PHY 2048L
Lab 4

Conservation of Spring and
Gravitational Potential Energy

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Updated: 8/23/2008
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Prelab

Answer all 6 of the questions on pages 131 and 132 of the lab manual. Note that question 5 requires the solution of a quadratic equation, so allow yourself enough time to do this and be sure to attach any worksheet you might require if you can't fit it in the space provided.

(Hint: Read the hint given in the problem.
You have a built-in check since you already know one of the two solutions. If your result from the quadratic formula does not include that one, you have made an error.)

Comment

You should use the adjustments available in the setup to position the meter stick so it is easy to read the length of the spring, particularly when it is fully extended and/or in motion. You might make some trial runs with the 500 g mass to see how it will work in various configurations. Be particularly careful to be sure you are using a point on the bottom of the spring that can be read consistently when the spring is stretched. See the picture at the bottom of this page.

There are various tricks that can be used to make an accurate measurement of position while the mass is in motion. All of them require repeated measurements for the same initial conditions, so care in setting the starting point the same way every time is very important. One trick is to use your finger as a placeholder near where you think the end of the spring stops while making repeated measurements. Anticipating where it will stop helps guide your eye and makes it easier to fine-tune your reading. Another, helpful when dropping the mass, is to use a horizontal ruler to locate where the bottom of the mass stops. Once sure of that, you can pull the mass down to this position and read where the bottom of the spring is located.

Apparatus

The photo below shows the apparatus as it should look when you arrive. When you are done with the lab, you should put it back the way you found it so then next group will also find it looking like this.

spring and meter stick support system

Each of the clamps is adjustable, and you should use them to position the various components so you can make your readings more easily. You can rotate them so everything is further from the edge of the table, and you can adjust the relative position of the spring and the meter stick. You might want the spring in front of the meter stick, or you might want it right next to the edge of the meter stick, depending on how you are going to make your readings. Try a few readings, adjust things so they work for you, and then start your actual lab measurements.

Procedure

The balance is provided so you can check the masses of the masses you will use. They are not all the same, with many being a bit light due to paint chipping off when mishandled. Ignoring these small differences can introduce a significant percentage error for the lighter masses you use and bias your results.

The photo below should give you some idea of the issues you will face when making a measurement. Here you see a fairly small mass on the spring, and I have added some lines in blue to help identify the key features in the photo without obscuring the spring itself. See below.

sample reading of the spring position

The very thin blue line near the center of the photo is one pixel below the bottom of the lowest point of the spring. It is used as a reference on the left and right where a thicker blue line has been aligned with the bottom of the spring where the reading should be made. It looks like this spring is at 10.5 cm. Notice that the reading is made at the bottom of the spring coil and not at the loop used to hang the mass or the bottom of the mass. I think this is the best location to use for reproducible measurements.

 
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