PHY 2048L
Lab 10

Use of the Buoyancy Apparatus

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Updated: 1/21/2013
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This is a somewhat modified version of Lab 18. We will only do parts 1 and 2 of the procedure on pages 191 and 192, omitting the part with an unknown liquid. However, we will extend part 1 by measuring the specific gravity of a total of 4 metal objects, so you will be given a different data table to use in this lab.

Warning
Be careful about spilling water, which can be a major safety hazard.
There is a mop to clean up spills.

Guiding Questions:

Note that "your" piece of wood has an identifying letter on it that must be part of a complete answer to this question in your abstract.

Preparing for the lab:

Read the theory for "Lab 18" (pages 189 to 191) in the lab manual before answering the questions on LON-CAPA.


Comments

Be sure to look at the section below about tying knots!

We will not do Part 3 of this lab. Instead, we will do 4 different metals in Part 1 (Aluminum, steel, Copper, and Brass) and enter those additional data in the space set aside for Part 3. Here is a link to a pdf file showing a hand-drawn version of the changes in the data sheet.

Setting up the Balance

The trickiest part of the experiment is how to measure the mass of an object while it is submerged in a beaker of water, since you can't put the beaker on the pan of the balance or the pan in the beaker, and you also cannot remove the pan of the balance without changing the calibration of the balance.

As you can see below, there is a simple solution available to us: an adjustable support platform you may not even know was there.

support in place above the pan of the balance

All you need to do is reposition the support platform above the pan, as shown in the picture. Be very sure it is level and secure before putting a full beaker on it. The balance should be set up this way when you arrive, and should be left this way when you leave.


Suspending the Mass

You use a piece of nylon string to hang the mass from the hook on the balance so it is completely submerged in the water and not touching the sides of the beaker. The two pictures below (taken during the lab in Spring 2002) show the experiment in progress as done by two different groups. There are some subtle differences between the two that you might look for, but the basics are the same.

rod suspended in beaker by group A rod suspended in beaker by group B

The rod can be tied on with either a slip knot or a fisherman's knot. Since you have to measure several rods, a slip knot works best. The loop at the top is made by taking a loop of string and tying an overhand knot in it.

Some hints: (1) start with a piece of string that is longer than you think you need, since it is hard to cut a string to make it longer, but easy to make it shorter; (2) tie the top loop last, since it is easier to adjust to get the mass at the right depth; (3) notice that there are two hooks on the beam of the balance [look at the two pictures again] and that this gives you an additional adjustment for the depth the masses are submerged.

 
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