IB Angular Momentum Lab

Model 1 - Constant Velocity Model (CV): (Proposed by John Mandlbaur)

The velocity remains constant.   Any change in velocity is illusory.  (i.e. It is still going the same speed when it is pulled in, but it has to go less distance per revolution, so it just looks like it is going faster, but it's not)

This model predicts that:
v1 = v2
Regardless of change in radius.  (i.e. Kinetic Energy is conserved)

Model 2 - Conservation of Angular Momentum (COAM): (Proposed by Sir Isaac Newton)

The angular momentum remains constant.  L = Iw:
so
I1w1 = I2w2
(mr2)(v/r) = (mr2)(v/r)
and finally
r1v1 = r2v2
So if you reduce the radius the velocity will increase.   (If you cut the radius in half when you pull the mass in, the velocity will double)

These models have very different predictions if you pull a mass in.  (The rubber stopper cannot both remain at a constant velocity, and also speed up)  We are going to use the Vernier Video Analysis App to test these two hypotheses.  Since both models have testable predictions, they are scientific models.  

Directions for getting data:

1. You and a partner  (Groups of three need to analyze two videos) set up a tripod (Or hold the phone suuuper steady with no side to side motion), one person filming, one person spinning the stopper.  Put the phone in landscape, and get close enough that the experiment takes up like 80% of the frame.  Film against a white background.  Visible on the wall behind you should be some sort of scale.  You could use two pieces of tape a meter apart, or if you are up against the whiteboard, we can measure the height of the whiteboard.

2. Wait until the person spinning the stopper has a period of just over a second per revolution, steady and level, and a radius of  just over a meter. Start filming and get 2 or 3 revolutions at the big radius, and then very quickly pull the stopper in to about a half a meter radius.  Pull it in just after it is either to your left or right, and quickly enough that you stop pulling it in by the time it is on the other side.  Film a couple more revolutions, and stop.  Try to keep the center as steady as possible right before and right after pulling it in.   Repeat this if you are a group of three.  Before you proceed, make sure that :

   1. 1. 1. 1. 1. You pulled the stopper in very quickly when it was either in front of you or behind you.

               2. You kept the center very steady

               3. The stopper was always in the frame

3. Upload the video to your Google drive, and open it with the Vernier Video Analysis App.  Set the scale (System) using your length reference.  Video Steps 3-7 .:. Key Link

4. Play the video through once, and find the part where you pull the stopper in.  

5. Back the video up to the half revolution before you pull the stopper in and using the "Add" tool, click on the stopper when the stopper is at one side.  Advance frame by frame, and about halfway between the one side and the other, click the center of rotation (the tube in your hands), and then advance frame by frame until the stopper is on the other side, and click again on the stopper.  We now have data on the radius and period before you pulled it in.

6. Keep advancing frame by frame until you are done pulling it in, and then do the same three clicks for the half period after, clicking on the stopper at one side, then on the tube in your hands, and then on the stopper on the other side after you have pulled it in.  Now we have data about the radius and period after pulling it in.

7. Screenshot the Video Analysis screen into the attached document where it says to.  Make sure you get the numbers from the data table into the screenshot.  We just need the time and the x values, so you can crop out the y values:

Directions for Calculating the Velocity before and after:

1. We will use the half period before pulling in the mass, and the half period right after to calculate the velocity right before and right after pulling in the stopper.  Generally, the velocity is 

   1. 1. 1. v = (pi)(r)/(half period)

2. That is, it has gone half way around (pi)(r) in the half period.

3. To get the half period before, subtract the time of the first click (when the stopper was at one side) from the time value for when it was at the other side (the third click).  The uncertainty for this is just the time between frames.  For normal video, this is 1/30th of a second, but less if you did slo mo.  Look it up.

4. To get the radius before, and its uncertainty, subtract the x value on one side before, from the x value of the tube (the center) in your hands before, and do the same with the center and the x value on the other side before.  You now have two radii.  Their average is what you use for the before radius.  Their difference divided by two is the uncertainty of the radius before.

5. Do the same calculations for the after half rotation period and radius.

6. Use v = (pi)(r)/(half period) to calculate the velocity before and after, and use your uncertainty calculation for multiplication or division to find the uncertainty of these values.

7. Show all these calculations.  Video Steps 3-6

Your completed lab must include:

• A screenshot of your video analysis

• Your calculated velocities before and after pulling in the mass (See the video)

• A properly calculated uncertainty of these velocities. (See the video)

• A conclusion. You will need to cite your calculations, and make a logical mathematical argument as to whether the Mandlbaur model (that velocity before = velocity after) is supported or refuted.  Avoid claiming that something is proven.  (All we can do is prove a model wrong)  You don't need to cite sources of error or suggest improvements.

E.C.  - We are really just trying to test the Mandlbaur model for this.  (i.e. the velocity either is or is not the same before and after).  For extra credit, calculate the value of (Radius)x(Velocity) before and after.  Also calculate the uncertainty of these values.  Sir Isaac Newton's model of Conservation of Angular Momentum says these should be the same.  Write a sentence or two about these calculations citing data.  Do they lie within their uncertainty of each other?  Show your calculations.

