# Project #14167 - physics lab

WRITTEN LAB ASSIGNMENT – Buoyancy

Name: _____________________________________                               Date: ____________

Student ID #: ________________

Conceptual Physics: Hewitt/Baird

Fluids Buoyancy and Floating

Pool Cubes 2: Buoyancy

Purpose

To investigate the nature of the buoyant force and to see the role it plays in determining whether or not an object floats

Apparatus

__computer __PhET sim, “Buoyancy” (available at http://phet.colorado.edu/en/simulation/buoyancy)

Discussion

When objects are immersed in a fluid, the fluid exerts a force on them. This is the buoyant force.

Procedure

INITIAL SETUP

Turn the computer on and let it complete its startup cycle. Locate and open the PhET sim, “Buoyancy,” and note that it opens in its “Intro” tab. Do not change the settings in the on-screen Blocks, Fluids, or Show Forces panels.

PART A: WOOD

Step 1: Measure (in the sim) and record the weight (W) of the 5.00-kg wood block.

Step 2:

a. Place the wood block in the water and notice that it floats.

b. When the wood block rests on the ground, the downward gravitational force is balanced by an upward normal (“contact”) force. When floating, the gravitational force is still there, but the normal

(“contact”) force is not. The force exerted by the fluid is the buoyant force (B). When floating, which force is greater: the gravitational force or the buoyant force? Defend your answer.

c. Record the value of the buoyant force acting on the wood block.

Step 3:

a. What volume of water does the wood block displace when it floats in the water?

b. What is the mass of that displaced water? (Note that the density of water is 1.00 kg/L.)

c. What is the weight of the displaced water? (g = 9.8 m/s2)

PART B: BRICK

Step 1:

Measure and record the weight (W) of the 5.00-kg block of brick (in air).

Step 2:

a. Measure and record the apparent weight (N) of the brick when submerged in water.

b. Does the brick appear to be ___heavier or ___lighter when submerged in water?

c. When submerged in water, the brick appears to have an additional force pushing it ___upward ___downward. This additional force is the buoyant force (B).

Step 3:

a. Draw a free-body diagram of the brick block at rest on the bottom of the pool to show how the weight, apparent weight, and buoyant force act on the block.

b. Calculate the size of the buoyant force. Show your work.

Step 4: What is the volume of the brick block? (Observe; do not calculate!)

Step 5: What is the volume, mass, and weight of the displaced water?

Summing Up

1. How does the buoyant force acting on an object compare to the weight of the water displaced by that object?

2. The wood floated; the brick did not. What condition—in terms of forces—is required for an object to float?

3. What is the condition for floating in terms of object and fluid densities?

Going Further

1. Fluid Density. Replace the water with oil (using the on-screen Fluids panel). Describe and execute a method to determine the density of the oil. (g = 9.8 m/s2)

2. Latent Bloomer. Switch the sim to the Buoyancy Playground tab. Select My Block from the on-screen control panel.

a. What is the density of the densest block you can create?

b.       i. Place that block in the water. It __floats __sinks __is neutrally buoyant.

ii. Now maximize its volume. It __floats __sinks __is neutrally buoyant.

iii. While at maximum volume, minimize its mass. It __floats __sinks __is neutrally buoyant.

c. Without dragging the low-density block or changing its mass or volume, how can you bring it to rest at the bottom of the pool?

3. Sim Surfing. Click the on-screen Reset All button. Select Two from the on-screen Blocks panel. Set

Block A to be an 8-L block of Styrofoam; set Block B to be a 2-L block of brick. Place the Styrofoam block in the water. Place the brick block on top of the Styrofoam block. The arrangement floats.

a. What is the mass of the stack?

b. What is the volume of the water displaced?

c. What is the mass of the water displaced?

d. Change the volume of the Styrofoam block to 6 L, 4 L and 2 L. What do you observe during this process?

e. Adjust the volume of the Styrofoam block until the stack has neutral buoyancy. Record the volume of

the block.

4. Describe, in one paragraph, your experiment.  A good summary explains succinctly (i) what you did, (ii) how you did it, and (iii) what you found out.

 Subject Science Due By (Pacific Time) 10/09/2013 12:00 am
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