# integration problem (pls help)

#### Maqi

If a ball is thrown vertically upwards from a platform 8 feet above the ground. After 2 seconds, the ball is 2feet below the platform. Find the initial velocity of the ball. (Acceleration due to gravity = -10m/s^2)

how????

#### romsek

MHF Helper
$s(t) = s_0 + v_0 t + \dfrac 1 2 g t^2$

$a \approx -10m/s^2$

$s_0 = 8ft$

$s(2) = 8-2=6ft$

Just plug everything in and you'll have some simple algebra to find $v_0$

1 person

#### Maqi

In still confused cause I keep ending up with 9m/s

6 = -5(2^2) +2Vo +8
9 = Vo

Last edited:

#### Cervesa

$v_0 = 9 \, m/s$ is the correct solution for the given parameters. If the initial velocity were 31 m/s, the projectile would take slightly more than 3 seconds to reach the top of its trajectory.

#### romsek

MHF Helper
In still confused cause I keep ending up with 9m/s

6 = -5(2^2) +2Vo +8
9 = Vo
oh... it looks like there's some units conversion to be done.

They give the initial and final positions in feet but the acceleration due to gravity in m/s.

Convert the positions to meters.

1 person

#### Maqi

Ohhhh okay thanks so much. I finally got it!
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