Measuring angles with a protractor

Mathematics · Geometry · ages 9-10

Name ______________________ Date ____________

Ben looks at an angle before he measures it. It is a thin sliver, much narrower than a right angle. Which reading could be its measure?
- 95°
- 170°
- 15°
Mia looks at an angle before she measures it. The opening is wider than a right angle but not flat like a line. Which reading could be its measure?
- 80°
- 120°
- 180°
Your protractor's center is on the vertex. What must point to 0 before you read anything?
- One ray of the angle
- The vertex point
- The curved edge
You place the protractor center on the vertex and one ray on 0. The other ray crosses at the 25 mark on the same row. The angle measures 25 degrees.
Circle one: True False
A protractor's printed baseline sits above its physical bottom edge. Sam aligns the physical bottom edge with a ray instead of the printed baseline, then reads the scale. Is his method reliable?
Circle one: True False
You draw one ray, place the protractor center on its end, and align 0 with the ray. You mark a point at the 78 line on the same row, then draw a second ray from the vertex through that point. What is the measure of the angle you drew?
Answer: ______________
Priya wants to draw a 110° angle. Which set of steps will work?
- Draw two rays first, then slide the protractor between them to find 110
- Draw one ray, set the center on its end with 0 on the ray, mark 110 on the same row, then draw the second ray through the mark
- Mark the 110 point, then draw both rays out from that point
Leo measures a wide, obtuse angle. He places the protractor center on the vertex and one ray on 0. The two rows show 62 and 118 where the ray crosses, and he reads 62. Which number should he have read?
- 118°
- 62°
- 90°
Priya puts the protractor's center on the vertex, but she forgets to line any ray up with 0. She reads 40° where a ray crosses. Why is 40° not a reliable measure of the angle?
- She read the wrong one of the two scale numbers
- No ray was on 0, so the counting started from the wrong place
- Her angle is too small for a protractor to measure
You draw one ray and place the protractor center on its end with 0 on the ray. You mark a point at 140 on the row opposite your aligned 0, then draw the second ray through it. What angle did you draw?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

Measuring angles with a protractor W1-mt_4MFUAsbx_6-s1

15° · A sliver narrower than a right angle is acute, so its measure is under 90°. Only 15° fits.
120° · Wider than a right angle but not flat means obtuse, so the measure sits between 90° and 180°. Only 120° fits.
One ray of the angle · After the center is on the vertex, you line up one ray with 0. Then you can read the mark where the other ray crosses.
True · With one ray on 0, follow that same row to where the other ray crosses. The reading is 25 degrees.
False · The printed baseline is the reference for 0 and 180. Aligning the physical bottom edge instead gives an unreliable reading, though a coincidentally correct number is possible.
78 · With the center on the vertex and 0 on the first ray, marking 78 on the same row and drawing through it creates a 78 degree angle.
Draw one ray, set the center on its end with 0 on the ray, mark 110 on the same row, then draw the second ray through the mark · Drawing works in a set order: one ray, protractor placed on its end, mark at 110 on the same row, then the second ray through the mark.
118° · An obtuse angle reads between 90° and 180°. Follow the row whose 0 is on the aligned ray. At this mark that row reads 118°, not 62°.
No ray was on 0, so the counting started from the wrong place · A reliable reading requires one ray on 0. With no ray on 0, the number at the crossing is not a reliable measure. A coincidentally correct number is possible, but the setup does not justify it.
40 · The row opposite your aligned 0 reads 140 at that mark. The row starting at your aligned 0 reads 180 minus 140, which is 40. So you drew a 40 degree angle.