)

  7. Kinetic Energy • Energy an object has by virtue of its motion • KE = ½ mv2(1/2 * mass * velocity squared) • Measured in units of Joules (J)

  8. A few short examples: • A bowling ball of mass 5.00 kg is raised 1.0 meter. What is its increase in potential energy? • PE = mgh • PE = (5.00 kg) * (9.8 m/s2) * (1.0 m) • PE = 49 J • If the same bowling ball is now dropped back down and is moving at 4.42 m/s, how much kinetic energy does it have? • KE = ½ mv2 • KE = ½ * (5.00 kg) * (4.42 m/s)2 • KE = 49 J

  9. Was that a coincidence? • It was no accident. Energy in nature is CONSERVED, like mass and momentum. It cannot be created or destroyed. What you put IN, you are likely to get OUT.

  10. Quiz • Ignoring friction, who does more work if the two masses are equal?

  11. Quiz • A cannon is fired upward. How does the speed of the cannonball just before it hits the ground compare with the speed just after it was fired?

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