Protractor errors

[Murray]

Hi all

I posted this in a recent Newsletter:

http://www.intmath.com/blog/letters/intmath-newsletter-pedigrees-resources-10307#puzzle

It involves two protractors that have fundamental flaws, leading to students' measuring angles incorrectly. While "a poor tradesman always blames his tools", this is a case where it would have been nice if the students spotted the problems in the first place.

I haven't had a completely satisfactory answer to it yet.

Anyone here like to have a go?

(I think this could be a good question to give to kids who've been using "proper" protractors for a while.)

Regards,

Murray

[Maria Droujkova]

I've had a protractor like the second one, and I think I fixed it by making a hole in it with the point of my compass ;-)

Cheers,
Dr. Maria Droujkova

NaturalMath.com
919-388-1721

-- .- - ....

[Paul Libbrecht]

Did you see through the hole?

Paul

‎

[Maria Droujkova]

On Wed, Jul 22, 2015 at 12:11 AM, Paul Libbrecht <pa...@hoplahup.net> wrote:

Did you see through the hole?

Paul

I would put the tip of the compass, or pencil's sharp end, through the hole and line it up with the vertex of the angle. Or just peek through it to line it up - but then, it would wiggle around annoyingly. Then I would rotate the protractor until the side of the angle lined up with 0. This Instructables page shows a protractor with a hole pre-drilled in the right place: http://www.instructables.com/id/Basic-Clinometer-From-Classroom-Materials/

I think the manufacturers just forgot the hole.

--

[Paul Libbrecht]

That does not work to measure an already drawn angle or?

paul

[Maria Droujkova]

Here's a video of using a protractor with a hole in it: https://www.youtube.com/watch?v=SFvtQN3LMWE

It works for already drawn angles. It's easier when the protractor is transparent, but if it's not, the angle's side just has to be long enough to stick out beyond the protractor.

When I was a child, I had a metal protractor I loved. Because I could also use it to slice apples and share them fairly with my friend.

On Wed, Jul 22, 2015 at 10:19 AM, Paul Libbrecht <pa...@hoplahup.net> wrote:

That does not work to measure an already drawn angle or?

paul

[Murray]

Thanks Maria for your inputs.

I like your apple story!

Regards

Murray