Need help by tonight!

NINHARDCOREFAN

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?

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NINHARDCOREFAN

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?

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Last edited:

NINHARDCOREFAN

anyway I tried that too got 12.4 for Vy and 28.3 for speed which is still wrong

Last edited:

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.

NINHARDCOREFAN

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

enigma

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What was the answer you got, NIN?

NINHARDCOREFAN

37.3 with my last calculation

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)

THANX A LOT.

enigma

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round off error?

plugged wrong number in?

NINHARDCOREFAN

yep damn system accepts only 1% error wrong so 37.3 was wrong. 38 was right

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...

NINHARDCOREFAN

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.

krab

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

enigma

Staff Emeritus
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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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