# Thread: integration problem (pls help)

1. ## integration problem (pls help)

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????

2. ## Re: integration problem (pls help)

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

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

$s_0 = 8ft$

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

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

3. ## Re: integration problem (pls help)

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

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

4. ## Re: integration problem (pls help)

$v_0 = 9 \, m/s$ 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.

5. ## Re: integration problem (pls help)

Originally Posted by Maqi
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.

6. ## Re: integration problem (pls help)

Ohhhh okay thanks so much. I finally got it!