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**Homework Statement**

Recent studies have raised concern about 'heading' in youth soccer (i.e., hitting the ball with the head). A soccer player 'heads' a size 5 ball deflecting it by 31.0°, and keeps its speed of 10.40 m/s constant. A size 5 ball has a mass of approximately 0.446 kg. What is the magnitude of the impulse which the player must impart to the ball?

After getting this incorrect, I was given the hint: "The impulse is the momentum imparted to the ball, and it changes the ball's momentum. Remember that momentum is a vector."

**The attempt at a solution**

My prof has done this question in class a number of times, however the answers he came to in class are not being accepted as correct answers.

The way we did it in class is as follows:

I = fΔt = Δp

I = (Ʃp)f - (Ʃp)i

I = (Ʃmv)f - (Ʃmv)i

I = m(mf-mi) = mΔv

"Since the velocity magnitude is the same before and after collision, only the change in direction contributes to the change in velocity."

Δv = v

10.4cos31 = 8.91 m/s

Original classroom answer:

I = (.446)(8.91)

I = 30972 N*s

Revised classroom answer:

|Δv| = (8.9 - 10.4)

I = Δp = mΔv = m(v2 - v1) = m(10.4mgcosθ - 10.4)

I = m(8.9 - 10.4)

I = -0.669

As I said, both of these answers are incorrect. Can anyone help explain what is wrong and what process needs to be followed to get the right answer? The question feels like it should be simple, but I just can't seem to spot what where my prof went wrong explaining it to the class.

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