## 2 ways to find the spring constant

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## More Uncertainty

Here is an updated video on uncertainty.  I included the answer to the quiz.

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## Uncertainty stuff

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## Projectile Motion

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## Friction Data

Since there were some students that weren’t able to collect data on the coefficient of friction, I decided to do it for you.  Here you go.

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## Lab Report Guidelines

September 5, 2019 1 comment

The first formal lab report is due at the end of the midterm exam.  I’m flexible on the actual format of the report, but I find that students like more guidance.  In general, you want to present your physics model along with the data and analysis that supports it.  Here are some sections that you could include.

Introduction

• This should be a one or two paragraph section.
• What is the main physics model that you are investigating (building)?
• It’s ok to describe the outcome.  Don’t worry about spoilers.  Just give a very brief overview of the whole report.

Data and Methods

• Write this as though you were giving details to another student in your lab such that they could reproduce your data.
• You don’t need to include “gather materials section”.  You don’t need to describe any equipment unless it is very different from other students in your lab.
• If your setup is important, you might want to draw a picture.  Don’t steal pictures from the internet.  That’s bad.
• For the data, you don’t have to show ALL your raw data – but you can if you think it’s important.
• You are probably going to want a graph.
• Don’t forget the units.

Model and Analysis

• Here is where you analyze the data and build a model.
• Maybe you already know what the model should be—that’s fine, you still should justify it with data.
• Show your calculations with enough detail that another student in lab could follow them.

Conclusion

• Do not say “I learn a lot”.
• Do not use the phrase “human error”.
• You should discuss the uncertainty in your data.
• Use your model in some other calculation.  See the questions at the end of the lab page.

Other notes and reminders

• If you copy or modify someone else’s lab report, that is cheating.  You will get a zero and an academic integrity report.  Don’t cheat.
• Oh, but you worked with another student—that’s fine.  You might have the same data, but not the same words.  Trust me, it’s obvious when you cheat.  Don’t cheat.
• Do not turn in hand written lab reports.  You can hand write the equations—that’s fine.
• Feel free to staple a graph in there.
• Don’t use stuff from online.
• Don’t make it long just because you think that’s better.
• Here is a sample lab report. https://plab193.wordpress.com/2018/01/31/lab-report-constant-velocity/
• There are no late labs.  If you don’t turn it in on time, it counts as a zero. You can turn it in early though.
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## Practice with graphs

Here is an example of plotting mass vs. volume to find the density.

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## Sample Lab Report

Here is a sample lab report so you can see what kinds of things to put in there.

https://plab193.wordpress.com/2018/01/31/lab-report-constant-velocity/

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## Don’t Forget About Lab Reports

Just some quick reminders.

• The week after Thanksgiving is the final exam (during the last week of class).  Labs don’t get an exam time during finals week.
• The final exam will cover everything in the semester (not just the second half).
• Your second formal lab report is due at the final exam (but you can use it on the final).  THERE ARE NO LATE LAB REPORTS ALLOWED. You can turn it in early, but not late.
• Double reminder: you can’t copy other people’s lab reports.  Please don’t do that—it’s just plain cheating.
• Notes on lab reports (from before).
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## Lab: Buoyancy

Suppose you place a mass on a spring scale and then partially lower it into a fluid.  If the object is at rest, there are three forces acting on it: the gravitational force, the force from the spring and the buoyancy force.

In the y-direction, the forces add up as:

$F_s +F_b -mg=0$

We can find the weight fairly easily (and it doesn’t change).  Also, the scale force can be found by reading the value of the spring scale.  That means we can calculate the buoyancy force from those two values.

As you dip the object into the fluid, the buoyancy force should increase.  According to Archimedes Principle, the buoyancy force can also be calculated from the volume of the object below water and the density of the fluid.

$F_b = \rho V g =\rho h A g$

In this case, the Greek letter rho is the density of the fluid.  Water has a density of about 1000 kg/m3. The submerged volume can be found as the product of the depth and the cross sectional area.

So, here’s what to do:

• Take a cylinder. Measure the length and diameter so that you can calculate the cross sectional area and the volume.  Use meters.
• Hang the cylinder with the scale – measure the mass and weight.
• Dip the object in a fluid.  Measure the depth and the spring scale reading.
• Repeat for many depths
• Make a linear plot using the depth and the spring scale reading.  Find the slope.

For fun: Suppose you have a beaker of water on a balance.  What happens to the scale reading as you partially submerge an object in the water?

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