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Need equipment and ideas for an experiment

  1. Aug 8, 2018 #1
    I require balls (about the size of a small marble) of different masses. I have no idea how to go about this and your help would be greatly appreciated!

    I also need an hemisphere (about 20cm in radius). I though of 3D printing the same, and it's extremely expensive. Any alternative ideas?
    Thanks a lot,
  2. jcsd
  3. Aug 8, 2018 #2
    Steel ball bearings are available in a variety of sizes in many hardware stores in the US. Various online suppliers can provide other materials with a wide range of diameters and densities. See: https://www.smallparts.com.au/store/categories/balls+categories/

    In the US, lead balls are available in many sporting goods stores because they are shot from muzzleloading rifles and pistols. I have some that I use in free fall experiments, because air resistance can be neglected with reasonable accuracy.

    The 20 cm hemisphere is harder. I'd work toward an experimental design where it was not needed. A good experimentalist figures out how to design affordable experiments. Including materials before you figure out how to affordably source them is bad form.
  4. Aug 8, 2018 #3
    The aim is to observe the time period of a sphere on a hemisphere with changing masses. It would be great if you could suggest any other ways to test the same. 20cm was chosen as the effect of damping on something smaller was significant leading to larger uncertainties.
  5. Aug 8, 2018 #4
    Building supply stores sell wooden balls (on a base) for decorating the tops of posts, some approaching 20 cm. You could probably saw off the top half.
  6. Aug 8, 2018 #5
    Cross posted, I didn't realize you needed a hollow hemisphere. Buy a cheap cardboard globe and cut it in half? Mold paper mache over a ball of suitable size?
  7. Aug 8, 2018 #6
    Try McMaster Carr - a good source for various materials in basic stock forms

    But even the local hardware store may have a variety like, cork, rubber, wood, stainless - etc... esp if you have one of those good old scholol types.
  8. Aug 8, 2018 #7
    So you need the inside of a hollow sphere? There are some reasonable suggestions above.

    But if your experiment can be done in 2 dimensions rather than three, a short section of PVC pipe in the appropriate diameter would be cheap and cheerful (or an end cap or coupling). Cylinders are often easier than spheres.
  9. Aug 8, 2018 #8


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    Staff: Mentor

    Buy a stainless steel salad bowl for the hemisphere. Easy. Look for standard size balls around 1cm diameter in wood, steel, lead, hollow plastic, etc. Have fun!
  10. Aug 8, 2018 #9


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    Science Advisor

    A small globe of the Earth or Moon for the (hemi)sphere.
    Other possibilities may turn up in School Supply, Arts and Crafts, or Home Decorating stores.
    And it probably doesn't even have to be a hemisphere, just concave with rotational symmetry.
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