I have completed an experiment on the effects of the wetness of a ball on its trajectory when projected. What the data I collected says that the more wet a ball is (determined by the mass of the ball after wetting), the farther away its first splotch/bounce will be from where it was released.

Here is an image of our launching mechanism AKA catapult: Launch Mechanism

How the mechanism works is that we put the ball into it (it's gripped by the tyres at the bottom) and then we let go of the string shown, from a controlled height. The mechanism then swings and hits a ruler, which causes the ball to jolt out of the tyre grip, launching through the air.

We then measure the distance from where it was released using a tape measure.

enter image description here

BTW distance is in metres. This shows that there is a positive correlation between both.

However, we were expecting quite the opposite:

We know from conservation of energy that all the balls should have the same initial velocity upon release from our catapult mechanism, because $v= \sqrt(2gh)$. Since we assume this, we know that all the balls have the same initial velocity upon release. After release, the only force (in an ideal world) acting on the ball is gravity/weight force. Thus the downwards acceleration of all the balls should be around $9.8 m/s^2$. This means that all the balls should land at exactly the same spot, and so make identical splotches.

What then is causing this difference in splotch distance? Could someone provide some insight, if not an answer, at least a point in the right direction.

  • $\begingroup$ Which part of your trebuchet is providing the elastic potential energy? $\endgroup$ Commented Oct 20, 2018 at 4:53
  • $\begingroup$ What is the ball made of? $\endgroup$
    – Farcher
    Commented Oct 20, 2018 at 5:51
  • $\begingroup$ @Farcher it's a standard tennis ball $\endgroup$ Commented Oct 20, 2018 at 6:49
  • $\begingroup$ @harshit54 it gives the ball gravitational potential energy, which is converted to kinetic as it swings down. $\endgroup$ Commented Oct 20, 2018 at 6:50
  • $\begingroup$ Where does the energy come from? You pull the string and then where does the energy get stored? $\endgroup$ Commented Oct 20, 2018 at 7:01

1 Answer 1


The first thing you should do is analyze your data thoroughly to make sure the positive correlation is statistically significant.

Did you account for any frictional forces between the ball and the tire grip as the ball is thrown out of the device? A friction force would have the effect of slightly decreasing the ball's speed as it's leaving the pendulum (I'm assuming from the picture that your device is similar to that of the Charpy impact test). The water, if covering the entire tennis ball, might be acting as a lubricant and decreasing the effects of friction, thereby allowing the wet ball to launch faster and travel farther than the dry one.


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