# Homework Help: Prjectile Motion

1. Apr 30, 2010

### Crebble32

A rifle fires a bullet at 300 m/s at a target that is up on a wall 100 m away. The rifle's line of sight creates a 30 degree angle with the horizon. What initial angle does the rifle need to be directed to hit the target?

The formulas I thought I needed are:

Vy=ViSIN(30) Here my Vi is the V-"not"
Vx=ViCOS(30)

I found that the height of the target is 57.75 m, and the hypotenuse is 115.5 m...

I was told to find time then use a kinematic equation. So I found t=.33. I came up with several different angles in several dfferent ways and they are all too large... My prof said that it is only raised a very little bit. I have had MANY failed attempts at this and just can't figure it out.

I appreciate any help possible!

2. Apr 30, 2010

### rl.bhat

Whenever solving problem for inclined plane, consider axis along the inclined plane as x-axis and perpendicular to it as y-axis. Then
Vi(x) = Vi*cos(θ-α)
a(x) = -gsin(α)
Vi(y) = Vi*sin(θ-α)
a(y) = -gcos(α) where α is the angle of inclination and θ is the angle of projection along the horizontal.
Now proceed as usual in the projectile motion.

Last edited: Apr 30, 2010
3. Apr 30, 2010

### Crebble32

I am just working in circles... just come up with the values I'm already given. I don't understand what formulas to use...

4. Apr 30, 2010

### rl.bhat

Time of flight T = 2vsin(θ-α)/gcosα
Horizontal displacement OX = vcosθ *T
Range along the inclined plane = R' = OX/cosα.
Angle of inclination α is given. Now find θ.

5. May 1, 2010

### Crebble32

Thank you so much for your help! I think I finally got it :)