Download Falling Water

Falling Water
Time: 40-60 minutes
Materials (per group):
Plastic straw (clear is the
best)
Pen or Pencil
Plastic cup (to hold colored water for
dripping the straw)
Newsprint, newspaper, or blank paper
(minimum of 12-16 sheets per team)
Per class:
1 or 2 large plastic pitchers to
distribute water
Water
Food Coloring
Marking Pens
Meter Stick
Hand towel or paper towels
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:
1. Fill the pitchers with water. Color the water with several drops of food coloring.
2. Divide the class into teams of three or four students each.
3. Show the students how to use a straw like a water dropper. 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.
4. 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.)
5. 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.
6. Have student teams record 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?
7. Have the class agree on four heights from which to drop the water, such as 30 cm,
60 cm, 90 cm and 1 m. Help students make a chart on a blank sheet of paper to
record the diameter of their water drop for each height. Draw a sample chart on the
board.
8. 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.
9. 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.
10. 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.
11. 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.
12. After teams have finished their drops, help them compute the average drop
diameter for each height.
13. Walk students through the process of creating a graph to illustrate the data from
their charts. Demonstrate on the board.
14. For upper grades: Using their graph results, have students make and test
predictions for what would happen at intermediate heights and heights greater than
1 meter. Are their predictions correct?
Dry everything as much as possible. Please save and reuse straws.
Discussion:
 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?
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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?
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 electrical energy. Since
the force of kinetic energy is dependent on the height and mass of the falling water, civil
engineers take this into consideration 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.
Extra Discussion (extra time only)
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 produce electricity.
In this activity, we are going to drop water from different heights to demonstrate how the
kinetic energy of water 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.