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Sometimes, understanding takes an email or two (or 46)

October 13, 2010

I often tell my students that they should feel free to email me with questions. I really love email because 1. I can respond to it on my schedule 2. It’s easy for me to deal with (I can type a response in seconds) and 3. It’s permanent. It’s pretty easy to go back, search my inbox and see who’s communicating with me.

So here’s a conversation between a student and me about some corrections to a test.

Student <> Tue, Oct 12, 2010 at 8:13 PM
To:
Hey Mr. B, I have a question about #4 a. on our BFPM test. For the position vs. time graph R2D2 is moving away from the origin right? And where did he start? At the origin? Or 7m away from it? and at 2 1/3 seconds, right?
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me <> Tue, Oct 12, 2010 at 8:16 PM
To: Student <>
His initial position is indicated by the t=0 on the photo
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Student <> Tue, Oct 12, 2010 at 8:16 PM
To:
Oh wait! He is moving towards the origin starting from 7m away right?
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Student <> Tue, Oct 12, 2010 at 8:20 PM
To:
what about the x=0 then? So he starts at t=0 which is 7m away from the origin which is x=0?

>>> me <> 10/12/10 8:17 PM >>>
His initial position is indicated by the t=0 on the photo

On Oct 12, 2010, at 8:13 PM, Student wrote:

> Hey Mr. B, I have a question about #4 a. on our BFPM test. For the position vs. time graph R2D2 is moving away from the origin right? And where did he start? At the origin? Or 7m away from it? and at 2 1/3 seconds, right?
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me <> Tue, Oct 12, 2010 at 8:22 PM
To: Student <>
yes.
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me <> Tue, Oct 12, 2010 at 8:22 PM
To: Student <>
x=0 is showing where the origin is.
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Student <> Tue, Oct 12, 2010 at 8:26 PM
To:
Thanks! I think I got it!

>>> me <> 10/12/10 8:23 PM >>>
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Student <> Tue, Oct 12, 2010 at 8:31 PM
To:
Sorry, another question. For 4. c. since the velocity never changes wouldn’t the answer be v sub 0= (3m/s) x t to the 0 power?
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me <> Tue, Oct 12, 2010 at 8:37 PM
To: Student <>
yes, but you can drop the t^0, since it is 1. Also, check your signs.
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Student <> Tue, Oct 12, 2010 at 8:40 PM
To:
My signs?
Also, sorry again, a for 1. d. Is one of the forces the non-contact gravitational force of the earth on the produce or the contact normal force of the basket on the produce, since they are equal but exerted in opposite directions. I think it is the non-contact gravitational force of the earth on the produce.

>>> me <> 10/12/10 8:37 PM >>>
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me <> Tue, Oct 12, 2010 at 8:42 PM
To: Student <>
Yes. The sign of the velocity.

On Oct 12, 2010, at 8:40 PM, Student wrote:

> My signs?
> Also, sorry again, a for 1. d. Is one of the forces the non-contact gravitational force of the earth on the produce or the contact normal force of the basket on the produce, since they are equal but exerted in opposite directions. I think it is the non-contact gravitational force of the earth on the produce.

This is not correct. What is the force that the scale measures? Imagine holding a spring scale in your hand and pulling it with your pinky. What force is is measuring?
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Student <> Tue, Oct 12, 2010 at 8:45 PM
To:
What is the sign of velocity? Is the answer just v= 3m/s? And are the forces for d. not the non-contact gravitational force of the earth on the basket, the non-contact gravitational force of the earth on the produce, and the non-contact magnetic force of the magnet on the basket.

>>> me <> 10/12/10 8:42 PM >>>
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me <> Tue, Oct 12, 2010 at 8:47 PM
To: Student <>
Go back and look at your graph. Is the velocity positive in your graph? Does this also agree with the motion you see in the illustration?

On Oct 12, 2010, at 8:45 PM, Student wrote:

What is the sign of velocity? Is the answer just v= 3m/s? And are the forces for d. not the non-contact gravitational force of the earth on the basket, the non-contact gravitational force of the earth on the produce, and the non-contact magnetic force of the magnet on the basket.

No. In d, you are looking for a SINGLE force. What is the force that the SS measures when you pull on it with your pinky. Describe this force first and you can answer this question.
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Student <> Tue, Oct 12, 2010 at 8:48 PM
To:
Oh! So v= -3m/s, I forgot about that

>>> me <> 10/12/10 8:48 PM >>>
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Student <> Tue, Oct 12, 2010 at 8:56 PM
To:
One final question! In 1. c. one of the forces acting down on the basket is the contact force of the produce on the basket, right? And wouldn’t this be a normal force since it is perpendicular?
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me <> Tue, Oct 12, 2010 at 9:01 PM
To: Student <>

On Oct 12, 2010, at 8:56 PM, Student wrote:

One final question! In 1. c. one of the forces acting down on the basket is the contact force of the produce on the basket, right? And wouldn’t this be a normal force since it is perpendicular?

yes. But I don’t think you ever answered my prev. question.
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me <> Tue, Oct 12, 2010 at 9:02 PM
To: Student <>
this is really important. In your correction, you should add a sentence or two describing how you can avoid sign mistakes like this. (this can be your deeper understanding)
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Student <> Tue, Oct 12, 2010 at 9:04 PM
To:
Isn’t it whatever force is pulling down on it? But there are more than one in this instance….

