More Physics Confusion

[jonthedit]

To find the initial velocity of this, what do I need to do?
Ignoring Air resistance and Air Friction:

I have found t, the total time it takes to reach the water, and the height maximum point, when the Vi = 0.
I am lost how to find the initial velocity.

I have tried

deltaX
/ = v
t

,

trying: 16.05 / 2 (the current t) = 8.025m/s

I have tried finding t for the triangle part I made only, being sqroot of (8.03/5).
This makes the Vi = 12.6649m/s, which I think is too big of a number :|

Finally, taking the total Xx distance, 13.9[total Xx]/2[original t]: I get Vi=6.95

 

[marcus134]

where's your gravity? 9.81m/s²

 

[jonthedit]

where's your gravity? 9.81m/s²

That has to do with...?
Also in this problem I get to assume gravity is 10m/s^2.

I just care about my initial velocity. Gravity should not matter to find this... right?

 

[marcus134]

he's diving, he obviously ain't gonna fly 8m high in the sky then hit a wall and fall straight into the water.
it's a parabolic movement, the horizontal movement is constant, and the vertical is accelerated.

 

[grossaffe]

You said you already know how long it takes for him to hit the water, no? We'll call that t1. So then your initial velocity in the X direction = 13.9/t1. From there, you should be able to do trigonometry with the angle of the dive.

 

[jonthedit]

he's diving, he obviously ain't gonna fly 8m high in the sky then hit a wall and fall straight into the water.
it's a parabolic movement, the horizontal movement is constant, and the vertical is accelerated.

I said that too, but with the math it does not matter.
I am ignoring air resistance and friction. For purposes of equations, I can assume he hits a wall, because I do not care about his Xy descent, as I already know the change in Xy.
The 30* angle and unknown speed provided will make it a superhuman jump and his speed is going to be ridiculous.

 

[marcus134]

you know (13.9 , -12)

X=Vo*T*cos(a)
Y=Vo*T*sin(a) -0.5GT²


unless you prefer this:

Y= - X²G/2Vo²cos²(a) + X tan(a)

http://en.wikipedia.org/wiki/Projectile_motion

 

[jonthedit]

Correct answer Vi = 12.6649m/s

 

[Veho]

The correct solution is Vi=6,34 m/s

Plug it back into the projectile motion formulae and all the numbers fit.

 

[jonthedit]

The correct solution is Vi=6,34 m/s

Plug it back into the projectile motion formulae and all the numbers fit.

Still not following this... I don't get how I find V0, I get V0y and V0x

 

[grossaffe]

Still not following this... I don't get how I find V0, I get V0y and V0x

V0 = sqrt((V0x)^2+(V0y)^2)
V0 = V0y/(sin(30 degrees))
V0 = V0x/(cos(30 degrees))

Pick your poison.