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Work

Work. (Pay special attention to words in BLACK) What is “Work”? According to physics… Work is a force applied for a certain distance. W=F D x. Work. A 50 Kg car is moved 40 m by a 30 N force. How much work is done to the car? 1200 Nm 1Nm = 1 Joule = 1 J So, W = 1200 J.

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Work

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  1. Work (Pay special attention to words in BLACK) What is “Work”? According to physics… Work is a force applied for a certain distance. W=FDx

  2. Work A 50 Kg car is moved 40 m by a 30 N force. How much work is done to the car? 1200 Nm 1Nm = 1 Joule = 1 J So, W = 1200 J

  3. W = FDx = maDx But, Vf2=Vi2+2aDx Rearrange: aDx= (Vf2-Vi2)/2 Substitute: W= maDx = m (Vf2-Vi2)/2 So: W = 1/2m Vf2-1/2mVi2 1/2mv2 has a special name: Kinetic Energy!

  4. Kinetic Energy Kinetic energy is another way that physics looks at motion: KE = 1/2mv2 Looking back… W=DKE So: Work is the change in Kinetic Energy.

  5. Kinetic Energy Reconsidering the car problem: If the 50 Kg car began at rest and experienced 1200 J of work how fast is the car traveling after the work is done? Vf =6.9 m/s

  6. Potential Energy Potential energy is anything that has the potential to do work to an object. A rock on top of a hill, a stretched slingshot, a hydrogen/Oxygen mixture etc. (What types of potential energy are each of the examples?)

  7. Gravitational Potential Energy: PEg = mgh m = mass, g = 9.8 m/s2, h = height

  8. Gravitational Sample • A 2000 Kg boulder sits 43m above the ground on top another larger boulder. What is the potential energy of the top boulder? PEg = 842,800 J

  9. Elastic Potential Energy PEe= 1/2kx2 k = spring constant = F/x (Hooke’s Law) X= displacement from the relaxed position F = Force

  10. A 45 Kg block is against a spring with spring constant 950 N/m. The spring is compressed 10 cm from the relaxed position. How much energy is stored in the spring? PEe = 4.8 J

  11. Conservation of Energy Energy cannot be created nor destroyed. The total mechanical energy initially must equal the total mechanical energy finally. SMEi =SMEf KEi+PEgi + PEei = KEf + PEgf + PEef

  12. Conservation of Energy KEi+PEgi + PEei = KEf + PEgf + PEef Spring? Moving? Up? Spring? Moving? Up? KE is the Energy of “MOVING” PEg is the energy of “UP” PEe is the energy in a “SPRING”

  13. A 2000 Kg boulder sits 43m above the ground on top another larger boulder. It falls! How fast is the boulder moving just before hitting the ground? Vf = 29 m/s

  14. A spring with a spring constant of 35 N/m is compressed 0.23 m from its relaxed position. Resting against this spring is a 0.08 Kg character. When the character is released, what is his maximum velocity? 4.8 m/s

  15. Pendulum • A pendulum has a 5.0 Kg mass attached to a 1.2 m long string. If the pendulum is brought back through an angle of 25o, what will the maximum velocity be? 1.4 m/s

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