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Download Physics 6.3 and 6.4
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Physics 6.3 and 6.4 Conservation of Energy and Power Mechanical Energy is Conserved • Energy cannot be created or destroyed, it can only be transformed. • In situations where friction can be ignored (doesn’t really happen), the system is ideal and the sum of the kinetic and potential energies is always to same. • The sum of the kinetic and the potential energy is equal to the mechanical energy, thus • E=PE + KE • Energy changes from kinetic to potential, however total energy stays the same • How does this relate to your roller coaster designs? Solving Problems • Calculating final speed (we did this in the roller coaster lab) • Take the initial height and the mass of the object to find the starting potential energy (this is all the energy that is going into the system) • The initial kinetic energy is zero at the starting height • PE initial + KE initial =PE final + KE final • Plug in what you know to solve for the unknown Solving Problems Cont. • At an amusement park, a swimmer uses a water slide to enter the main pool. If the swimmer starts at rest, slides without friction, and descends through a vertical height of 2.31 m, what is her speed at the bottom of the slide? • A crow drops a 0.11kg clam onto a rocky beach from a height of 9.8m. What is the kinetic energy of the clam when it is 5 meters above the ground? • What is the velocity at that point? Section 6.4 Power • Power is the amount of work done over an amount of time. • Think about running up the stairs vs walking up the stairs, the same amount of work is done, however it is much more exhausting to run than walk. • Power=Work/time • Units: Watts (W) Power Units • You produce more power when you run up the stairs, compared to when you walk because your force stays the same but the time to run up the stairs decreases. • P=W/t (time decreases, increasing the Power) • 1 Watt=1 J/s • To understand the value of a Watt, a fluorescent light bulb has the power of 23 Watts • You can also look at Horsepower, which is 746 Watts Human Power Output Example: an 80kg person walks up a flight of stairs in 22 seconds, and the altitude gain is 3.7 meters. • What is the work done? • What is the power? • Convert to horsepower? • The greatest power most people can produce over a sustained amount of time is about ½ Horsepower or 370 W. Practice Problem • Complete practice #48 on page 213 Power Depends on Force and Speed • • • • P=W/T P=Fd/t P=F(d/t)=Fv V=P/F
