# mechanics helpWatch

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Thread starter 1 month ago
#1
https://imgur.com/a/Oapbtq7

I am correct as to what I is, for some reason they are using vectors. and i am using magnitude instead, so 0.2(6.57) -0.2(12) is the same as what they have in vector form but there is no negative for the j component. Doing what i did i get a negative impulse but they dont. i dont understand how to get it right, id rather not use vectors as the main textbook only uses them when you have to and i dont see where my method is wrong. do you have to use vectors for this question? thanks
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1 month ago
#2
(Original post by Gent2324)
https://imgur.com/a/Oapbtq7

I am correct as to what I is, for some reason they are using vectors. and i am using magnitude instead, so 0.2(6.57) -0.2(12) is the same as what they have in vector form but there is no negative for the j component. Doing what i did i get a negative impulse but they dont. i dont understand how to get it right, id rather not use vectors as the main textbook only uses them when you have to and i dont see where my method is wrong. do you have to use vectors for this question? thanks
Their work is fine, positive y (and impulse) is uowards
In your working at the bottom, it seems that the ball is heading in the same direction before and after. The "-" is for the change in momentum, but the velocities are both positive? There must be a sign change if it bounces.
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#3
(Original post by mqb2766)
Their work is fine, positive y (and impulse) is uowards
In your working at the bottom, it seems that the ball is heading in the same direction before and after. The "-" is for the change in momentum, but the velocities are both positive? There must be a sign change if it bounces.
but its still going in the same direction? like if you hit a tennis ball and it bounces it still has the same direction
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1 month ago
#4
(Original post by Gent2324)
but its still going in the same direction? like if you hit a tennis ball and it bounces it still has the same direction
Not in the vertical direction.
Its going into, then away from the wall.
It changes direction.
If unsure, just resolve directions for the velocity. In the horizontal direction, its the same, in the vertical its different.
Last edited by mqb2766; 4 weeks ago
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Thread starter 4 weeks ago
#5
(Original post by mqb2766)
Not in the vertical direction.
Its going into, then away from the wall.
It changes direction.
If unsure, just resolve directions for the velocity. In the horizontal direction, its the same, in the vertical its different.
so do i need to find the vertical component for the velocity before and after and then do mv-mu?
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4 weeks ago
#6
(Original post by Gent2324)
so do i need to find the vertical component for the velocity before and after and then do mv-mu?
Yes, the horizontal component is unchanged.
The bounce only happens in the vertical component
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Thread starter 4 weeks ago
#7
(Original post by mqb2766)
Yes, the horizontal component is unchanged.
The bounce only happens in the vertical component
ok do ive found that the vertical component before is 3root6 - 3root2 by doing sin75 x 12. vertical component after is sin61.81 x 6.57.
using those 2 numbers in mv-mu im still not getting the right answer, even using sin75 x 12 x 1/2 doesnt give the same answer either
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4 weeks ago
#8
(Original post by Gent2324)
ok do ive found that the vertical component before is 3root6 - 3root2 by doing sin75 x 12. vertical component after is sin61.81 x 6.57.
using those 2 numbers in mv-mu im still not getting the right answer, even using sin75 x 12 x 1/2 doesnt give the same answer either
6sin(75)
and
-12sin(75)
are the two vertical velocities - after and initial, as in the book?
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Thread starter 4 weeks ago
#9
(Original post by mqb2766)
6sin(75)
and
-12sin(75)
are the two vertical velocities - after and initial, as in the book?
how do they get the -12sin75?
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4 weeks ago
#10
(Original post by Gent2324)
how do they get the -12sin75?
The velocity going perpendicular into the wall
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