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Unit 4 Notes

Unit 4 Notes. LOL Charts. There will be an L at each time you want to show how the energy in the system is distributed. Between each L, the O shows energy entering or leaving the system. Energy commonly leaves a system in the form of heat and sound. Common Types of Energy.

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Unit 4 Notes

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  1. Unit 4 Notes

  2. LOL Charts There will be an L at each time you want to show how the energy in the system is distributed. Between each L, the O shows energy entering or leaving the system. Energy commonly leaves a system in the form of heat and sound.

  3. Common Types of Energy • Eg is gravitational potential energy. It is proportional an object’s height above the earth’s surface. • Ek is kinetic energy. It is the energy of a moving object. • Eel is elastic energy. It is the energy stored in a spring or other stretchy object. • Eth is thermal energy. It is proportional to temperature. • Ech is chemical energy. This is stored in chemical bonds. We rarely use this in physics.

  4. Pie Charts • The size of the slices shows how the energy is distributed. • When energy leaves the system, we show the amount that left as Eint

  5. Work and Energy W = F ∆x Work is force multiplied by change in position. ***The force and ∆x have to be in the same direction*** Unit: Joules (J) = Newtons x meters (Nm) When a system does work, it gives energy to something else. When work is done on a system, it receives energy. The amount of work done is also the area underneath an F vs ∆x graph

  6. Gravitational Energy • Fgis basically constant. • If I lift up an object to a height h, I do work on it and give it energy • W = Eg= Fg ∆x • Eg= mgh

  7. Spring Energy • In our lab, we found that a spring force is proportional the change in its length from equilibrium (Hooke’s Law) • The area under its F vs ∆x graph is the area of a triangle. Specifically, Eel=1/2 k (∆x)2

  8. Example: A spring has a spring constant of 500N/m. Sketch a graph showing F vs∆x. How much energy is stored in the spring when it is stretched 0.5m? A = 1/2Bh = ½*0.5m*250N = 62.5J

  9. Kinetic Energy Ek=1/2 m (v)2 As you can see, Ek is directly proportional to mass and to velocity squared.

  10. Dissipated Energy The amount of energy dissipated due to friction is equal to the force of friction multiplied by the displacement. Ediss= Ff∆x

  11. Conservation of Energy • Energy is never created or destroyed, but it does change forms • The initial energy of a system plus any energy added to the system is equal to the final energy of the system plus the energy that leaves the system. • E0 + Einput = Ef+ Edissipated

  12. Example A 2kg ball rolls off a ledge as shown. Solve for its velocity just before it hits the ground. Eg+ Ek= Ek mgh + 1/2 m(v0)2 = 1/2 m(vf)2 2kg(9.8N/kg)(2m) + 1/2 2kg(2m/s)2 = 1/2 2kg(vf)2 Vf= 6.57 m/s

  13. Power • Power is the rate at which energy is used. • P= = Units: J/s = Watts • P=Fv ***Remember that the Force and velocity have to be in the same direction***

  14. I push a 5kg box upward 2m in 2s. What is the power rating for this situation? I add Egto the system, so P= = P = (5kg)(9.8N/kg)(2m)/(2s) P = 49W

  15. Goku exerts a force of 1,500N on Vegeta causing him to move 13m in 2s. What is Goku’s power level? P = Fv = F P = 1500N * 13m / 2s P = 9750W !!!!!!THAT’S OVER 9000!!!!!

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