HPP Activity A30.v1
What’s your potential?
Exploration
GE 1.
1. Have you heard of the greenhouse effect, as it applies to earth's climate? If
so, describe it here.
2. What do you know about the issue of global warming?
3. Is there a connection between global warming and the greenhouse effect?
Global warming is an issue of great current interest. There is now fairly conclusive evidence that
the average surface temperature of the earth has increased by about 0.6 [ºC] since 1900. There is
some controversy over why this increase is occurring. The majority of climatologists think that
human activity is a major contributor to this increase. The issue is related to our energy
production and use policy, because most of our energy sources for human activity are based on
fossil fuels. When these fuels are used, they produce greenhouse gases, which can result in an
increase in surface temperature.
If increases in greenhouse gases are indeed contributing to global warming, then physics might
provide help in solving the global warming problem by developing energy technologies that do
not produce greenhouse gases. One such technology is already in use: hydropower. Humans
have used this source of energy for more than 2000 years.
Let's explore this source of energy. Consider a simple application used by our ancesters: the
waterwheel. Here is a conceptual figure.
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity A30.v1
GE 2.
1. Why does the wheel move in this situation?
2. Does the water do work on the wheel?
3. Why does the water move?
4. Why does water come out of the faucet when you open the valve at home?
5. If you have water stored in a tower, as
shown, does it have the potential to do
work?
6. If there are 1000 [kg] of water in the
tower, and it is 30 [m], what do you think is
the potential work that water can do?
Maybe that last question was a bit difficult. Let's look at it again.
GE 3.
1. If you lift a 1 [kg] sample of water through a height of 2.0 [m], at constant
velocity, what force must you exert on the water?
2. How much work must you do?
3. If the sample of water now falls from a height of 2.0 [m], what is the work
done by gravity on the water?
Activity Guide
 2010 The Humanized Physics Project
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HPP Activity A30.v1
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4. Can the falling water do work on some other object? Give an examle.
5. Now, if you carry the 1 [kg] sample in a pail at a steady speed up a ramp
(angle of 30, height = 2.0 [m]) what force must you exert on the water to
move it up the ramp? What is the direction of this force?
6. How much work do you do in this case? Compare this work to that done in
question 2.
7. If the the pail of water is allowed to slide down the ramp (assume the ramp
is very slippery), how much work is done by gravity on the pail of water?
Compare this value to that in question 3.
8. Will any other ramp angle give a different answer for the work done?
Invention
GE 4.
1. Consider the following diagrams. Does the work done by gravity depend
on how the pail moves from yi to yf? Explain why you think so.
yf
yi
x
2. Try to develop a general formula for the work done by gravity on the pail
of water as the pail moves from yi to yf.
Activity Guide
 2010 The Humanized Physics Project
HPP Activity A30.v1
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Explain your formula to your instructor.
3. The work done by gravity on an object can be expressed as the change of a
function that depends only on vertical position. What is that function? If
you need help with this, ask your instructor.
We call this function the gravitational potential energy function, Ug.
Application
GE 5.
1. Let’s relate what we know about potential energy to hydroelectric power
generation. Consider the hydroelectric power plant in sketched below.
h
For Hoover Dam, h is 160 meters. It produces 2 x 109 Joules of electrical
energy every second. If the conversion is 100 % efficient, what is the
mass of the water that must be flowing through the turbine every second?
2. What provided the Work that gave the water this potential energy in the
first place?
Activity Guide
 2010 The Humanized Physics Project