Download Hands-on Activity: Falling Water Created by: Integrated Teaching

Hands-on Activity: Falling Water
Created by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado
at Boulder
Summary
Students drop water from different heights to demonstrate the conversion of
water's potential energy to kinetic energy. They see how varying the height
from which water is dropped affects the splash size. They follow good
experiment protocol, take measurements, calculate averages and graph
results. In seeing how falling water can be used to do work, they also learn
how this energy transformation figures into the engineering design and
construction of hydroelectric power plants, dams and reservoirs.
Engineering Connection
Engineers design hydroelectric dams to take advantage of the conversion of
water's potential energy to kinetic energy, and then mechanical energy to
Bridal Veil Falls,
electrical energy. Since the force of kinetic energy is dependent on the height Yosemite National Park,
and mass of the falling water, civil engineers take this into consideration
California.
when designing dam turbines. High kinetic energy (caused by a great amount
of water or falling from a high distance) results in incredible force on the
turbine blades, so engineers design the blade and the assembly to withstand
this force. Engineers use this same understanding of energy transfer when
they design roller coasters.
Grade Level: 4 (3-5)
Time Required: 50 minutes
Keywords: average, dam, energy, graph, hydroelectric power, kinetic energy, measurement, potential energy, turbine, water
Learning Objectives:
After this activity, students should be able to:

Describe an object's varying potential energy as being dependent on its height above the Earth's
surface.

Explain the path of potential energy converted into kinetic energy as objects fall.

List ways that the energy of falling water is used to do work.

Describe how engineers use the potential and kinetic energy measurements of water to design
reservoirs and hydroelectric power plants.
Materials List
Each group needs:

Plastic straw (clear is best)

Cup or glass (to hold colored water for dipping the straw)

Meter stick

Pencil or pen

Marking pens

Newsprint, newspaper or blank paper (minimum of 12-16 sheets per team)
For the entire class to share:

1 or 2 plastic, gallon-size (3.78 liter-size) milk containers to distribute water

Water, preferably colored with food coloring

Falling Water Worksheet, one per team
Introduction/Motivation
Who knows what energy is? (Answer: Something that can do work). Have you ever heard of potential or
kinetic energy? Well, potential energy is simply the "stored" energy of an object. An object's potential
energy depends partly on its height and gravity. Kinetic energy is the energy of the object in motion. As
an object falls, or moves down an incline, its potential energy is converted into kinetic energy. So, a
falling object (or one rolling or sliding down a hill) loses its potential energy as it increases its kinetic
energy.
When we think of water, potential energy is the energy of the water as it sits calmly
in a reservoir or lake. The kinetic energy of water is when it is rushing through a
river, down a waterfall or through a hydroelectric power plant. The force of the
kinetic energy is dependent on the height and mass of the falling water.
Hydroelectric is when water is used to produce electricity. Civil engineers design
and build dams and hydroelectric power plants that use the energy of falling water
to turn the blades of turbines to generate electricity that we can use in our homes.
So, it is actually the kinetic energy of water moving through a power plant that can
Installation of huge
produce electricity.
water turbines at
In this activity, we are going to drop water from different Ft. Loudoun Dam
heights to demonstrate how the kinetic energy of water in Tennessee.
The new technology of
free-flow hydro works a
lot like a wind farm,
except with underwater
turbines.
changes when it hits the floor. Engineers must
understand how height affects the amount of kinetic
energy so they create a reservoir that holds the proper amount of water and
design a hydroelectric power plant that produces the right amount of
electricity. Since water falls too fast for us to measure the velocity (or kinetic
energy) of the water directly, we will measure the size of the splash produced
to show us how much kinetic energy is there.
Vocabulary/Definitions
Diameter:
The straight-line distance across a circular area, through its
center point.
Energy:
The ability to do work.
Hydroelectric
power plant:
A power plant that uses water turbines to generate
electricity.
Hydroelectricity:
Electricity produced by the energy of running water.
Kinetic energy:
The energy of motion. For example, a spinning top, a
falling object and a rolling ball all have kinetic energy. The
motion, if resisted by a force, does work. Wind and water
both have kinetic energy.
Mechanical
energy:
Energy that can be used to do work. It is the sum of an
object's kinetic and potential energy.
Potential energy: Potential energy is the energy stored by an object as a
result of its position. For example, a roller coaster at the
top of a hill, or water being held behind a dam.
Renewable
Energy that is made from sources that can be regenerated.
energy:
Sources include solar, wind, geothermal, biomass, ocean
and hydro (water).
Turbine:
A machine in which the kinetic energy of a moving fluid is
converted into mechanical energy by causing a series of
buckets, paddles, or blades on a rotor to rotate.
Procedure
Before the Activity

Gather materials and make copies of the Falling Water Worksheet.

