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I'm in the IB program in my high school, and we are doing an internal assessment in physics. It must be about some physical property of a bouncy ball. I decided to determine how the average density of a ball affects its bounce height. Does density actually affect bounce height, or am I starting a lab that is pointless?

Phlebas
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  • So, a tennis ball bounces higher because of its greater elasticity, not because of its density, which is lower than the baseball. Is that right? – Phlebas Mar 05 '17 at 14:22
  • Yes, that's correct. Density will have some loose correlation, but the material properties will be what's important. That won't have direct density dependence. – JMac Mar 05 '17 at 14:33
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    As an IB student once, just a tip: read the IA criteria again and again... Make sure you religiously follow all the criteria they set and exactly how they want lab reports to look like. Be rigorous in your discussion of errors; they are not expecting a ground-breaking discovery, they just want to see you rigorously do an experiment and discuss it well. – JamalS Mar 05 '17 at 15:01
  • Basket balls bounce higher than tennis balls but are less dense. Steel balls are denser than both but bounce higher still. See https://www.teachengineering.org/activities/view/cub_energy_lesson03_activity3. – sammy gerbil Mar 07 '17 at 20:02

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I think this is a pointless lab activity because density is not something which can be varied easily without affecting other properties such as elasticity and diameter - eg by choosing a different type of ball.

Although it is gratifying to come up with an original line of research, an easier and more reliable strategy is to build on the work of others at your school - which is what usually happens in academic/industrial research. Find out what projects have been done before, and look for ways of improving on them. Most projects contain evaluations which point out shortcomings in the method and include suggestions for future research.

If you are still stuck for ideas, "The Physics of Basketball" cited by akhmeteli contains quite a few suggestions. After a little digging on the internet I found :

Other ideas :

  • modelling drag on a ball to confirm it is proportional to area and speed or speed-squared;

  • measuring the deformation of a ball as a static load is placed on it, to verify that Hooke's Law is obeyed.

Pick a couple of options and discuss them with your teacher.

sammy gerbil
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Density does affect the bounce height (to some extent), in particular, due to air drag: the higher the density, the less significant is the effect of air drag.

akhmeteli
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    Odds are any drag differences due to density will be overshadowed by material effects. It seems like it would be difficult to see the influence of drag unless you had two materials of significantly different density with very similar elastic properties. Not saying this answer is bad either, just pointing out how little the significance would likely be in experiments. – JMac Mar 05 '17 at 14:40
  • @JMac: according to https://books.google.com/books?id=bd6HXRDV0fgC&pg=PA11&lpg=PA11&dq=basketball+air+drag&source=bl&ots=aL-zZTVwg5&sig=pQ0xukvAiusIPckmkyqG1rTB3EY&hl=en&sa=X&ved=0ahUKEwjimNfzz7_SAhVU7WMKHYvaCJYQ6AEIOTAE#v=onepage&q=basketball%20air%20drag&f=false , the air drag is of the same order of magnitude as (approximately one third of) gravity for a basketball at 10m/s . To achieve this velocity, one needs to drop a basketball from a height of about 5m, if I am not mistaken. So I am sure the effect of air drag is not difficult to demonstrate using what's available in a school lab. – akhmeteli Mar 05 '17 at 15:11
  • @JMac: One can choose two balls that have the same bounce height for a small height and compare the bounce heights for a higher height. One can also choose balls that are much smaller than a basketball, so the relative significance of air drag is higher. So the experiment seems quite feasible. I agree that dependence of the bounce height on the average density is not the most interesting topic, but it was the OP who chose it, not me:-) – akhmeteli Mar 05 '17 at 15:14
  • I'm just saying the effect of drag vs. density won't really be what you measure either. As density changes the material properties are likely to change as well. This will have an effect on the elasticity of the material, so much of what you measure by changing density is not due to drag, but a combination of many effects. You would need to deliberately design the experiment to determine the effects of air drag due to density, density itself will not be a good measure for the drag effects. – JMac Mar 05 '17 at 15:35
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The bounce height tells you about the fraction of energy lost from the moment the ball was dropped, until it has rebounded.

This can include air drag (especially important for very light objects like beach balls), the surface you bounce on (sandy beach or marble floor?) , the degree of deformation of the ball during the bounce (more deformation, like a poorly inflated ball, will lead to greater loss) the actual material of the ball (some materials are more lossy than others) and the nature of the interface between ball and surface (friction can play a part - lower friction can lead to higher bounce).

It may be difficult to use just "density" without the other factors confounding the result (for example, a lower density material might deform more - is it the deformation or the density that caused the difference?). I think inflation pressure of the same ball would be an easier experiment to do successfully. Combine it with an investigation of friction (use sandpaper face-up or face-down, or spray the floor with teflon spray) and you would have an interesting project. You can use a water manometer to determine pressure in the ball with good accuracy.

Have fun.

Floris
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