# Homework Help: Projectile motion: find distance a ball would land

1. May 2, 2016

### phys1213

1. The problem statement, all variables and given/known data
I'm trying to come up with an equation to determine where a ball would land (basically the distance it moves) from a spring loaded projectile launcher set up on a table. I'm looking for "d", and I know the spring constant, compression, mass of the ball, height the ball starts at, angle the launcher is set at, and whatever else I can measure using a meter stick, balance, and protractor. There aren't any numbers just known variables

2. Relevant equations
Conservation of Energy eqn (at least the version I learned in class): Efinal-Eintial=Einput-Eoutput
F=ma
Fg=mg
sinθ=voy/vo
cosθ=vox/vo
KE: 1/2mv2
Spring: 1/2kx2
3. The attempt at a solution
I attempted to use conservation of energy but I get stuck trying to figure out where d goes in to be able to solve for it. Also, other online resources use a conservation of energy eqn that has different terms than what I was taught, but I'm assuming they are all the same.
System: Ball and Earth
Initial time: just after ball leaves launcher
final time: just before ball hit ground
Efinal=1/2mvf2
Einitial= 1/2mvo2+1/2kx2+mgH
Einput-Eoutput=0
1/2mvf2-1/2mvo2-1/2kx2-mgH=0
And then I'm stuck trying to figure out how the distance goes into this. I'm wondering whether I need to integrate the velocity with respect to time and relate that to the distance since the distance the ball travels is the velocity*time. Any help is appreciated!!

2. May 2, 2016

### BvU

Hi phys,

Not bad for a first post. Yes, you need some projectile motion equation to complete this. From initial Einitial= 1/2mvo2+1/2kx2+mgH (where vo = 0 ?) you get v0. And your projectile trajectory is uniform motion horizontally (needing t) and uniformly accelerated vertically (which gives you a quadratic equation for t) .