# Projectile Motion

1. Dec 19, 2008

### knightryder

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

Hello everyone - here's my question: I am doing an assignment on projectile motion and I will be launching some balls from a compressor (that I have not seen yet!). I know how to calculate how much work will be done by the air expanding (will obv. depend on how much pressure is in the compressor); but i do not know how to go from there:

do i say that the work done will equal the kinetic energy of the ball and just use the simple projectile motion formulae? If i do, isn't that neglectling the time taken for the ball to accelerate?
If I take acceleration into account, i guess i would use momentum of air hitting ball = momentum of ball (elastic collision) then use f=ma on the ball. This will give me the acceleration of the ball. But how do I use the acceleration of the ball upwards with gravity taken into account? This would mean the acceleration would be highest at the launch, be 0 somewhere midair, decrease to 9.8 m/s then stay there? Perhaps there is an easy equation that I am forgetting (I would never rule that out!!!)

2. Relevant equations
ummm.. thats what I am wondering..
if my first guess is right, then I use Ke= 1/2 m v2,
if my second guess is right , then I use Newton's 3rd.

3. The attempt at a solution

Please read part 1.

Thanks for your help.
P.S. If possible use metric units- I am Grade 11 Australia.

2. Dec 19, 2008

### LowlyPion

Welcome to PF.

I suspect you already know, but here are the kinematic equations for motion once the ball is in flight.
https://www.physicsforums.com/showpost.php?p=905663&postcount=2
And they an also be used to calculate your acceleration and initial velocity depending on what you are measuring.

As to your question about how complex ... that can be as complicated as you want it to be.

Your biggest problem will likely be in getting accurate measurements and determining your error tolerances. The easiest way to get a good crack at what your initial velocity is would be to fire the ball straight up and time its total time of flight. That of course ignores air resistance, which is certainly a factor, but it can give you a ball park working initial velocity.

Good luck.

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