# Homework Help: What am I doing wrong? Simple Projectile motion calculations.

Tags:
1. Nov 4, 2014

### dylanjames

Ok, I have used every calculator I can find, typed it in several different ways for the past three hours and at this point I am giving up and asking for help.

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

A rock is thrown from the edge of a 14 meter high cliff with a velocity of total magnitude of 7m/s. Find the final velocity with which the rock hits the ground if it was launched initially upward.

2. Relevant equations
OK so we have
delta D - 14m (downward, negative)
V1 - 7m/s
V2 - Solve for
A - 9.8m/s^2 (down) (let upward be positive so this would be negative)
delta T - Solve for.

For equations - (upward throw)
Vfinal^2 = 7^2 + 2 (-9.8^2)(-14)

So V final = square root ( (7^2) + 2(9.8^2)(-14)
Final answer I got is 52.32.

This is a practice question so they list the answer and the answer is supposedly plus or minus 17.98m/s.

What am I doing wrong???!?!?

I thought maybe calculator when squaring a negative number does not turn it into a positive (ie. -9.8x-9.8 = positive number) and therefore threw off answer but when done that way it is multiplied with negative 17 and I cannot ask for square root of a negative..

Been trying to troubleshoot this too long and its driving me nuts! Any and all help is SO appreciated.

2. Nov 4, 2014

### Staff: Mentor

Hi dylanjames. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

Get rid of that ^2 around acceleration, it doesn't belong.

Last edited by a moderator: May 7, 2017
3. Nov 4, 2014

### dylanjames

........thank you. boy doesn't that make me feel stupid!!!!!