# Energy - Mr. Woods' Science Classes

Energy Part 5 Conservation of Energy Conservation of Energy There are some things that are never created or destroyed, but are transformed from one form into another. Mass is conserved in a chemical reaction, the mass in is equal to the mass out. Energy is conserved - It is changed from one form into another. Conservation of Energy

The total mechanical energy (kinetic and potential) in a system stays the same unless an outside object does work to add or take away energy from the system. Mechanical Energy = EK + Eg + Es Work input (Win) = work done to add energy to a system. Work output (Wout) = work that the system does on something else. Work of friction (Wf) = work that friction does on the system to turn mechanical energy into heat Conservation of Energy

When we put it all together, we get the smallest AND largest equation we have used so far: Ei=Ef Eki+Egi+Esi+Win=Ekf+Egf+Esf+Wout+Wf Remember: W=Fxxx Ek = 1/2 mv2 Eg=mgh Es=1/2 k(xx)2 Conservation of Energy

Example: A ball falls from a height of 2 meters in the absence of air resistance. Eg to Ek: The ball is losing height (falling) and gaining speed. Thus, the internal force (gravity) transforms the energy from Eg (height) to Ek (speed). Conservation of Energy Example: A skier glides from location A to location B across a friction free ice.

Eg to Ek: The skier is losing height (the final location is lower than the starting location) and gaining speed (the skier is faster at B than at A). Thus, the internal force (gravity) transforms the energy from Eg (height) to Ek (speed). A large chunk of ice with a mass of 15 kg falls from a roof 8 m above the ground. Fxind the kinetic energy of the ice when it reaches the ground and the speed of the ice when it reaches the ground. m=15kg

h=8m g=9.8m/s2 Ek=? v=? Ei=Ef SO Eg=mg Eg=(15)(9.8) h Eg=1,176 J (8)

Ei=Eg=mgh & Ef=Ek at ground Ek=1/2mv 2 1176=1/2(15) 2 v2 2352=(15)v 156.8=v2 12.52 m/s=v

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