Why does the protractor turn when I drag the base arm?

Because that is what lining a protractor up means. If the scale stayed flat you could read any angle off it without lining anything up, and lining it up is most of what the skill is.

Can I measure a reflex angle?

Yes. The scale goes all the way round to 360, so a reflex angle reads straight off rather than having to be worked out from the other one.

How do I get the class to estimate first?

Turn Reading off. The number and the name of the angle both go, so nothing on the board gives it away. Set an angle, ask for estimates, then turn it back on.

Can I set an angle exactly?

Yes. Leave Snap on and set Steps of to 1, then the plus and minus buttons move a degree at a time and a drag lands on whole degrees.

Protractor

Measure · Year 3–Year 6

Drag an arm and read the angle. Hide the number and estimate first.

Loading…

About the protractor

Two arms from one point, both draggable, with a protractor laid over them. The protractor's zero sits on the base arm, so whichever way the angle is turned the moving arm points at its own reading.

That is the bit children get wrong. Lining the instrument up and reading from the right end is most of the skill, and a protractor drawn flat on the page lets a class read an angle off it without ever doing either. Drag the base arm and the whole scale turns with it.

It is a 360 degree protractor rather than the usual semicircle, so reflex angles read straight off. Teaching them as "measure the other one and take it off 360" is a subtraction lesson wearing a geometry lesson's clothes, and angles round a point are on the Year 6 curriculum.

Turn the reading off and the class estimates. The name goes with it, because "reflex" gives the answer away as surely as the number does. Turn the protractor off as well and what is left is a bare angle, for naming or for asking whether it is more than a right angle.

How to use it on your whiteboard

1Drag either arm, or tap where you want it. The nearer arm moves.
2The plus and minus buttons open and close the angle. Turn it rotates the whole thing without changing the angle.
3Steps of sets how much a button moves, and what a drag snaps to. Snap off gives you every degree.
4Show hides the protractor, the reading, or both.
5Tap Fullscreen so the angle fills the board.

Questions

Why does the protractor turn when I drag the base arm?: Because that is what lining a protractor up means. If the scale stayed flat you could read any angle off it without lining anything up, and lining it up is most of what the skill is.
Can I measure a reflex angle?: Yes. The scale goes all the way round to 360, so a reflex angle reads straight off rather than having to be worked out from the other one.
How do I get the class to estimate first?: Turn Reading off. The number and the name of the angle both go, so nothing on the board gives it away. Set an angle, ask for estimates, then turn it back on.
Can I set an angle exactly?: Yes. Leave Snap on and set Steps of to 1, then the plus and minus buttons move a degree at a time and a drag lands on whole degrees.