Solving for Horizontal Projectile Motion: Mechanics Final Question

[Femme_physics]

This is one of the questions at the mechanics finals that I skipped.

Homework Statement

http://img155.imageshack.us/img155/1702/mecccs.jpg

In the drawing is described the trajectory of a stone cast from a cliff at point A in an initial horizontal velocity V. During its motion the stone passes close to an obstacle at point B.

A) Calculate the minimal velocity V for the stone not to hit obstacle B
B) Calculate the velocity of the stone at point B (magnitude and direction
C) Calculate the horizontal distance from point A till it hits the ground

The Attempt at a Solution

From some reason I'm getting an error. I think the last formula I used should have an opposite Yi and Yf for it.

http://img683.imageshack.us/img683/2923/anattempt.jpg

 

[SteamKing]

In the first part of your solution, you found out how long the stone took to fall 10 m under the force of gravity. The horizontal distance of 40 m must be traveled by the stone in the same amount of time it fell vertically 10 m.

Here is a great big hint: the horizontal distance traveled by the stone is affected only by the initial velocity. There is no influence from gravity.

Can you find out what the initial horizontal velocity of the stone is in order for it to clear the top of the obstacle?

 

[Femme_physics]

Can you find out what the initial horizontal velocity of the stone is in order for it to clear the top of the obstacle?

Well, yes, it should be in the formula I posted, but from some reason I'm getting an error

 

[I like Serena]

Hi Fp!

You introduced a minus sign (sign - not symbol) in your equation that shouldn't be there.

And then you can't take a square root of a negative number.
That will give you an error.

 

[Femme_physics]

You introduce a minus sign (sign - not symbol) in your equation that shouldn't be there.

That's what I'm talking about. Why shouldn't it be there if the equation tells me to just plug in the values? Is there something wrong with it? Because I just copied it straight out of the formula sheet.

 

[I like Serena]

You seem to have the wrong formula.

For the horizontal motion you should use x_f = x_i + v_x t.

Btw, I don't recognize the formula you used. Where did you get it?
I can't think of any use for that formula.

 

[Femme_physics]

http://www.scribd.com/doc/57839921/All-Dynamics-Formulae

Second page at the top

זריקה אופקית

which means horizontal projectile motion

I could get Vi through this formula, so I can think of a use!

 

[I like Serena]

I see.
Yes that would work for horizontal projectile motion.
However, the formula should have delta y on the left side which is positive.

 

[Femme_physics]

Hmm...so it really should be

Delta Vi - delya Yf and not the other way around?

 

[I like Serena]

It should be: y_0 - y or y_i - y_f.

(Btw, this is assuming that the y-axis points upward, which it should.)

 

[Femme_physics]

Aha! So, that was my error, or should I say, THEIR error, since I only followed the formula. I daresay I made no mistake here, they did!

Thank you