>>> me <> 10/12/10 9:02 PM >>>

On Oct 12, 2010, at 8:56 PM, Student wrote:

> One final question! In 1. c. one of the forces acting down on the basket is the contact force of the produce on the basket, right? And wouldn’t this be a normal force since it is perpendicular?

yes. But I don’t think you ever answered my prev. question.
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me <> Tue, Oct 12, 2010 at 9:07 PM
To: Student <>
Yes!

No, there is only 1 force pulling down on the spring scale in both cases. What is it? Note the spring scale and basket are different things.
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Student <> Tue, Oct 12, 2010 at 9:11 PM
To:
I suppose the non-contact gravitational force of the earth on the basket. Where does the non-contact magnetic force come into play?

>>> me <> 10/12/10 9:07 PM >>>
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me <> Tue, Oct 12, 2010 at 9:16 PM
To: Student <>
No. It might help to make a fbd for the spring scale. What objects are touching it.
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Student <> Tue, Oct 12, 2010 at 9:18 PM
To:
The ceiling and the basket below are touching it.

>>> me <> 10/12/10 9:17 PM >>>
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Student <> Tue, Oct 12, 2010 at 9:19 PM
To:
Oh wait! I think that it might be the contact tension force of the spring on the basket!
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me <> Tue, Oct 12, 2010 at 9:22 PM
To: Student <>
yes. You can assume the scale is massless, so no Grav force. Now which of the two forces you mentioned does the scale measure?
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me <> Tue, Oct 12, 2010 at 9:22 PM
To: Student <>
why?
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Student <> Tue, Oct 12, 2010 at 9:24 PM
To:
Oh, the non-contact magnetic force? What about the produce?

>>> me <> 10/12/10 9:22 PM >>>
yes. You can assume the scale is massless, so no Grav force. Now which of the two forces you mentioned does the scale measure?

On Oct 12, 2010, at 9:18 PM, Student wrote:

> The ceiling and the basket below are touching it.
>
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me <> Tue, Oct 12, 2010 at 9:27 PM
To: Student <>
no, you were right before. But why?
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me <> Tue, Oct 12, 2010 at 9:27 PM
To: Student <>
this is right—but why?
On Oct 12, 2010, at 9:19 PM, Student wrote:

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Student <> Tue, Oct 12, 2010 at 9:31 PM
To:
Because the tension force is a similar to the normal force that a table
exerts on a book as it sits there. It has the same value in Newtons of
all of the forces acting down on the basket, just is the opposite
direction.

>>> me <> 10/12/10 9:27 PM >>>
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me <> Tue, Oct 12, 2010 at 9:34 PM
To: Student <>
You’re on the right track. I think you have the answer to the problem, but you don’t have the force that the scale measures. And I don’t think you quite have why those forces are equal.
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Student <> Tue, Oct 12, 2010 at 9:53 PM
To:
So it’s not the contact tension force of the spring on the basket? They
are equal because the spring has to hold up the basket and to do that it
has to exert the same force but pulling up!

>>> me <> 10/12/10 9:35 PM >>>
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> >>>>>>> Sorr> changes wouldn’t the answer be v sub 0= (3m/s) x t to the 0 power?
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me <> Tue, Oct 12, 2010 at 9:54 PM
To: Student <>
The answer to the question is the tension force of the spring on the basket. But this is not the force that the spring measures. Why not?

Your Why answer is close, but not quite.

How does the force of the spring on the basket compare to the force of the basket on the spring? Why?
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Student <> Tue, Oct 12, 2010 at 9:59 PM
To:
They must be equal, right?

>>> me <> 10/12/10 9:55 PM >>>
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>> gra>>>>>>> This is not correct. What is the force that the scale measures?
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me <> Tue, Oct 12, 2010 at 9:59 PM
To: Student <>
WHY? This is the big question….
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Student <> Tue, Oct 12, 2010 at 10:01 PM
To:
The spring has to keep the basket from falling down!

>>> me <> 10/12/10 10:00 PM >>>
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>>>>>>>> On Oct 12, 2010, at 8:40 PM, Juli>>> non-contact gravitational force of the earth on the produce or the
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me <> Tue, Oct 12, 2010 at 10:01 PM
To: Student <>
no. It’s connected to a fundamental law….
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Student <> Tue, Oct 12, 2010 at 10:03 PM
To:
Since the basket pulls down on the spring the spring must pull back up
with an equal force but in the opposite direction

>>> me <> 10/12/10 10:02 PM >>>
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>>> baske>>>> that the SS measures when you pull on it with your pinky. Describe
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me <> Tue, Oct 12, 2010 at 10:04 PM
To: Student <>
yes. What fundamental law is this?
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Student <> Tue, Oct 12, 2010 at 10:04 PM
To:
N3

>>> me <> 10/12/10 10:04 PM >>>
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>>>>>>>>>> What is the sign >>> earth
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me <> Tue, Oct 12, 2010 at 10:05 PM
To: Student <>
now you’ve got it.
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Student <> Tue, Oct 12, 2010 at 10:06 PM
To:
Thanks Mr. Burk! If I don’t have time to turn in my corrections tomorrow
I will plan to turn them in Thursday!

>>> me <> 10/12/10 10:05 PM >>>
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>>>>>>>>> yes. But I don’t think you ever answer>>>>>>>>>> Go back and look at your graph. Is the velocity positive in
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me <> Tue, Oct 12, 2010 at 10:06 PM
To: Student <>
no prob.
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