What is the impulse on the ball from the wall?

[A_lilah]

Homework Statement

a 250 g ball with a speed v of 6.6 m/s strikes a wall at an angle θ (the angle between the wall and the path of motion) of 30° and then rebounds with the same speed and angle. It is in contact with the wall for 6 ms. What is the impulse on the ball from the wall?

Homework Equations

J is the impulse
J = mvfinal - mvinitial
the change in momentum (P) = Jnet
P = mv

The Attempt at a Solution

First I broke the velocity vectors into components:

6.6m/s * cos(30) = 5.716m/s
6.6m/s * sin(30) = 3.3 m/s, so

Vo = 3.3i + 5.716j (m/s)
and
Vfinal = 3.3i - 5.716j (m/s)

and the momentum of each is the mass of the object * the velocity:

(.25kg)Vo = Po = .825i + 1.428j
(.25kg)vfinal = Pfinal = .825i - 1.428j

so Jnet = Pfinal - Pinitial = (.825-.825) i + (-1.428 -1.428)j
so Jnet = -2.856 in the j direction
so the answer should be -2.856, but it is not (and it is not 2.856 either)...

any help is appreciated.
Thanks!

 

[physixguru]

Have a re-look at your v(initial).