Download Physics 6.3 and 6.4

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
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P=W/T
P=Fd/t
P=F(d/t)=Fv
V=P/F