# Homework Help: Real quick help needed with KINEMATIC EQUATION

1. Dec 10, 2012

### riseofphoenix

So I'm given...

In the y-direction
vo = 0 m/s
vf = 7.5 m/s (is the speed of the ball thrown straight up)
g = -9.81 m/s2 (when it falls back down)

yo = 0 m
yf = 1.5 m (at this height above the ground, the bat connects with the ball)

t = ?

I tried doing this...

yf = yo + vyot + (1/2)gt2
1.5 = 0 + (0)t + (1/2)(-9.81)t
1.5 = -4.905t
0.30581 = t

:(
Help?

2. Dec 10, 2012

### TSny

Projectile motion starts just after the ball leaves the bat. So, the initial velocity is not zero. The final velocity would be the velocity the ball strikes the ground (not given).

Similarly, think about the initial and final values of y.

3. Dec 10, 2012

### riseofphoenix

Ok... So

vo = 7.5 m/s (initial speed of the ball when thrown straight up)
vf = ____ m/s (the final speed when the ball strikes the ground)
g = -9.81 m/s2 (when it falls back down)

yo = 1.5 m
yf = 0 m (at this height above the ground, the bat connects with the ball)

t = ?

Like that?

4. Dec 10, 2012

### TSny

Yes. Good.

5. Dec 10, 2012

### riseofphoenix

yf = yo + vyot + (1/2)gt2

0 = 1.5 + 7.5t + (1/2)(-9.81)t2
0 = 1.5 + 7.5t - 4.095t2
0 = - 4.095t2 + 7.5t + 1.5

[ -b ± √(b2 - 4ac) ] / 2a
= [ -7.5 ± √(7.52 - 4(-4.095)(1.5)) ] / 2(-4.095)
= [ -7.5 ± √(56.25 +24.57) ] / -8.19
= [ -7.5 ± √(80.82) ] / -8.19
= ( -7.5 ± 8.989 ) / -8.19
= ( -7.5 - 8.989 ) / -8.19
= -16.48999444 / -8.19
= -16.48999444 / -8.19
t = 2.013 s

What did I do wrong??

Last edited: Dec 10, 2012
6. Dec 10, 2012

### TSny

Looks like just a slip. Half of 9.81 is not 4.095

7. Dec 10, 2012

### riseofphoenix

Oh yeah...i just realized that..the answer is supposed to be A :)

Can you help me out with one more?

I tried doing....
a2 + b2 = c2 after making my triangle...

3.22 + b2 = 122
b2 = 144-10.24
b = √(133.76)
b = 11.56

And then I tried doing

12*sin (18.9) = 3.89 m

8. Dec 10, 2012

### haruspex

Don't you think gravity might come into it somewhere?