Discern initial V w/o components?

[afronicus]

Discern initial V w/o components!?

Homework Statement

Ok, ball traveled 188m (618 ft) before landing on the ground. Assume ground is flat, and ball V_i is from 45^o, disregard air resistance, and Starting position is .9m above ground.
Find V_i

I KNOW )))) a= 9.8 m/s²
X_i= 0
X_f=183
Y_i= 0.9m
ANGLE= 45⁰
V_y (@ highest point) = 0

Homework Equations

Ok so the equations I'm using are:
A---V_f= V_i + (a)(t)

B---X_f= X_i+ V_i+ (1/2)(a)(t)²

C---(V_f)²=(V_i)²+2a(X_f-X_i)

D---X_f-X_i=((V_i+V_f)/2)(t)

The Attempt at a Solution

So I've been racking my brain over this for the past hour, and I keep failing to discover the answer. My equations are left with 2 variables. What am I missing?? :/

 

[rock.freak667]

Homework Statement

Ok, ball traveled 188m (618 ft) before landing on the ground. Assume ground is flat, and ball V_i is from 45^o, disregard air resistance, and Starting position is .9m above ground.
Find V_i

I think you've overloaded your brain with information and equations, so let's take it slowly.

Split the initial velocity V_i acting at 45°, into x and y components.

The first line says

ball traveled 188m (618 ft) before landing on the ground.

So this means it traveled horizontally 188 m. So what does this tell you about the time taken for the motion? (Hint: horizontal velocity remains constant throughout, so distance = speed*time)

Write the time,t, in terms of V_i.

When you get this, then we will consider vertical motion.

 

[iRaid]

Vx=x/t so that will help you.

 

[afronicus]

hi, thanks a bunch for your reply. Ok, so my V<sub>i in components is Vix=Vi cos(45)
&
Viy=Visin(45)
...
Ok, and time can be written as:

t =( Vfx/y / a) + Vix/y.

(Vx)(t)=188 m

apparently there is numerical value you're supposed to get...I opted to have Mastering Physics show me the answer, said it was 41.3. I just can't figure out how.
...</sub>

 

[afronicus]

A man, my formula B is off, initial veloss should be multiplied by time. Thought I had figured out out... Not so much.

 

[rock.freak667]

(V<sub>x)(t)=188 m

apparently there is numerical value you're supposed to get...I opted to have Mastering Physics show me the answer, said it was 41.3. I just can't figure out how.
...</sub>

A man, my formula B is off, initial veloss should be multiplied by time. Thought I had figured out out... Not so much.

So your time would be

[tex]t = \frac{188}{V_i cos 45}[/tex]

I can't really read how you manipulated formula B, but considering y-direction, the initial displacement y₀ you were given. And you know that y = 0 when t = 188/(V_icos45°).

Just plug it in and solve for V_i.