Momentum of a Tennis Ball at an Angle

[PeachBanana]

Homework Statement

A tennis ball of mass = 0.060 kg and speed = 26 m/s strikes a wall at a 45° angle and rebounds with the same speed at 45° .

Homework Equations

Δp = pfinal - pinitial
horizontal velocity = (26 m/s)cos 45°

There's a picture if you all care.

The Attempt at a Solution

pintial = mv
pintial = (0.060 kg)cos 45°
pinitial = 1.1028 kgm/s

pfinal = - pintial

I made this relationship because it is now going in the opposite direction.

pfinal = -1.1028 kgm/s

Δp = -1.1028 kgm/s - 1.1028 kgm/s = -2.2056 kgm/s

The answer is positive and I cannot understand why.

What is the direction of the impulse given to the ball?

A. Upward
B. Downward
C. Left
D. Right

I've eliminated B and C but I'm still unsure. The sine and cosine of forty-five degrees are the same.

 

[gneill]

Momentum is a vector. It's magnitude is m*v, where v is the speed.

The change in momentum would be found by finding the momentum vectors for before and after and finding the difference.