What equation to use for simple kinetics quesitons?

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Alipepsi
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For the questions below, which kinematic equations would I use? I have already tried these on my own but can not find a way to match the answers with the answer sheet, the answers on the answer key are RIGHT, as they are approved by my teacher.
Okey so the questions are
1) A baseball is thrown up vertically at 25m/s. What velocity will the ball have after:
a)1.0s
b)2.0s
c)3.0s

2) A soccer ball is kicked straight up at a speed of 30m/s. How long will it take to reach a height of 5.0m?
It would be great if you showed me how you got your answers, if not, I will really appreciate the equations, THANKS!
 

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Tiny, I'am confused =(
I've looked over the equations but can not figure out what equation to use. PLEASE HELP ME I have a test after march break =S
 
a=vf-vi/t

Then just derive it :P
 
Vf^2=vi^2+2a(d)
v=sq root 30^2-2(9.81)(5)
v= 28.3m

THEN

Solve for time using the equation: t=(v-vf)/a

answer is 0.17seconds ;D

THANKS FOR YA HELP MAN! :D
 
I see, can you help me with how to know what equations to use for questions. Like I was confused for what equation to use for the second problem. Do I look for all the values I'am given with the values in all the equatiosn then pick the one? Ik my quesitons is silly :P
 
I always write down a list of all the given values when I start a problem. Then I compare them to the equations that I know.
 
tiny-tim said:
You're welcome! :smile:

But quicker would have been s = ut + 1/2 at2 :wink:​

So you have to derive for t right?

This is what I got:

t= sq root s-ut/0.5a

This is correct? Really need to know this asap
 
yes, you'd have to solve for t
 
SHISHKABOB said:
yes, you'd have to solve for t

did I derive for t correctly there?
 
you'd have to use the quadratic equation to solve for t in that case.

notice how t= sq root s-ut/0.5a still has a t ion both sides of the equation.
 
SHISHKABOB said:
you'd have to use the quadratic equation to solve for t in that case.

notice how t= sq root s-ut/0.5a still has a t ion both sides of the equation.

If I was to use quadratic, then how would I get my A, B, and C values from solving that equation?
 
well the equation is also this:

1/2 at2 + ut - s = 0

so if t is the variable, what are the constants?
 
Alipepsi said:
did I derive for t correctly there?

You can check for yourself by substituting your formula for t back into the original equation to see if everything cancels out algebraically. Or you can use it to calculate t, then substitute it and your given quantities back into the original equation to see if everything cancels out numerically.