PHYS 242

Buoyancy Lab

1. Introduction

In this laboratory, we will study the phenomenon of buoyancy and measure the buoyant force on an

object by a liquid. When an object displaced in contact with any fluid (liquid or gas), there are forces

exerted by the fluid on the object’s surfaces. (These forces are related to the pressure at the surface.)

We describe the summation of these forces as the buoyant force. The buoyant force is an upward contact force whose magnitude is related to the volume of the object and the density of the surrounding

fluid.

There are many aspects of the buoyant force that are confusing for students. First, the formally defined

buoyant force is often confused with the more qualitative and intuitive notion of buoyancy. Consider

two cubes of the same size but di↵erent mass. We often refer to the less dense cube as being ‘more

buoyant,’ but it turns out that the two cubes will displace the same volume of liquid. Thus, the buoyant

forces on the two cubes are actually equal.

Archimedes’ principle states that the magnitude of the buoyant force on an object is equal to the

weight of the fluid displaced by the object. This principle is often misinterpreted as stating that the

buoyant force is related to the weight of an object, but the weight of displaced fluid depends only on

the volume of the object, the density of the fluid, and the strength of the gravitational field.

Lab Objectives: after completing this lab, students should be able to:

1. Describe and use a procedure for measuring the volume of liquid displaced by an object.

2. Describe and use a procedure for determining the magnitude of the buoyant force on an object

that would sink.

3. Describe and use a procedure for determining the magnitude of the buoyant force on an object

that would float.

4. Predict which quantities do and do not a↵ect the magnitude of the buoyant force. Physics homework help.

Equipment

For this lab, you will be using the following simulation:

https://iwant2study.org/lookangejss/02_newtonianmechanics_4massweightdensity/ejss_model_

buoyancycase/buoyancycase_Simulation.xhtml

We will also be using various videos, the links for which are on iLearn.

Here are two related simulations, which we will not use for this lab, but you might find interesting:

• https://amrita.olabs.edu.in/?sub=1&brch=1&sim=72&cnt=4

• https://go-lab.gw.utwente.nl/production/splash/build/splash.html?preview=

(C) 2020 SFSU. Adapted from Copyright 2005 by S.E. Kanim, M.E. Loverude, and L.G. Ortiz. DO NOT DISTRIBUTE. 1

PHYS 242 Buoyancy

2. Measuring Displaced Volume

In this lab we will be using a buoyancy simulation from iwant2study.org, as well as videos of the Phet

Simulation on Buoyancy. In many of the experiments in this lab you will be asked to make predictions

and to check those predictions with the simulation. In each case, write down both your initial prediction

and your observations. Resolve any inconsistency before proceeding to the next part.

(a) Displacement and mass

i. Open the simulation at:

https://iwant2study.org/lookangejss/02_newtonianmechanics_4massweightdensity/

ejss_model_buoyancycase/buoyancycase_Simulation.xhtml

ii. Set ⇢1 (the density of the liquid) = 1.0 We’ll call this liquid “water.”

iii. Set ⇢2 (the density of the block) = 1.5

iv. Set the height of the block so that it is that the maximum height above the liquid.

v. Click on the depth tab and set the depth of the liquid so that it is just below the bottom of

the block when it is at maximum height. We want the block to be able to be fully submerged

in the liquid when we lower the height later. See the picture below for reference.

vi. Which of these scales currently tells you the weight of the block (top or bottom)? What is

the weight of the block? Assume the units are Newtons.

(b) Methods of determining volume

i. We will call our ⇢2 = 1.5 block “Block A.” Now lower the block to its minimum height. It

should be nearly submerged. What happens to the water level? Use the faint gray horizontal

line as a reference point.

(C) 2020 SFSU. Adapted from Copyright 2005 by S.E. Kanim, M.E. Loverude, and L.G. Ortiz. DO NOT DISTRIBUTE. 2

PHYS 242 Buoyancy

ii. Now consider a block with a density of 2.5. Let’s call this block “Block B.” Raise the height

of the block so it is once again out of the water. Set ⇢2 = 2.5. What is the weight of this

block? Has the volume of the block changed?

iii. PREDICT: If Block B is submerged in water, will the final water level be greater than, less

than, or equal to the final water level when Block A was submerged?

iv. Check your prediction by lowering Block B into the water until it is submerged. Physics homework help. Describe

what happened. Was your prediction correct?

v. PREDICT: How would the volumes of water displaced compare if a single block were lowered

to di↵erent depths?

vi. To test our prediction, watch the video “1 – Block at Di↵erent Depths” on iLearn, in which a

block is submerged at di↵erent depths. For now, don’t worry about the buoyancy force, just

pay attention to the changing water level. What do you notice? Was your prediction correct?

vii. The di↵erence in the initial and final water level reading is referred to as water displaced.

What is the value of the volume of water displaced by the block in the video when it is fully

submerged?

(C) 2020 SFSU. Adapted from Copyright 2005 by S.E. Kanim, M.E. Loverude, and L.G. Ortiz. DO NOT DISTRIBUTE. 3

PHYS 242 Buoyancy

(c) Summary of displaced volume

i. When the block is completely submerged, how does the volume of the block compare to the

volume of liquid displaced? The volume of the block can be found in the upper left-hand side

of the screen in the video.

ii. What determines the volume of liquid displaced by a submerged object? (Does it depend on

the object’s mass, its volume, or its depth?)

iii. PREDICT: How would the volumes of liquids displaced compare if you submerged one aluminum block in water and an identical aluminum block in a liquid with a di↵erent density

than water (e.g oil)? Explain.

iv. Test this by watching “2 – Block in Di↵erent Liquids” on iLearn, in which an aluminum block

is submerged in di↵erent liquids. Once again, ignore the buoyant force for now and just focus

on the displacement of the liquid. Was the fluid displaced di↵erent as a result of the changing

fluid? Was your prediction correct? Physics homework help.

