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# Energy

Energy. Energy is the ability to do work. Energy of motion is called kinetic energy . Energy unit is the Joule. Kinetic energy = ½ x mass x velocity 2 KE = ½ mv 2. If we have a mass of 2 and a velocity of 3, what is the kinetic energy? KE = ½ mv 2 KE = .5 x 2 x 3 2 KE =.5 x 2 x 9

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## Energy

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1. Energy

2. Energy is the ability to do work. • Energy of motion is called kinetic energy. • Energy unit is the Joule.

3. Kinetic energy = ½ x mass x velocity2 • KE = ½ mv2

4. If we have a mass of 2 and a velocity of 3, what is the kinetic energy? • KE = ½ mv2 • KE = .5 x 2 x 32 • KE =.5 x 2 x 9 • KE = 9

5. KE = ½ mv2 is for kinetic energy in a straight line.

6. The Work-Energy Theorem states that when work is done on an object, its kinetic energy changes, and the work done equals the change in kinetic energy. • Work = change in kinetic energy

7. Work = KEf – KEi • Since KE = ½mv2 • Work = ½ mfvf2 – ½ mivi2

8. If a 100 kg truck accelerates from 20 to 40 m /s. What work is done by the engine? • Work = ½ mfvf2 – ½ mivi2 • Work = ½ 100x402 – ½ 100x202 • Work=.5x100x1600 - .5x100x400 • Work = 80,000 – 20,000 • Work = 60,000 J

9. Potential Energy

10. Energy that is dependent upon height is called potential energy. • Potential energy equation 1 = mass x gravity x height • Potential Energy = PE • PE = mgh

11. A ball is thrown up. At the top of the path, how fast is the ball moving? • 0 m/s – Not moving • How much kinetic energy does the ball have at the top of the path? • 0 – since it is not moving

12. If the ball is thrown to a height of 5 meters and the ball has a mass of 10 grams, what is the PE? • Which equation? PE = mgh or PE = wh • PE = mgh • PE = 10 g x 10 m/s2 x 5 m/s • PE = 500 J

13. If gravity is the only force causing the potential energy, than the energy is called gravitational potential energy. • A system can also have potential energy due to a spring.

14. Potential energy that is stored in a spring is called elastic potential energy. • The equilibrium of the spring is its unstretched position – where the spring has no force.

15. Equation for elastic potential energy = ½ (spring constant) (spring displacement)2 • The spring constant is represented by k • The spring displacement or distance the spring is stretched is represented by x • PEelastic = ½ kx2

16. Spring constant of 180,000 and the spring is compressed .15 m, what is the elastic potential energy? • PEelastic = ½ kx2 • PEelastic = ½ 180,000 x .152 • PEelastic = ½ 180,000 x .0225 • PEelastic =2025 J

17. As a mass accelerates on a spring, the spring’s stored PE is converted to kinetic energy. • When the mass reaches the equilibrium position where x = 0, the Potential energy = 0 and the kinetic energy is at the maximum.

18. The mass will continue to vibrate back and forth, changing potential energy to kinetic energy until the mass stops moving. • This process with the mass vibrating right to left and back again, changing kinetic to potential and potential to kinetic is called simple harmonic motion.

19. When potential energy is converted to kinetic energy, what forces take away some of the energy? • Friction, - Energy is transformed into heat • The law of conservation of energystates that in an isolated system, energy is conserved – it cannot be created or destroyed.

20. Total energy 1 = PE + KE • Total energy 2 = mgh + ½ mv2

21. What is the total energy of a ball that has KE of 10 and PE of 10? • Which Equation; TE = KE + PE or TE = ½ mv2 +mgh? • TE = KE + PE • TE = 10 + 10 • TE = 20

22. What is the total energy of a ball that has a mass of 250 and a height of 25 with a velocity of 40? • Which Equation; TE = KE + PE or TE = 1/2mv2 +mgh? • TE = ½mv2 +mgh • TE = ½ 250(40)2 + 250(10)25 • TE = ½ 250 (1600) + 250(10)25 • TE=200,000+62,500 =262,500 J

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