Need help by tonight!

  • #1
For number 11(please see the attachment), I keep messing up and I don't know what I'm doing wrong.

To find the answer I know I have to do this:
Speed = sqrt(Vxsquared + Vysquared)

I know that velocity of x is gonna stay constant, so its v is 25.5

And for the velocity of y I did this(Vy=Vo*sin(ang)-gt):
Vy= 25.5*sin(29.1)-9.8(4.41) (time is right)
Vy= -30.82

so after plugging these values into the speed equation, I got this answer: 40.

What's wrong with this?
 

Answers and Replies

  • #2
For some reason, I can't attach the file. Here is the question.

Given: g = 9.8 m/s2
A stone is thrown from the top of a building upward at an angle of 29.1(degrees) to the horizontal and with an initial speed of 25.5 m/s. The height of the building is 40.7 m.

What is the speed of the stone just before it strikes the ground?
 
  • #3
enigma
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EDIT: ACK hold on... misread...
 
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  • #4
anyway I tried that too got 12.4 for Vy and 28.3 for speed which is still wrong
 
Last edited:
  • #5
enigma
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25.5m/s is the magnitude of the total velocity, therefore, it cannot be the same as the x component.
 
  • #6
Damn it got it wrong again, I calculated Vx by 25.5*cos29.1 and still got the answer wrong. I only have 2 chances left to getting it right
 
  • #7
enigma
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What was the answer you got, NIN?
 
  • #8
37.3 with my last calculation
 
  • #9
enigma
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That's a bit off my answer. Are you rounding off anywhere?

Vx = 25.5*cos(29.1) = 22.3
Vy = -30.8

sqrt(Vx^2+Vy^2)
 
  • #11
enigma
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round off error?

plugged wrong number in?
 
  • #12
yep damn system accepts only 1% error wrong so 37.3 was wrong. 38 was right
 
  • #13
enigma
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Is your prof doing anything to make sure that people aren't copying each others' work?

Submitting the HW to a website just seems like it's a quick easy way for unscrupulous students to cheat...
 
  • #14
LOL, last year the university was giving out hw by handouts, everybody cheated then. That's why they have HW on the web now. Everyone's numbers are different.
 
  • #15
krab
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Don't worry about x and y components: Use conservation of energy.

(1/2)mv_f^2=(1/2)mv_i^2+mgh

or

v_f=sqrt(v_i^2+2gh)=sqrt(25.5^2+2*9.8*40.7)=38.05
 
  • #16
enigma
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krab,

I have a feeling that NIN hasn't gotten to that topic in his classes yet. It is still early in the semester.

You're right, though. Conservation is the way to go.
 

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