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How-To: Teach Elementary Science

Finding the Volume of an Irregularly Shaped Object Using the Displacement Method
Natasha Cooke

Volume is a property of matter that requires accurate measurement.  Students are often confused when you have to find the volume of an object that does not have a defined shape.  There are formulas to find the volume of a square, rectangle, cylinder, etc. But how do you find the volume of a ring? A key? Paper clips? These are all irregularly shaped solid objects.  One easy way to find the volume also involves some simple subtraction- the displacement method. Using a known amount of water and adding the object, you can subtract the starting volume from the ending volume to determine the volume of water displaced (the amount of water that moved up when the object was placed in a beaker). Using the sample below as a guide, have your students find some small objects capable of fitting in a beaker, graduated cylinder, or measuring cup.

Directions: Using the following materials: beaker/graduated cylinder/measuring cup, 2 marbles (or any small object/s), and water, follow the instructions below:

Sample problem: What is the volume of 2 marbles?

1) Add 50 ml of water to the beaker (choose any starting volume). The volume of the water is 50ml.

2) Put 2 marbles into the beaker.

►Record the ending volume of the water + two marbles here:      54   mL

3) What is the volume of the 2 marbles?  ►  4     mL

54 ml
-  50 ml
=  4 ml

 Starting Water Volume Ending Volume of Water + Object/s Ending Volume - Starting Volume Volume of Object/s 50ml 54 ml 54 ml -50 ml 4 ml 50ml 50ml 50ml 50ml 50ml

You can use this simple exercise to begin a unit on measurement. If you do, let me know how it works out.