- #1

- 34

- 0

## Homework Statement

A stone is thrown from a cliff 100 meters high with an initial velocity v

_{0}= 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

## Homework Equations

I selected these:

v

_{x0}= v

_{0}cos(53)

v

_{y0}= v

_{0}sin(53)

Then I applied these t the following equations:

x

_{f}= v

_{x0}t

y

_{f}= y

_{0}+ v

_{y0}t+ (1/2)(-9,8)t

^{2}

t= ( -b± √(b

^{2}- 4ac) ) / 2a

## The Attempt at a Solution

So, for part a), using the above equations I found:

v

_{x0}= v

_{0}cos(53)

v

_{x0}= 25cos(53) = -22,96

x

_{f}= v

_{x0}t

x

_{f}= -22,96t

v

_{y0}= v

_{0}sin(53)

v

_{y0}= 25sin(53) = 9,898

y

_{f}= y

_{0}+ v

_{y0}t+ (1/2)(-9,8)t

^{2}

0= 100 + 9,898t+ (1/2)(-9,8)t

^{2}

Then, with that last equation, I used this equation: t= ( -b± √(b

^{2}- 4ac) ) / 2a

to find t= -3,62s

When the answer SHOULD BE 7s........ What am i doing wrong???

then, i tried b) using v

_{f}= 0

using the equation(H is maxheight):

v

_{f}

^{2}= v

_{0}-2(9,8)(H-100)

0 = 25

^{2}-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 x

_{f}?

d) Its supposed to be (15

**i**-48,6

**j**) m/s but i forgot how to calculate vectors.

ANYWAY PLEASE HELP!!! Its easy for you, but for me, this is hard. ;(