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