Physics IAL Unit 4 problem in Momentum?

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mehrabhossain
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By using Newton's Second Law, I am getting the answer B, but the markscheme says the answer is D, both the answers are similar, instead of that - sign. Now I am unable to find out how the - sign is coming into the equation?Name:  Screenshot 2021-01-17 150440.png
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Stonebridge
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(Original post by mehrabhossain)
By using Newton's Second Law, I am getting the answer B, but the markscheme says the answer is D, both the answers are similar, instead of that - sign. Now I am unable to find out how the - sign is coming into the equation?Name:  Screenshot 2021-01-17 150440.png
Views: 10
Size:  106.0 KB
It's because the change in momentum is final value minus initial value.
If the initial value (perpendicular to the wall) is (plus) p sin theta, then the final value, being in the opposite direction, is minus p sin theta

Final value minus initial value is minus 2p sin theta
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ilovephysmath
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(Original post by mehrabhossain)
By using Newton's Second Law, I am getting the answer B, but the markscheme says the answer is D, both the answers are similar, instead of that - sign. Now I am unable to find out how the - sign is coming into the equation?Name:  Screenshot 2021-01-17 150440.png
Views: 10
Size:  106.0 KB
Keep in mind that when they say momentum ‘p’ that means the momentum is positive i.e. the velocity is taken to be in the positive direction. This means that when the ball bounces off, it’s momentum (the vertical component in this case) will be equal in magnitude to the vertical component of the momentum with which the ball struck the wall, but in the opposite direction. So, the vertical component of its momentum as it bounces off will be -psinθ.
Now you can just use F = (final momentum - initial momentum)/t and you’ll get the answer along with the correct sign.
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