Fill the containers with water. Color the water with several drops of food coloring.
With the Students
1. Divide the class into teams of three or four students each.
2. Show the students how to use a straw like a water dropper (see Figure 1). Stick the straw in a
glass of water. Place a finger on the top of the straw. Pull the straw out of the glass, while still
holding your finger over the top. Hold the straw over a piece of paper and remove your finger.
3. Ask the students how they can all be sure to use the same amount of water for each trial.
(Answer: By marking a line on the straw and always refilling it to that
point.)
4. Discuss with students methods for measuring the diameter of a
splash. Have the students agree on a standard class method for
determining the diameter of a splash. Have students use a pencil or
pen to outline the splash, and label it with their name, the drop
height, and the splash diameter.
5. Have student teams describe on the worksheet what they predict will
happen to the splash as it is dropped from increasing heights. Do they
expect the size of the splash to increase or decrease with higher drop
heights?
6. Have the class agree on four heights from which to drop the water,
such as 30 cm, 60 cm, 90 cm and 1 m (note this on the worksheet).
Teams measure the diameter of the water splash for all four heights.
Each team member drops the water once from each height. Thus,
teams with three members have 12 data points and teams with four
members have 16 data points.
Figure 1. How to use a
straw like a water
dropper.
7. Students take turns doing each part of the activity. One student holds the meter stick. One
student drops the water using the straw. A third student measures the splash diameter.
8. Hold the bottom of the straw at the height by first placing it next to the
correct height on the meter stick and then moving it several centimeters away, being
careful to keep the straw at the same height. On the paper, write the height, diameter
and team member name, and record the data on the worksheet.
9. After one person drops the water, the team members switch places and
repeat the process. The student measuring the diameter of the splash moves the
paper so the water falls on a dry piece.
10. After teams have finished their drops, have them complete the worksheet by calculating
averages and creating a graph of their average data.
11. Conclude with a class-wide comparison of each team's results (table data and graphs) and a
concluding discussion, as described in the Assessment section.
Attachments
Falling Water Worksheet
Troubleshooting Tips
Make sure students fill their straw to the mark before each trial, otherwise the amounts will be
inconsistent.
This activity works best if the teams drop the water onto a hard, flat surface.
Shorter students may require chairs or stools on which to stand when they drop the water.
Assessment
Pre-Activity Assessment
Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs
down for false. Tally the votes and write the totals on the board. Give the right answer.

True or False: An object's potential energy depends on its height above the Earth's surface.
(Answer: True)

True or False: Kinetic energy is the energy of something staying very still. (Answer: False. Kinetic
energy is the energy of an object in motion.)

True or False: An object's kinetic energy increases as its speed increases. (Answer: True)
Discussion Question: Ask a discussion question to get students to think about the upcoming activity.
After soliciting answers, explain that these questions will be answered during the activity. Ask the
students:

What makes a good experiment? Only one variable can be changed at a time in a good
experiment. Why?
Activity Embedded Assessment
Prediction: Ask students to predict the outcome of the activity before the activity is performed. What
will happen to the splash as it is dropped from increasing heights? Do you expect the size of the splash
to increase or decrease with higher drop heights?
Design Decisions: As a class, design the parameters for your experiment, including determining standard
methods to ensure experimental consistency in the following:

Amount of water being dropped

Measurement of the splash diameter

Height from which to drop water
Worksheet: Have the student teams record measurements, answer questions, make calculations and
graph data on the worksheet. Review their answers to gauge their mastery of the subject.
Post-Activity Assessment
Concluding Discussion: Ask the students and discuss as a class:

What patterns do we see in the results? How might we have altered the experiment or our
methods to make a better experiment? What conclusions can we draw?

What happens when we increase the height from which the water is dropped? What if a light
bulb is dropped from 1 cm off the floor? From 2 meters off the floor? How does the potential
energy and the resulting force (kinetic energy) change in these two cases? What if you dropped
a glass into the sink? Does the height at which it is dropped make a difference in whether it
breaks or not? How is a short waterfall different from a very high waterfall?

How would engineers use this understanding to design hydroelectric power plants? How would
engineers use this understanding to design roller coasters? How is the force you feel on a roller
coaster different if you are coming down a steep section compared to a level section?
Diagramming: Ask the students to illustrate the concept of energy transfer through drawing.

Draw the transformations of energy that occurs when an object is dropped. (Answer:
Potential energy due to the object's height above the Earth is converted to kinetic energy as it
falls. Note: Point out to students that when the object strikes the Earth, the kinetic energy is
converted into other forms of mechanical and potential energy, such as sound or heat energy. It
may be appropriate to explain that energy may change its form, but there is no net change to
the amount of energy. This is called conservation of energy.)

Draw the transformations of water energy as it goes from a reservoir through a hydroelectric
plant.
Engineering Application: Have students reflect on the following engineering applications for kinetic and
potential energy. This assignment is suitable for a class discussion or a journal entry.

Use your understanding of kinetic energy to explain why engineers place some hydro generators
in fast-moving waters. (Answer: Fast moving waters have more kinetic energy to transfer to the
blades of a turbine, making them spin faster than they would in slow moving waters.)

Use your understanding of potential and kinetic energy to explain why engineers build large
dams for hydroelectric power plants. (Answer: The dams hold back large amounts of water. The
water is higher than the turbines so it has potential energy. As the water moves downward to
the turbines, its potential energy is transformed to kinetic energy, turning the blades of the
turbine.)
Activity Extensions
Test how changing the mass of the water drop affects the size of the splash produced. Try different sized
drops for each height.
Math Problem Solving: Present the class with the following problems and ask the students to determine
which case has the greater kinetic energy when it hits the ground. (Note: Greater height or mass =
greater kinetic energy.)

A 2-ml drop of water dropped from 2 meters, or a 2-ml drop of water dropped from 1 meter?
(Answer: The 2 ml drop of water dropped from 2 meters.)

A 4-ml drop of water dropped from 3 meters, or a 2-ml drop of water dropped from 3 meters?
(Answer: The 4-ml drop of water dropped from 3 meters.)

A 1-liter bottle of water dropped from 2 meters, or a 1-liter bottle of water dropped from 6
meters? (Answer: The 1-liter bottle of water dropped from 6 meters.)
Test how the angle at which water falls affects the size of the splash produced. See the Foundation for
Water and Energy Education's Turning to Hydropower activity, "Will changing the slope of falling water
affect how fast it flows?" at: http://www.fwee.org/TG/unit_2c.pdf.