Efficiently Solve Motion Problems Using Energy Conservation

[majormuss]

Homework Statement

I just came across this question and i think its giving me a real hard time.
This is part of a bigger question so I'll provide the other statements below
7 The maximum height reached by the ball is
approximately
(1) 1.2 m (3) 4.9 m
(2) 2.5 m (4) 8.5 m

Homework Equations

t=1 s and therefore half way is 0.50s
V(i)=8.9m/s
angle is 35 degrees
Viy= 4.9m/s

The Attempt at a Solution

First of all, why is it that I can't use certain formulas in Solving some motion problems even though everything seems to work ok. I tried using this formula...d=V(i)t +1/2at^2... but my answer turns out wrong. and this one rather gives me the answer...Vf^2=Vi^2 + 2ad...why is that?

 

[ekkilop]

Since you know the initial velocity, why not try to solve it using energy conservation?

 

[tiny-tim]

Hi majormuss!

(try using the X² tag just above the Reply box )

t=1 s and therefore half way is 0.50s
…
Viy= 4.9m/s

First of all, why is it that I can't use certain formulas in Solving some motion problems even though everything seems to work ok. I tried using this formula...d=V(i)t +1/2at^2... but my answer turns out wrong. and this one rather gives me the answer...Vf^2=Vi^2 + 2ad...why is that?

All the standard constant acceleration formulas will work.

I expect you got t = 0.5 s wrong … where did that come from?