9 pm, and still dont understand

[megg0230]

Homework Statement

A rifle bullet is fired at an angle of 30 degrees below the horizontal with an initial velocity of 800 m/s from the top of a cliff 80 m high. How far from the base of the cliff does it strike the level of ground below
g=9.8
initial velocity- 800 m/s
y=80 m

Homework Equations

People i ask say..
R=(initial velocity squared x sin(2 theta)
g

The Attempt at a Solution

i did
800^2 x sin2(30)
9.8

i get 56556.76106-which seems way to high
Then i did Soh Cah Toa
Sin=Opp/Adj
56556.76106/80
and got 706.9595133

I really just don't understand what i am doing, and need help. bad. this is due tomorrow.

 

[pooface]

Hello welcome to Physicsforums! Hope you will learn here and also teach us what you know!

For the problem:
(1)You must resolve the initial velocity into their X and Y coordinates using trigonometry.
(2)Then use kinematic equations to solve the problem.

 

[megg0230]

I don't even know any of that.
We never learned Kinematic equations.
Im very slow at this, and am now regretting taking this class.

 

[pooface]

Hey don't despair! I can teach you what I know. I am learning this myself.

Kinematic Equations:
S=volt+0.5at^2
S=Vft-0.5at^2
S=((Vo+Vf)/2)*t
Vf=Vo + at
Vf^2=Vo^2 + 2aS

S=Displacement in metres
Vo=Initial Velocity in metres/second
Vf=Final Velocity in metres/second
a=acceleration in metres/second squared
t=time in seconds

For all kinematic problems, write your 5 variables down and write the known values in.

Velocity is a vector quantity. It has magnitude and DIRECTION. direction is usually indicated by an angle as in your problem.

Velocity of the bullet is 800m/s angle 30degrees below the horizontal. Draw a picture and see if you can resolve the velocity into X and Y coordinates.

Have a look at this site:
http://www.glenbrook.k12.il.us/gbssci/phys/Class/vectors/u3l2c.html

Then we will see if we can solve the problem. Please come back.

 

[rocomath]

I don't even know any of that.
We never learned Kinematic equations.
Im very slow at this, and am now regretting taking this class.

http://www.mathlinks.ro/Forum/latexrender/pictures/b/0/2/b02ab4a44802a89197dc4fcc4947b1ec33ebdf9d.gif

 

[pooface]

Here is a better site.

http://id.mind.net/~zona/mstm/physics/mechanics/curvedMotion/projectileMotion/generalSolution/generalSolution.html

 

[rocomath]

use the 2nd equation to find time

keep in mind the bullet is being fired DOWN, so your y value must be negative

when you find time, disregard the negative value

then use the 1st equation to find x