# Projectile motion

1. Oct 20, 2009

### look416

1. The problem statement, all variables and given/known data
A ball is projected horizontally from the top oF A CLIFF WOTH SPEED U. iF AIR RESISTANCE IS NEGLECTED AND G IS ASSUMED to be 10 ms-2, the velocity of the ball is found to be 50ms-2, 3 seconds later. What is the value of u

2. Relevant equations
s=ut+1/2at2
s=vt
v2=u2+2as
v=u+at

3. The attempt at a solution
well the problem is i do not know to use which components either x or y
if i use x, but in projectile motion and newton 1st law, the speed of x will not change as there is no other force to push or pull the ball
however, usually we use the initial speed for x only, and for y is 0
so how X.X

Last edited: Oct 20, 2009
2. Oct 21, 2009

### look416

well no1 can solve it ? ==

3. Oct 21, 2009

### rl.bhat

Ball moves horizontally without any change in its velocity U
Ball falls vertically withe zero initial velocity.
What is its velocity V in the vertical direction after 3 seconds?
U and v are perpendicular to each other.
What is the resultant of these two velocities?
Equate this resultant velocity to 50 ms^-1. , and find U

4. Oct 21, 2009

### look416

well this is what i have done
v=u+at
v=0+(9.8)x(3)
v=$$\frac{147}{5}$$
using Pythagorus theorem,
a2+b2=c2
($$\frac{147}{5}$$)2+b2=502
so the b=40.44
yep i got the answer thx so much