# Homework Help: 9 pm, and still dont understand

1. Oct 22, 2007

### megg0230

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

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

3. 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
56556.76106/80
and got 706.9595133

I really just dont understand what i am doing, and need help. bad. this is due tomorrow.

2. Oct 22, 2007

### 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.

3. Oct 22, 2007

### megg0230

I dont even know any of that.
We never learned Kinematic equations.
Im very slow at this, and am now regretting taking this class.

4. Oct 22, 2007

### pooface

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

Kinematic Equations:
S=Vot+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 [Broken]

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

Last edited by a moderator: May 3, 2017
5. Oct 22, 2007

### rocomath

Last edited by a moderator: May 3, 2017
6. Oct 22, 2007

### pooface

Here is a better site.

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

Last edited by a moderator: May 3, 2017
7. Oct 22, 2007

### 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