# Math Help - gravity

1. ## gravity

Am studying for a upcoming test and am a little lost on the whole gravity and acceloration thing.

The question am working on is like this
"I ball is thrown stright up with a speed 25m/s"
The first part of this question is how high does the ball go.
I figured that to be 31.98m

25/9.81 = 2.55 The ball will reach 0 speed after 2.55s

then i divided 25/2 and got 12.5m/s

then 2.55*12.5 = 31.89m

now the hard part, I need to know what the times are when the ball reaches half the heigth.

am not really good at using all the formulas, could somone please explane the process very basicaly to me?

2. Originally Posted by JBswe
Am studying for a upcoming test and am a little lost on the whole gravity and acceloration thing.

The question am working on is like this
"I ball is thrown stright up with a speed 25m/s"
The first part of this question is how high does the ball go.
I figured that to be 31.98m

25/9.81 = 2.55 The ball will reach 0 speed after 2.55s

then i divided 25/2 and got 12.5m/s

then 2.55*12.5 = 31.89m

now the hard part, I need to know what the times are when the ball reaches half the heigth....
hej,

from your results I know that you have used the equation:

$h(t)=25 \cdot t - \frac12 \cdot 9.81 \cdot t^2$ to calculate the height. You know the maximum height already $h_{max} = 31.2\ m$. Use this value to calculate the time when the ball has reached half of the maximum height:

$15.6 =25 \cdot t - \frac12 \cdot 9.81 \cdot t^2$. Now solve for t. You must get 2 results because the ball reaches the half height when going up and a second time when it is falling down.

For confirmation only: I've got: $t_1 \approx 0.728\ s~\vee~t_2 \approx 4.369\ s$

3. am not really good with the formulas, if i solve for T with this formula

$15.6 =25 \cdot t - \frac12 \cdot 9.81 \cdot t^2
$

do i move all the T's over like 2t^2?

4. Originally Posted by JBswe
am not really good with the formulas, if i solve for T with this formula

$15.6 =25 \cdot t - \frac12 \cdot 9.81 \cdot t^2
$

do i move all the T's over like 2t^2?
no, move the 15.6 over. then use the quadratic formula to find the value of t