How Can I Calculate the Magnitude of Momentum in a Game of Pool?

[VitaX]

Homework Statement

Homework Equations

Px = mVcostheta
Py = mVsintheta
Theta = tan-1(y/x)
Magnitude of P = (Px^2 + Py^2)^(1/2)

The Attempt at a Solution

Px = .165*2.84cos64.6 = .201 N*s
Py = .165*2.84sin64.6 = .4233 N*s
a) Theta2 = tan-1(-.4233/.201) = 64.6 degrees

b) (.201^2+.4233^2)^(1/2) = .4686 N*s

My work for part a may just have been luck or not needed but I obviously did something wrong since my answer in part b is wrong. Did I use wrong numbers for part b? Or am I supposed to calculate something else then find the magnitude?

 

[housemartin]

well, part (b) asks you to find change in linear momentum, not the magnitude of linear momentum (what you have done). See in the picture, linear momentum is a vector, those two arrows represent linear momentum before ball hits the wall and after. Directions are different, aren't they? ;] so linear momentum has changed, not in magnitude, but in direction. As problem statement says, only y part of momentum has changed. So just find this change in momentum

 

[VitaX]

well, part (b) asks you to find change in linear momentum, not the magnitude of linear momentum (what you have done). See in the picture, linear momentum is a vector, those two arrows represent linear momentum before ball hits the wall and after. Directions are different, aren't they? ;] so linear momentum has changed, not in magnitude, but in direction. As problem statement says, only y part of momentum has changed. So just find this change in momentum

Wait so if I get what you are saying. It is just putting a negative sign on the Momentum I found in the y direction of part a and having that be my answer for part b? To me that's the only logical answer unless I'm not understanding something. Is that right or wrong?

 

[housemartin]

first, balls momentum along y-axis was directed in positive direction, after bounce - in negative. So Py(1) = +.4233 N*s and Py(2) = -.4233 N*s. What is the change in momentum then?

 

[VitaX]

Is the change in momentum = P1*P2cos(theta)