3. Determining the magnitude of the buoyant force

(a) Consider two identical blocks hanging from a scale, one submerged in water and one in air.

(C) 2020 SFSU. Adapted from Copyright 2005 by S.E. Kanim, M.E. Loverude, and L.G. Ortiz. DO NOT DISTRIBUTE. 4

PHYS 242 Buoyancy

i. PREDICT how the scale readings would compare. Is the reading of the scale supporting the

block in water greater than, less than, or equal to the reading of the scale supporting the

block in air?

ii. Check your prediction using the simulation. Change ⇢2 back to 1.5 and ⇢1 to 1.0. Check

the reading on the top scale when the block is in air and when it is submerged. Were your

predictions correct?

iii. What does your result imply about the direction of the force exerted on the block by water?

The force exerted by the water is known as the buoyant force. There is a buoyant force exerted

by the air as well, but this force is small enough that it is typically ignored. In some cases, like a

hot air balloon, the buoyant force of the air cannot be neglected. For the remainder of this lab,

ignore buoyant forces by the air unless advised otherwise.

(b) Draw a free-body diagram for each block in section 3(a). (Use the notation B for the buoyant

force.)

Check your free-body diagram with your instructor before continuing.

(c) Use your free-body diagram and Newton’s second law for an object at rest to write an equation

that relates the weight of the block to the other forces. (Hint: what is the net force acting on

each block?)

i. Block submerged in water

(C) 2020 SFSU. Adapted from Copyright 2005 by S.E. Kanim, M.E. Loverude, and L.G. Ortiz. DO NOT DISTRIBUTE. 5

PHYS 242 Buoyancy

ii. Block in air

iii. If the reading on the scale for the submerged block is T1 and the reading on the scale of the

block in air is T2, write an equation for B in terms of T1 and T2.

iv. The equation in the previous question suggests a procedure for determining the magnitude

of the buoyant force on a completely submerged object. By measuring the scale reading (the

tension force) on the block when it is in water and when it is in air, you can simply determine

the magnitude of the buoyant force. Briefly describe the procedure you will follow.

v. Try out your equation with our Block A (⇢2 = 1.5).

Check your results with your instructor before you proceed.

4. Variables that influence the buoyant force

(a) Mass

i. PREDICT how the buoyant forces would compare for two blocks of di↵erent mass but equal

volume submerged in water.

ii. Check your prediction using the video “3 – Di↵erent Mass” on iLearn. The magnitude of the

buoyant force is given by the size of the pink arrow. Was your prediction correct?

(C) 2020 SFSU. Adapted from Copyright 2005 by S.E. Kanim, M.E. Loverude, and L.G. Ortiz. DO NOT DISTRIBUTE. 6

PHYS 242 Buoyancy

(b) Volume

i. PREDICT how the buoyant forces would compare for two objects of di↵erent volume but

equal mass submerged in water.

ii. Check your prediction using the video “4 – Di↵erent Volumes” on iLearn. Was your prediction

correct?

**ORDER A PLAGIARISM-FREE PAPER NOW**

(c) Depth

i. PREDICT how the buoyant force would compare for two identical blocks submerged at different depths (e.g. near the surface and near but not touching the bottom).

ii. Check your prediction using the video “1 – Block at Di↵erent Depths” on iLearn. For this

question, ignore the box that is only partially submerged.

(d) Density of object

i. PREDICT how the buoyant forces would compare for two objects of the same density but

di↵erent size. Physics homework help.

ii. Check your prediction using the video “5 – Same Density” on iLearn. Was your prediction

correct?

(e) Density of fluid

i. Honey has a density greater than that of water. PREDICT whether the buoyant force on an

object submerged in honey is greater than, less than, or equal to the buoyant force on the

same object submerged in water.

(C) 2020 SFSU. Adapted from Copyright 2005 by S.E. Kanim, M.E. Loverude, and L.G. Ortiz. DO NOT DISTRIBUTE. 7

PHYS 242 Buoyancy

ii. Check your prediction using “2 – Block in Di↵erent Liquids” on iLearn. Was your prediction

correct?

(f) Summarize your results from this section by identifying the factors that influence the magnitude

of the buoyant force exerted on a submerged object by a fluid. (Recall that you tested mass,

volume, depth, density of object, and density of fluid.)

5. Measuring the buoyant force on an object that sinks

(a) In this section, we’ll be looking at the buoyant force on small (3 L), medium (6 L), and large (9

L) aluminum blocks. For each block:

i. Fill in the columns for block mass, weight, and buoyant force in the table. You can find

the mass and buoyant force using the video “6 – 3 Sizes” on iLearn. (Mass is in the upper

left-hand corner of the screen).

ii. Determine the volume (using the video) and weight of water displaced by the block when

completely submerged. (Hint: the density of water is 1 g/cm3

). Fill in the remainder of the

table below. Physics homework help.

Block Water Displaced

Volume (L) Mass (kg) Weight (N) Buoyant Force

(N) Volume (L) Weight (N)

3 L

6 L

9 L

(b) Generalize your results by answering the following questions about objects that sink:

i. Is the volume of water displaced by a submerged object greater then, less than, or equal to

the object’s own volume