Who Sank the Boat?
Inquiry activity based on the book by Pamela Allen
Created by Shawn Reintjes, The Science House, NCSU

 Who Sank the Boat? Followup Activity - Buoyancy

Buoyancy Followup Activity

The scientific phenomenon you just explored is called “Buoyancy”. Buoyancy is defined as: the force exerted by a fluid, like water, in an upward direction. In principle, it states that an object will float if its density is less than that of the fluid, and will sink if greater. Every liquid exerts an upward force on objects immersed in it. This upward force is called buoyant force. This phenomenon is called buoyancy. The Archimedes Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object. To further understand what buoyancy is and how it works. You can conduct the following experiment:

Buoyancy

When an object is immersed in water, it pushes water aside. The buoyant force of water on the object reduces the weight of the object. In this experiment, you will determine and compare the buoyant force on an object and the weight of the water pushed aside (displaced) by the object for three objects. The Greek mathematician Archimedes first studied this relationship during the third century B.C.

Objectives

In this experiment, you will

·   Use a Force Sensor to measure the weights of objects in and out of water.

·   Determine the buoyant force of water on each object.

·   Determine the weight of water displaced by each object.

·   Compare buoyant force to weight of water displaced for each object.

·   Use the results of the experiment to explain why objects sink or float.

Materials

Computer

String

Vernier computer interface

Metal mass with a hook

Vernier Force Sensor

50 g piece of clay

 

Metal rod included with Force Sensor

Wooden pencil

Ring stand

100 mL graduated cylinder

Right-angle clamp

Water

Procedure

  1. Obtain a metal mass, a 50 g piece of clay, and a pencil. Roll the piece of clay into a hot-dog shape that will fit into the 100 mL graduated cylinder.
  2. Connect the sensor to the computer interface. Set the range switch on the Dual-Range Force Sensor to 10 N. Start the Vernier data-collection program and open the file “32 Buoyancy” from the Middle School Science with Vernier folder.
  3. Zero the Force Sensor.
    1. Fasten the Force Sensor to a ring stand as shown in Figure 1.
    2. Tie a piece of string to the hook of the Force Sensor.
    3. Click Zero to zero the sensor.
  4. Measure the weight of the object in air.
    1. Attach the metal mass to the string on the Force Sensor.
    2. When the readings stop changing, record the weight in the data table.
  5. Collect data in water.
    1. Fill the graduated cylinder with enough water to cover the object.
    2. Read and record the volume of the water alone (to the nearest whole mL).
    3. Lower the object into the graduated cylinder.
    4. If the object sinks, lower it until it is covered with water but not touching the bottom. If the object floats, let it float.
    5. Record the weight of the object in water.
    6. Read and record the volume of the water plus object.
  6. Repeat Steps 4–5 for the clay and the pencil. Change the amount of water, if necessary.

Data

Object

Metal

Clay

Pencil

Weight in air (N)

 

 

 

Weight in water (N)

 

 

 

Volume of water alone (mL)

 

 

 

Volume of water + object (mL)

 

 

 

Processing the Data

  1. Find the buoyant force on each object (weight lost). Subtract the weight in water from the weight in air. Show your work here and record the results in the table in Question 3.
  2. Find the volume of water displaced by each object. Subtract the volume of water alone from the volume of the water + object. Show your work here and record the results in the table in Question 3.
  3. Find the weight of water displaced by each object. Multiply the volume of water displaced by 0.01. This will give you an approximate value for the weight of the water in Newtons. Show your work here and record the results in the table below.

Object

Metal

Clay

Pencil

Buoyant force (weight lost) (N)

 

 

 

Volume of water displaced (mL)

 

 

 

Weight of water displaced (N)

 

 

 
  1. How does buoyant force compare to the weight of water displaced for each object? Note: The answer to this question is known as Archimedes’ Principle.
  2. For which objects was buoyant force less than the weight in air? For which objects was buoyant force equal to the weight in air?
  3. Explain how buoyant force determines whether an object sinks or floats in water.

Extensions

1.   Repeat the experiment using saltwater. Compare your saltwater results with the results of your first experiment.

2.   Investigate why boats float when made of materials that would normally sink. Shape and float a boat made from your clay. Measure the volume of water that it can hold. Compare this volume to volume it displaced as a lump of clay. Explain your results.

3.   Have a contest with your classmates to see whose clay boat, described in Extension 2, can hold the most mass without sinking.

 

For further explanations go to http://www.pbs.org/wgbh/nova/lasalle/buoybasics.html

To try your hand at some buoyancy brainteasers http://www.pbs.org/wgbh/nova/lasalle/buoyancy.html

Also go to Nova’s site “The Voyage of Doom” to explore the mysteries of shipwrecks http://www.pbs.org/wgbh/nova/lasalle/

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