Remember - we only disprove models or hypotheses in science.  Results can support a particular model or not.  

Old Stuff

Using a PC in the back of the room:

• How to use LoggerPro to get radius and period from a video 

• How to do the calculations

@TheAnimammal

4 years ago

 M. van Heerden  The “chase” you are cutting to is exactly what I have been describing. It is an appeal to tradition logical fallacy. You are not addressing my argument logically, you are merely contradicting my conclusion which is a formal logic fallacy. It is illogical to contradict the conclusion of a logical argument.

To address a logical argument, it is necessary to show false premisses or show illogic or accept the conclusion drawn. You have not shown false premiss, you have not shown illogic. Therefore you must accept the conclusion drawn, otherwise you are evading my argument.

Your suggestion that the extra energy exerted by the professor will correlate with the change in (angular) energy predicted is simply false. It is an unconfirmed hypothesis. There is no scientific experiment which directly confirms that angular momentum is conserved in a variable radii system. ie: You are making unsupported claims.

There is however empirical evidence which confirms my claims exactly. If we are to conserve angular energy then the perpendicular component of velocity will remain constant in magnitude. We can then apply the equation W=v/r to the circular motion before and to the circular motion after the radius change. So my prediction when we halve the radius is that W will double as opposed to the current physics prediction that W will quadruple. W=v/r, so halving the magnitude of the radius and keeping the velocity constant in magnitude will result in doubling the angular velocity.

I assure you that any conservative measurement that we make of any reasonable ball on a string demonstration will result in a two fold increase to W if we are to reduce the radius to half. I have presented such an experiment by a third party experimenter which confirms my prediction exactly. Please see example 2 here: www.baur-research.com/Physics/measure.html. At 5:40 he announces his genuine result. He is so shocked to discover that angular momentum is not conserved that he begins yanking harder and harder until he can achieve his desired result but he never actually achieves it because he overshoots. In any event, measurements made while under the influence of motivated reasoning are not valid. The only reliable, repeatable, genuine and valid result is the initial result provided at 5:40. This result is exactly a two fold increase which is exactly what I predict.

If you cannot show invalid premisses or illogic, then addressing my argument requires that you accept the conclusion drawn.

Please address my argument?

Where we stand right now is that I have provided a logical proof which remains undefeated and there is third party empirical confirmation which is not contested by any counter evidence.

This is as far as there is to go in science. 

As things stand, it is a scientifically proven fact that angular momentum is not conserved because angular energy is.

Show less

Hey John - I actually did a little experiment - to test these two models.  Yours, that the mass on the string does not increase its velocity when it is pulled in, and the model of conservation of angular momentum, that the velocity of the mass times its radius remains constant.  (i.e. if you pull it halfway in, the velocity will double)  You can read the paper I wrote here: 

https://docs.google.com/document/d/1xqV9lwVJ50HlfocVIXhYwmp2iWJre2d0baOnG_DDHIg/edit?usp=sharing

You can access the videos I made and the spreadsheet I used here:

https://drive.google.com/open?id=1rCI5R_a3RXCrZJmoUhQonoWPWQpHn_mB

The experiment is far better controlled and more carefully measured than the one you cite as a refutation of hundreds of years of well accepted classical physics.  

In short - I disprove entirely the constant velocity model, and there is room in the error bars for the theoretical model.  SO angular momentum seems to be conserved after all.

I am sure that you consider yourself a rational man. I ask that you consider what I have to say with an open and rational  mind.

@TuHSPhysicsTTSD

4 years ago (edited)

 @TheAnimammal  SO here we are:

I have a model that is

1. Well accepted and settled science

2. Supported by a carefully executed experiment

3. Consistent with the Work-Energy Theorem

4. Agrees with Newton's Laws of motion

You have a model that

1. Refutes hundreds of years of basic fundamental Physics

2. Is based on nothing

3. Violates the Work-Energy Theorem

4. Violates Newton's Laws

5. Is refuted by my careful experiment

I am now going to resort to the most virulent ad hominem attack possible:  (You have forced me to this - Trigger warning - what follows is not pretty)

Your mother was a hamster and your father smelt of elderberries!

https://www.youtube.com/watch?v=cAy4zULKFDU

@TuHSPhysicsTTSD

1 year ago

 @TheAnimammal  Check out the new lab: https://sites.google.com/a/ttsd.k12.or.us/tuhsphysics/home/htp-ib-physics/rotational-mechanics/ib-angular-momentum-lab?authuser=0

And the Video Rationale: https://www.youtube.com/watch?v=8QHvXLD5a7A

It's dedicated to you

Hey - We are all going to test your scientific model: https://sites.google.com/a/ttsd.k12.or.us/tuhsphysics/home/htp-ib-physics/rotational-mechanics/ib-angular-momentum-lab?authuser=0. Check out the new lab.  They are going to spin the stopper on a string and do video analysis.  They can calculate the velocity before and after pulling it in.  They will also calculate their uncertainty.  May the best model win.  The velocity either stays the same, or it does not. @TheAnimammal