# Homework Help: Help with a basic physics problem!

1. Nov 22, 2009

### Chaalie

Hey guys!
This is my first post on this forum, though I've been looking through it for a few months now. I am in a basic Grade 11 physics course, and I'm having a little trouble with this one. So, here goes...

1. The problem statement, all variables and given/known data
A cannonball (mass=15.5kg) is shot out of a cannon on Earth (assume it is shot at a height of 0m). The maximum height that it reaches is 20.1m. What is the speed of the cannonball at the maximum height?

2. Relevant equations
The different equations I know that might be useful are...
Eg=m*g*h
V=d/t
A=v/t
d=V(initial)*t + (A*(t)^2)/2
d=[(V(initial)+V(final))*t]/2

3. The attempt at a solution
My attempts have been a little rough, but here is my best one...
F=mg
=15.5*9.8
=151.9N

A=F/m
=151.9N/15.5
=9.8m/s^2

Vf^2=Vi^2 + 2A*d
= 0 + 2(9.8)(20.1)
Vf^2= 393.96
Therefore, Vf = 19.84842563m/s

I have a feeling I've done something wrong...any input?
I'd still like to figure it out myself, so if somebody could just point me in the right direction, that'd be great!

Charlie

2. Nov 22, 2009

### cavalier

The the maximum height, its vertical velocity is zero. It has only horizontal velocity, and horizontal velocity depends on the velocity at which it was fired. It isn't possible to solve this problem with the information you are given.

3. Nov 22, 2009

### Redbelly98

Staff Emeritus
Correct. We need to know the initial speed or angle, or the initial horizontal component of velocity, to solve the problem.

4. Nov 24, 2009

### Chaalie

Thanks guys!
I'll check with my teacher to see if he missed anything.