# Homework Help: Physics, find the time it takes.

1. Dec 11, 2011

1. The problem statement, all variables and given/known data

How long does it take to reach the ground? And what is the angle of impact with the ground?

2. Relevant equations

3. The attempt at a solution
Δy = Voy * t + (1/2)ay * t2
t = (-5 m/s) ± √(5 m/s)2 - 4(1/2)(-9.81 m/s2)(-100m) / 2(1/2)(-9.81 m/s2)

t = -5 m/s ± (4.54)

Is it suppose to be 4.54 or -4.54? And am I suppose to add or subtract the two variables to find t??

2. Dec 11, 2011

### Simon Bridge

Sometimes you will get two possible solutions, you ignore the one that is not physical or does not correspond to your situation.
In this case, your math is for an inverted parabola - this will intersect the "ground level" in two places - one to the left of the start, and one to the right. Part of the exercize is to get you to pick the one that corresponds to your problem.

Hopefully in your actual work you will have more explanatory bits.
eg. from your numbers, it looks like b^2-4ac < 0 and the division by -9.8 should make the first term in t positive.