Calculating initial velocity for object

[babbagee]

Calculating initial velocity for object...

I will give the problem first and then explain what i am having trouble with.

a.) With what speed must a ball be thrown directly upward so that it remains in the air for 12.6 seconds?
b.) What will be its speed when it hits the ground?
c.) How high does the ball rise?

a.) For the first part i just used the equation, v=v₀ + at, and I assumed that v = -v₀, so my question is do i use -9.8m/s² or +9.8m/s². But after thinking about it I think I should use -9.8m/s² because when the ball reaches its maximum hight it falls back with a negative velocity, right. If someone else has another way of think about this please tell me. Oh, and I got 61.7 m/s, can some one confirm that.

b.) The velocity will be the same as the intial velocity but negative.
c.) I did this one and used -9.8m/s², and i got 193 meters, can someone confirm that also.

I am having a hard time chosing weather to use -9.8m/s² or +9.8m/s². I guess what I am trying to say is what is a simple of thinking about this.

Thank you

 

[dpsguy]

In kinematics questions ALWAYS take g=-9.8m/s2.Reason being that gravity always acts downwards.So if you take downwards vel. as -ve you must also take g to be -ve.

 

[babbagee]

ok, i understand

can someone confirm my answers.

thanks