Why do objects float or sink?
Summary
Students will use models to gain an understanding of the principles of buoyancy and how
they apply to technologies used to explore the ocean
Learning Objectives
Students will be able to:
 Explain positive, negative and neutral buoyancy
 Design a ROV that has positive, negative or neutral buoyancy
Background
While students may be aware that very heavy ships float and submarines have the ability
to dive or surface, the principles of buoyancy many not be understood. This series of
activities engages students in the principles before explaining the science behind the
experiences. It seeks to engage students in prediction, hands-on experiments, conclusions
and then modifications to change the outcome. With this inquiry-based presentation,
students can participate in the process of the scientific method that naturally leads to new
questions and ideas of testing them. Also, students are taught principles of pressure,
density, and stability, which are applied to the remotely operated vehicles necessary for
science exploration.
Why does an object float? To answer this question
return to the principles of pressure with depth. Water
has a much higher density than air and can therefore
exert more pressure on objects. This is true not only
when an object descends in the water but also when an
object is supported in the water by floating. The
pressure of all of the water below an object pushes up
on the object. That pressure is greater than the
downward pressure exerted by gravity. A boat, while
its materials may be more dense that the water uses it’s
shape to distribute that pressure in such way that the
upward pressure exceeds the downward pressure. Not
until the objects overcomes the pressure of the water
(like when the Titanic filled with water) does the object
sink.
Buoyancy force is the upward force exerted on an object. Archimedes’ principle
states that any floating object displaces its own weight of fluid. Thus any object that
floats does so because the force pushing it up is equal to the weight of the fluid that was
displaced. As a sinking ship takes on water, that water is no longer being displaced, but
sucked in. The boat increases its weight, giving gravity a stronger edge. Eventually, the
boat’s weight overcomes the upward pressure and goes down. The Archimedes principle
did not consider surface tension of fluid, but as this only modifies the amount of fluid that
is displaced, the principle of buoyancy holds true.
Objects that float on or toward the surface
of the water are said to be positively buoyant.
Objects that sink are negatively buoyant. And
objects that hover, that is neither rise or fall, are
said to be neutrally buoyant. The experiments
done in this lesson allow students to discover
these various positions. Any submersible vehicle
must be designed to achieve negative buoyancy so
that it can dive, positive buoyancy so that it can
ascend, and neutral buoyancy if it is to move
along at a certain depth to gather information.
Sources for information:
http://en.wikipedia.org/wiki/Buoyancy
(sketch of forces and buoyancy information)
http://www.lakesidepress.com/pulmonary/books/scuba/sectione.htm
(sketch for negative, netural and positive buoyancy)
Materials
Large Rubbermaid type tubs to hold water (shared by groups of 3-5 students)
Water bottles with caps
Funnel
Measuring cup or graduated cylinder (a simple plastic cup can be substituted with fill line
drawn on)
** ½ “ PVC schedule 40 plain end pipe cut into 3” to 5” lengths
** ½” PVC schedule 40 3 way tees
** ½” PVC schedule 40 4 way tees
** pool-noodle type floats (just cut into various lengths no longer than 5”)
Note: To make a small ROV box like model takes about 90” of pvc pipe with eight 3
way tees and four 4 way tees. Cost: $10.00/ROV for supplies. To reduce cost, eliminate
the 4-way tee and use straight tee and elbows instead. Students will have to get
creative…which is good!
** Or these materials can be found in the ROV kit sent from USC. 
What to do
WATER BOTTLE – FLOAT, HOVER or SINK?
1. Prepare some dishpan sized tubs with water (number depends on number of students)
2. Assign small groups to tubs (2-3 students in a group)
3. Pass out the Student Worksheets
4. Give students empty water bottles, measuring cup that will hold ~50 ml of water,
funnel (if needed), chart
5. Have students complete the questions 1-3 on the worksheet
6. Students begin experiment by putting in the empty bottle and recording what they see
in the first row of the worksheet. Have them circle FLOAT, HOVER or SINK and
draw what they see (where the water bottle is relative to the water level.)
7. Add water to the bottle and students record what they see in the second row of the
worksheet.
8. Repeat step 7 until the bottle is full.
9. When bottle is full of water, students complete final questions on page 3 of the
worksheet.
ROVs
1. Pass out ROV model kits to students
2. Have the students put the PVC pipes in the water and record their observations in row
1 of the ROV worksheet
3. Have the students put the floats in the water and record their observations in row 2 of
the worksheet.
4. Have students assemble PVC pipes however they would like to create an ROV.
(Pictures of possible ROV configurations can be found below.)
a. Optional discussion: Show all ROVs to class and have students talk about
their ROV
b. Why did they design it that way
c. What challenges did they have? What was easy?
Here is an example of one configuration for the ROV.
5. Have students put them in the water and record their observations in row 3 of the
worksheet.
a. Why did the ROV sink?
b. What should they add to the ROV to make it float?
6. Have the students add 1 float around a PVC pipe of the ROV. What happened? Why
is it lopsided?
7. Have the students add another float on the opposite side of the first (to make the
floatation evenly distributed. Put it in the water again and record their observations in
row 4 of the worksheet.
8. Have the students add 2 more floats (now you have 1 float on each of the top pipes),
put it in the water and record their observations in row 5.
9. Have the students add the last 2 floats (you may have to double up on some pipes),
put it in the water and record their observations in row 6.
Discussion
Use the figures on the next page to describe negative, neutral and positive buoyancy.
Buoyancy means the water underneath lifts up or pushes up on the object
 If the object is too heavy for the pressure, it sinks (negative buoyancy)
 If the object is really light it floats on top of the water (positive buoyancy)
 If the object is not too heavy or too light, it hovers in the water like a submarine
(neutral buoyancy)
Compare the pictures of the Water bottles and ROVs from the students’ worksheets to the
figure on negative, neutral and positive buoyancy. Have the students decide what
buoyancy they observed in their experiments.
Extension
How could you make the ROV sink then float again?
Scientists want to make the ROV sink to the ocean floor then hover around the study site
then at the end of the dive, make the ROV float to the surface again. How do they do
that? Well, they have weights AND floats on the ROV. The weights help the ROV sink to
the bottom. They drop a few weights to help it hover. Then at the end of the dive, they
drop all their weights and the floats on the ROV help it float to the surface.
Pictures indicating Types of Buoyancy
Examples of ROV configurations