Physics Problem :D

[Dr.Gusta]

When the arm is at 35 degrees the shooter is at about 25in off the ground.

[raychensg]

Just take a length of surgical tubing and find a spring scale. If you line it up with a meter stick or ruler, you can experimentally find the curve for surgical tubing's force per distance stretched.

It shouldn't take much time, and can probably be extrapolated to different lengths pretty simply.

[Mike Schreiber]

http://www.chiefdelphi.com/forums/sh...2&postcount=12

This spreadsheet will be useful to you. You can determine your initial velocity using energy (as others were saying).

.5 * k * x^2 = .5 * m * v^2

I recommend getting your surgical tubing spring constant experimentally by loading your mechanism with weights so m*g = k*x if you know k and you know how far you're pulling it back then you can calculate velocity. Be sure to include the weight of your launching mechanism when calculating velocity, it is being accelerated too.

[Ether]

Originally Posted by Mike Schreiber

You can determine your initial velocity using energy (as others were saying).

.5 * k * x^2 = .5 * m * v^2

In most catapults and kickers, a large portion of the spring energy is not imparted as kinetic energy to the ball. Thus this equation will give a substantially over-optimistic estimate of ball speed if "x" is the total amount the tubing is stretched (including pre-tensioning stretching).

You can increase the available spring energy by adding more strands or tubing and/or pre-tensioning the spring (or surgical tubing) with the launcher (or kicker) against the hard stop in the "fired" position.

[Mike Schreiber]

Originally Posted by Ether

In most catapults and kickers, a large portion of the spring energy is not imparted as kinetic energy to the ball. Thus this equation will give a substantially over-optimistic estimate of ball speed if "x" is the total amount the tubing is stretched (including pre-tensioning stretching).

You can increase the available spring energy by adding more strands or tubing and/or pre-tensioning the spring (or surgical tubing) with the launcher (or kicker) against the hard stop in the "fired" position.

That is true for catapults, but I was under the impression that a linear launcher (like I imagined they were using if they were asking about angle from an arm) would be more efficient at energy transfer. With the exception of friction what would prevent it from imparting energy? Deformation of the ball?

[Ether]

Originally Posted by Mike Schreiber

That is true for catapults, but I was under the impression that a linear launcher (like I imagined they were using if they were asking about angle from an arm) would be more efficient at energy transfer. With the exception of friction what would prevent it from imparting energy? Deformation of the ball?

Friction, deformation, and...

When the catapult/kicker/punch hits its hard stop, all its kinetic energy at that point is lost and not transferred to the ball.

[Aur0r4]

Don't forget the issues related to momentum transfer and impulse in a VERY inelastic collision. Then add the time-domain issues of the rebound of the ball off your shooter...

There's many, many variables that COULD be added to the problem. I have strongly encouraged our programming team to consider the law of diminishing returns with regard to how much they try to analyze.

We are only shooting a ball 20' into a goal. We're not putting a hypersonic dart through a tank turret a mile away while the gun platform is bounding over terrain

Lots of physics going on, but we only need to know the parts that contribute to the shot within the limits of the robot's own tolerances.