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Impulse of a ball hitting the wall

  • Thread starter miniradman
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  • #1
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Homework Statement


Calculate the change in momentum of a 120g pool ball hitting at 9m/s and rebounding at 6m/s. It rebounds at an equal and opposite angle of 45 degrees.

Pi = 9m/s
Pf = 6m/s
angle = 45 degrees

Homework Equations





The Attempt at a Solution


I have done all my calcs in the picture below which shows a diagram

I have concluded that the impluse was 10.8166 N, somehow I don't think I have done this right
 

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Answers and Replies

  • #2
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I have concluded that the impluse was 10.8166 N, somehow I don't think I have done this right
Well, you're right that you're wrong. Given that the impulse [itex]I = m\Delta v[/itex], consider breaking the velocities into orthagonal components. Which component will the normal force change? :wink:
 
  • #3
tiny-tim
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welcome to pf!

hi miniradman! welcome to pf! :wink:
I have done all my calcs in the picture below which shows a diagram

I have concluded that the impluse was 10.8166 N, somehow I don't think I have done this right
looks fine :smile:, except that you've not yet multiplied by the mass :wink:

(and do you need to specify the direction of the https://www.physicsforums.com/library.php?do=view_item&itemid=340" also?)
 
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  • #4
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hi miniradman! welcome to pf! :wink:


looks fine :smile:, except that you've not yet multiplied by the mass :wink:

(and do you need to specify the direction of the https://www.physicsforums.com/library.php?do=view_item&itemid=340" also?)
I don't think so...

Also, I have trouble remembering which one to flip around and make a negative? Is there are certain rule of guideline I have to follow to do this?

Thanks ;)
 
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  • #5
tiny-tim
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hi miniradman! :smile:
I have trouble remembering which one to flip around and make a negative? Is there are certain rule of guideline I have to follow to do this?
i'm not sure i follow :confused:

if the momentum before is p = (a,b), and the momentum after is q = (c,d),

then the change in momentum is q - p = (c-a,d-b) :wink:
 
  • #6
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What I did, was I flipped the 6m/s around so it becomes -6m/s (beause I took them way like vectors). Because it is on an angle of 45 degrees, I added the vectors head to tail

is this right?
 
  • #7
tiny-tim
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Homework Helper
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hi miniradman! :wink:

yes, you can either subtract the original vectors, tail-to-tail, or flip one so that it becomes adding, which is head-to-tail :smile:
 
  • #8
195
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awesome! Thanks mate ;)
 

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