Hailstones and Momentum
1. The problem statement, all variables and given/known data
I'm working on a similar problem and someone else solved the problem (along with the steps). I looked at what they did but I'm really confused on 2 of the steps. Here is the problem: In a 30s interval, 500 hailstones strike a glass window of area 0.60 m2 at an angle of 45o to the window surface. Each hailstone has a mass of 5.0 g (0.005 kg) and a speed of 8.0 m/s. If the collisions are elastic, find the average force. 3. The attempt at a solution vx = v cos 45o (Why is it not sin 45?) vx = (8.0 m/s)(0.707) = 5.66 m/s px = m vx = (0.005 kg)(5.66 m/s) = 2.83 x 10  2 kgm/s I understand everything up to this next step. What I don't understand is why is the momentum in the x direction multipied by two? px = 2 px = 5.66 x 10  2 kgm/s F = pxtot/t F = (500)(5.66 x 10  2 kgm/s)/30 s F = 0.943 kgm/s2 F = 0.943 N 
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A substantial understanding of trig is going to be crucial throughout your physics course(s). Review the unit circle, if you're having trouble with this concept. 
OH, I see! But what is the reason for multiplying Px two times?

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perfect sense. thank you!

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