Impulse of a ball hitting the wall

[miniradman]

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

 

[da_nang]

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 $I = m\Delta v$, consider breaking the velocities into orthagonal components. Which component will the normal force change?

 

[tiny-tim]

welcome to pf!

hi miniradman! welcome to pf!

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 , except that you've not yet multiplied by the mass

(and do you need to specify the direction of the https://www.physicsforums.com/library.php?do=view_item&itemid=340" also?)

 

[miniradman]

hi miniradman! welcome to pf!


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

(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 ;)

 

[tiny-tim]

hi miniradman!

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 …

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)

 

[miniradman]

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?

 

[tiny-tim]

hi miniradman!

yes, you can either subtract the original vectors, tail-to-tail, or flip one so that it becomes adding, which is head-to-tail

 

[miniradman]

awesome! Thanks mate ;)