## Due 3:30 pm, Thursday, October 1st

Physics 105, Fall Term, 2009

Did you complete the reading assignment?

Yes
No

## KEY CONCEPTS:

1) The total energy within a closed system is conserved.

2) Power is the rate of transfer or energy.

## Exercise 1

A closed system is specific area isolated from the rest of the universe. No heat or energy can be transferred from inside a closed system to outside of the system. Although a perfectly closed system does not exist, we can approximate some systems as closed systems.

Go back to: http://phet.colorado.edu/sims/energy-skate-park/energy-skate-park_en.jnlp This applet gives a perfectly closed system. Click the 'Bar Graph' button to display the energy in bar graph form. Click the 'Potential Energy Reference' checkbox and place the reference at the lowest point of the track.

Pick up the the skater and place him at the top of the ramp. Let him go. How does the initial gravitational potential energy of the skater compare to the gravitational potential energy of the skater as he reaches his peak at the other side?

Initial is more
Initial is less
They are the same

How does the initial gravitational potential energy of the skater compare to the kinetic energy of the skater as he reaches the lowest point of the track?

Initial gravitational potential is more
Initial gravitational potential is less
They are the same

Where is the total energy of the system the greatest?

Top of the track
Bottom of the track
Half way between the top and bottom of the track
It is the same at each location

In real life situations, you won't return to the same height that you started at just by standing on the skateboard because of friction. Click the 'Track Friction' button near the bottom right of the applet and increase it to about half way up the scroll bar.

Pick up the the skater and place him at the top of the ramp again. Let him go. How does the initial gravitational potential energy of the skater compare to the gravitational potential energy of the skater as he reaches his peak at the other side?

Initial is more
Initial is less
They are the same

How does the initial gravitational potential energy of the skater compare to the kinetic energy of the skater as he reaches the lowest point of the track?

Initial gravitational potential is more
Initial gravitational potential is less
They are the same

Explain what causes the results of the 2 questions above.

Where is the total energy of the system the greatest?

Top of the track
Bottom of the track
Half way between the top and bottom of the track
It is the same at each location

Why is friction considered to be a nonconservative force?

## Exercise 2

Power is generally a very straight forward topic, as it is just the amount of work done in a certain amount of time. Where confusion often comes in is when it is applied to power companies. Power companies make you pay by Kilowatt-hour, or 1000 Watts *hours.

What is the the basic unit of power? (select all that apply)

Joules
Watts
Joules/seconds
Watts-seconds

What is the basic unit of energy?

Joules
Watts
Joules/seconds
Watts-seconds

What is a power company charging you for?

Power
Energy

Here is an interesting and simple application that can help you see how you use power in your house and how the power company will bill you for it: http://jersey.uoregon.edu/vlab/Work/index.html. No questions for this one, this is for your own benefit in knowing how power companies work and the significance of power ratings on appliances.

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Was there anything that you didn't understand in the reading assignment?  What was confusing to you?