johnjohn320
Diamond Member
Last one for awhile, I promise. 🙂 I'm actually doing well in the class, believe it or not, but every now and then a problem stumps me. Like this one:
A ball thrown vertically upward is caught by the thrower after 2.00 seconds. Find the initial velocity of the ball and the maximum height it reaches.
OK, so obviously I have for the ball from the time it leaves the thrower to its apex
a=-9.81 m/s/s
vf=0
vi=?
d=?
t=1 sec (am I wrong in this?)
I have two formulas, vf^2 = vi^2 + 2ad ; and d = vit + 1/2at^2
Since using one or the other leaves multiple variables open, I substituted one into the other, getting me
vf^2 = vi^2 + 2(a)(vit + 1/2at^2)
I solve all this out, come to 0 = vi^2 - 19.62vi -4.905 I use the quadratic equation and get 19.62 +/- 20.11 over 2. Doing this gets me a vi of 19.something or -.455 . The book has 9.80 m/s as the vi. Grrrr....how did they get that?
I appreciate any help you can give me, but please, don't just solve the problem cause you know how, I'd really appreciate it if you can look at my work and see where I went wrong...
Thanks!
EDIT-nevermind...friend reminded me of good ol' (vf-vi)/t=a
God I'm an idiot sometimes...
A ball thrown vertically upward is caught by the thrower after 2.00 seconds. Find the initial velocity of the ball and the maximum height it reaches.
OK, so obviously I have for the ball from the time it leaves the thrower to its apex
a=-9.81 m/s/s
vf=0
vi=?
d=?
t=1 sec (am I wrong in this?)
I have two formulas, vf^2 = vi^2 + 2ad ; and d = vit + 1/2at^2
Since using one or the other leaves multiple variables open, I substituted one into the other, getting me
vf^2 = vi^2 + 2(a)(vit + 1/2at^2)
I solve all this out, come to 0 = vi^2 - 19.62vi -4.905 I use the quadratic equation and get 19.62 +/- 20.11 over 2. Doing this gets me a vi of 19.something or -.455 . The book has 9.80 m/s as the vi. Grrrr....how did they get that?
I appreciate any help you can give me, but please, don't just solve the problem cause you know how, I'd really appreciate it if you can look at my work and see where I went wrong...
Thanks!
EDIT-nevermind...friend reminded me of good ol' (vf-vi)/t=a
