# Reviewing Newtonian Mechanics

Bashyboy
b]1. Homework Statement [/b]
1. Two seconds after being projected from ground level (y=0 m), a projectile is displaced
x=40 m horizontally and y=53 m vertically above its launch point. What are the (a) horizontal
and (b) vertical components of the initial velocity v of the projectile? (c) At the instant the
projectile achieves its maximum height above ground level, how far is it displaced horizontally
from the launch point?

## The Attempt at a Solution

I am working on part a). I was able to determine the horizontal component; however, I am unable to ascertain the vertical component of velocity. I tried applying kinematic equations, but with no avail. I tried to employ a symmetry argument, by finding the velocity acquired as the projectile falls to earth from a vertical distance of 53 m, but then I realized that I don't know the velocity at this point, nor can I suppose that the speed is zero, because it isn't.

What should I do?

## Answers and Replies

voko
There is a formula relating initial velocity, uniform acceleration, displacement and time. You know three of these, thus should be able to find out the remaining unknown.

Bashyboy
Are you speaking of this formula: vi = (y -.5at^2)/t. If so, how can I apply it when I need to set t = 0, in order to determine the initial velocity?

voko
At what value of ##t## do you know ##y##? Why would you let ##t = 0## instead?

Bashyboy
I figured that I would set t = 0, because that is the instant whose velocity I am trying to find.

voko
At ## t = 0 ##, ## y = 0 ##, so your equation is ## 0 = v_i \cdot 0 - g \cdot 0^2 / 2 ##, which is useless. But you are given data at ## t ## different than 0, so use that.