(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

"A ball is thrown straight upward with a velocity of 18 m/s. How much time passes before the ball strikes the ground? Disregard air resistance."

v_{i}= 18 m/s

a = -9.8 m/s^{2}

t = ?

[not sure: v_{f}= 0 m/s]

2. Relevant equations

v_{f}= v_{i}+ at

3. The attempt at a solution

v_{f}= v_{i}+ at

t = [(v_{f}- v_{i})/a]

t = [(0 m/s - 18 m/s)/-9.8 m/s^{2}] = 1.8 s

Concerns:

--First, I understand that if I have final velocity as 0 m/s, that refers to when the ball has reached its maximum height--not its pre-impact final velocity.

--Second, with the first comment in mind, I'm not sure my approach is appropriate for this particular problem--and would appreciate guidance if it's not.

--Third and finally, some of my classmates insist that the correct answer is about double the answer I arrived at because "you just multiply the answer by 2 since it's only gone half of the distance" as I have conveyed above. I understand this thinking to some degree, but its simplicity seems to suggest the ball is projected from the ground--rather than simply thrown from what I'm gathering is knee-height. I would love to get your thoughts.

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

2. Relevant equations

3. The attempt at a solution

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Projectile: questioning need to double time

**Physics Forums | Science Articles, Homework Help, Discussion**