Morris Minor Owners Club

Hi,
I'm doing physics coursework which involves looking at the size of craters made from different ball bearings dropped from different heights. I've got most bits sorted now but I'm not entirely sure how I'm going to measure the size of the crater. Does anyone have any ideas of tools and methods that can be used to accurately measure the size of a crater made in sand?

Thanks

What scale are you talking about Andy?

Vernier gauge might be useful, as you can measure both depth and width albeit on small scale.

Calipers and a steel rule ?

remove ball bearing carefully with a magnet...

Drip molten wax into the hole, when its cooled pull it out and you have a cast of the hole created. You can them measure the volume buy dunking into a full beaker of water and measuring the volume displaced or just measure the length using a caliper or steel rule ;)

How does that sound

Just ranging between 3mm and 12mm ball bearings so its very small scale, thats what makes it difficult. I'm just using simple callipers at the moment but having trouble measuring the depth accurately as the sand is so soft.

Matt wrote:remove ball bearing carefully with a magnet...

Drip molten wax into the hole, when its cooled pull it out and you have a cast of the hole created. You can them measure the volume buy dunking into a full beaker of water and measuring the volume displaced or just measure the length using a caliper or steel rule ;)

How does that sound

Interesting! Wonder what my teacher would say to that...
I doubt the wax would actually stay in shape though while it dries, would just seep through the sand

I would take photos of the crater, next to an object like a coin or something. then zoom into the photo on the computer until the coin is true to life size on the screen, and you can now measure the diameter with a ruler without messing up the crater.

Picky

If you can measure the width of the crater without touching the sand ie: vernier calipers as has been suggested, you can then work out the depth as you know the dia of the bearing. Just a thought...

Sidney'61 wrote:

Matt wrote:remove ball bearing carefully with a magnet...

Drip molten wax into the hole, when its cooled pull it out and you have a cast of the hole created. You can them measure the volume buy dunking into a full beaker of water and measuring the volume displaced or just measure the length using a caliper or steel rule ;)

How does that sound

Interesting! Wonder what my teacher would say to that...
I doubt the wax would actually stay in shape though while it dries, would just seep through the sand

I agree with Matt and no the wax wont go through the sand as it more or less starts to cool on touch, if you get really stuck I can always ask my eldest as she is head of physics at a local school.

Use a builders/decoraters laser measuring tool

If you cant get access to a laser range finder (like wot builders use), or if the wax deforms the crater then you may need to resort to trigonometry to calculate the depth of the hole.

You should be able to use a laser pointer clamped into one of those chemistry stands to give you the angles between two fixed points and the centre of the crater ... calculating the depth is then quite similar to calculating the height of a hill given two angles ;)

http://www.nzmaths.co.nz/geometry/shape ... tions.aspx (Session 3)

Thanks all, I'll have a look for one of those laser measuring tools, theres probably one laying around somewhere.
I need to actually be able to measure the depth rather than just calculate it so that I can prove that it all fits in with my theories and predictions etc..
Thanks for all the ideas everyone it's all definitely food for thought.

How about measuring the height of the ballbearing above the sand with a lazer fixed perpendicular to something like a slide rule zero'd at the edge of the sand where there is no displacement. You don't have to disturb the bearing to get an accurate reading then.

You can calculate the displacement and diameter of the crater from that but I can't remember the formula.

Thinking about it, I'm assuming that the bearing does not disappear completely below the sand.

Of course if you could make the ball bearings out of ice...