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 March 31st, 2017, 02:34 AM #1 Member   Joined: Mar 2017 From: Tasmania Posts: 36 Thanks: 2 now enough info A tennis ball traveling at 20ms/s south-east hits a wall and bounces back at a velocity of 16ms/s south-west. Listing any assumptions you make, estimate a the inital momentum of the ball b the change in momentum of the ball I get how to calculate inital momentum and change in momentum but I just do not think enough information is given for the question I must be missing something help plz
 March 31st, 2017, 03:17 AM #2 Senior Member   Joined: Feb 2016 From: Australia Posts: 1,597 Thanks: 546 Math Focus: Yet to find out. You are correct, there isn't enough information given. But the question also asks to list your assumptions, so it's up to you to come up with some reasonable assumptions. My assumptions would be for the directions the ball travels. Draw your set of axis. South east would -45 degrees from the positive horizontal axis, similarly, south west would be -135 degrees from the positive horizontal axis (-45 from negative horizontal axis).
 March 31st, 2017, 04:22 AM #3 Member   Joined: Mar 2017 From: Tasmania Posts: 36 Thanks: 2 Does that also mean give the tennis ball a mass?
 March 31st, 2017, 04:26 AM #4 Senior Member   Joined: Feb 2016 From: Australia Posts: 1,597 Thanks: 546 Math Focus: Yet to find out. Not necessarily. Try solve symbolically first. Things tend to cancel out.
March 31st, 2017, 06:08 AM   #5
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Quote:
 Originally Posted by Posher A tennis ball traveling at 20ms/s south-east hits a wall and bounces back at a velocity of 16ms/s south-west. Listing any assumptions you make, estimate a the inital momentum of the ball b the change in momentum of the ball
$p_0 + \Delta p = p_f$

 March 31st, 2017, 03:48 PM #6 Member   Joined: Mar 2017 From: Tasmania Posts: 36 Thanks: 2 Isn't momentum defined as p = mv? I'm still a little confused, sorry. Last edited by skipjack; April 1st, 2017 at 08:06 AM.
March 31st, 2017, 04:38 PM   #7
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Quote:
 Originally Posted by Posher Isn't momentum defined as p = mv? I'm still a little confused, sorry.
Yes, momentum is a vector with the same direction as velocity ...

$\vec{p} = m\vec{v}$

Last edited by skipjack; April 1st, 2017 at 08:07 AM.

 April 1st, 2017, 05:26 AM #8 Member   Joined: Mar 2017 From: Tasmania Posts: 36 Thanks: 2 So how am I meant to figure out p0 if I'm only given velocity? I must be missing something from your diagram. Last edited by skipjack; April 1st, 2017 at 08:05 AM.
April 1st, 2017, 07:47 AM   #9
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$p_0 = 20m$, where $m$ is the mass of the tennis ball ...

Quote:
 Modern tennis balls must conform to certain criteria for size, weight, deformation, and bounce criteria to be approved for regulation play. The International Tennis Federation (ITF) defines the official diameter as 6.54–6.86 cm (2.57–2.70 inches). Balls must have masses in the range 56.0–59.4 g (1.98–2.10 ounces).

 April 1st, 2017, 07:28 PM #10 Member   Joined: Mar 2017 From: Tasmania Posts: 36 Thanks: 2 I believe that this question is rather debatable as the weight has not been given would I be right in assuming this

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