# Throwing a ball off a cliff and calculating its time, speed and using its degree.HELP

1. Nov 4, 2011

### MissJewels

1. The problem statement, all variables and given/known data
A stone is thrown from a cliff 100 meters high with an initial velocity v0 = 25 m / s and at an angle of 53° projection from the horizontal axis. determine:
a) the time before it reaches the ground
b) the maximum height
c) its horizontal reach
d) its speed when it hits the ground

2. Relevant equations
I selected these:
vx0 = v0cos(53)
vy0 = v0sin(53)

Then I applied these t the following equations:
xf= vx0t
yf= y0 + vy0t+ (1/2)(-9,8)t2

t= ( -b± √(b2 - 4ac) ) / 2a

3. The attempt at a solution
So, for part a), using the above equations I found:

vx0 = v0cos(53)
vx0 = 25cos(53) = -22,96
xf= vx0t
xf= -22,96t

vy0 = v0sin(53)
vy0 = 25sin(53) = 9,898
yf= y0 + vy0t+ (1/2)(-9,8)t2
0= 100 + 9,898t+ (1/2)(-9,8)t2

Then, with that last equation, I used this equation: t= ( -b± √(b2 - 4ac) ) / 2a

to find t= -3,62s
When the answer SHOULD BE 7s........ What am i doing wrong???

then, i tried b) using vf= 0
using the equation(H is maxheight):

vf2 = v0 -2(9,8)(H-100)
0 = 252 -16,9H +1690
H = 136,98 m
And the right answer should be 120m...... GRR !

As for c), I don'T quite understand how to measure it... do i find xf?

d) Its supposed to be (15i -48,6j) m/s but i forgot how to calculate vectors.

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

2. Relevant equations

3. The attempt at a solution

2. Nov 4, 2011

### iRaid

Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

These equations usually make problems much easier:

$x(t) = v_{o}cos\theta$

$y(t) = v_{o}sin\theta - \frac{1}{2}gt^{2} + h$

vo: Initial velocity
h: height

3. Nov 4, 2011

### Staff: Mentor

Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

Quadratic equations have two roots. Sometimes only one of them will make physical sense for a given situation.

EDIT: Also, make sure that your calculator is set for DEGREES rather than RADIANS if your angles are given in degrees!

Last edited: Nov 4, 2011
4. Nov 4, 2011

### MissJewels

Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

Thanks for the replies, but i was wondering if someone could perhaps check my work? Tell me what im doing wrong, if i miscalculated? I'm still in a nut.
Also, I have tried finding the two roots for the quadratic equation, and neither give me the proper answer.

5. Nov 5, 2011

### SteamKing

Staff Emeritus
Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

Well, you could show us your calculations after you have fixed the known errors.

6. Nov 8, 2011

### MissJewels

Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

I got it! I've been repeating the same silly mistake over and over again.