# A ball is thrown in the air @ 30m/s

1. Jul 5, 2009

### rdmfresno

1. The problem statement, all variables and given/known data
A ball is thrown straight up with an initial speed of 30m/s,
(a) Show that the time it takes to reach the top of its trajectory will be 3 seconds.
(b) Show that it will reach a height of 45m (neglecting air resistance).

2. Relevant equations
Earth's gravity = 9.8 meter/second² (Professor said to just round it to 10).
Equation unknown.
d=1/2(g)t² ?

3. The attempt at a solution
(a) Played around with Subtracting negative -9.8m/s² (is it considered negative since the ball gets thrown up? - Instead of it coming down?).

(b) 45m = 1/2(9.8m/s²)t² - I figure I need to know "t" before I could solve this.

Last edited: Jul 5, 2009
2. Jul 5, 2009

### LowlyPion

Consider what the definition is of acceleration. It is the rate of change of velocity.

If something starts out at 30 m/s and slows 10 m/s every second, it will reach 0 m/s in how many seconds?

Here are some formulas for you:
https://www.physicsforums.com/showpost.php?p=905663&postcount=2