# Homework: vertical motion in gravity

1. Oct 2, 2006

### JUka

i have this assignment. a ball is throun in to the air withe the speed equal to 49m/s and i need to find out after how many seconds it will reach 102,9 m. i used the formula we were given in class height=initial speed*time-[(Gravitational acceleration* time^2)/2]

P.S. could you use these terms cuz i learn physics in romanian not in english

2. Oct 2, 2006

### semc

i believe the equation is +at^2 and not -at^2....? alrite bascially u already have the ans so what seems to be the problem?the gravitational acceleration os 9.81....u just hav to sub in everything given to get the ans???

3. Oct 2, 2006

The part of the equation involving acceleration is 1/2*a*t^2 in general, but since a = -g, it is -1/2*g*t^2, so the equation is correct.

4. Oct 2, 2006

### Saketh

The formula is
$$x_f - x_i = v_0 t + \frac{at^2}{2}$$
It comes from integrating acceleration as a function of time twice. If you know integration, then you don't need to memorize this formula, because it is so easily derived.

You have all of the information; you have the formula as well. You are correct.
$$102.9 - 0 = 49t - \frac{1}{2}(9.8)t^2$$

Just remember that you need to use the quadratic formula to get the solutions from the quadratic equation that results from substituting in the numbers.