Solving for Velocity: Common Mistakes and How to Get It Right 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 going to 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?

 

[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?

 

[enigma]

EDIT: ACK hold on... misread...

 

[NINHARDCOREFAN]

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

 

[enigma]

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]

What was the answer you got, NIN?

 

[NINHARDCOREFAN]

37.3 with my last calculation

 

[enigma]

That's a bit off my answer. Are you rounding off anywhere?

V_x = 25.5*cos(29.1) = 22.3
V_y = -30.8

sqrt(Vx^2+Vy^2)

 

[NINHARDCOREFAN]

THANX A LOT.

 

[enigma]

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]

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 homework 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]

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.