Perimeter and Turns
What this lesson covers
- A polygon is a closed shape with straight sides.
- Perimeter is the distance all the way around a shape.
- A turn spins around a point.
- Clockwise turns to the right, like clock hands; anticlockwise turns to the left.
What is a polygon?
- A polygon has straight sides that join up to close the shape.
- Triangles, squares, rectangles, pentagons and hexagons are polygons.
- A circle is not a polygon, because its edge is curved.
Try it: polygon or not?
Perimeter is the distance around
- Imagine walking all the way around the edge of a field.
- That distance is the perimeter.
- For a polygon, measure every side, then add them up.
Measure each side, then add
- A rectangle 5 cm long and 3 cm wide has four sides: 5, 3, 5 and 3.
- 5 + 3 + 5 + 3 = 16 cm.
| Shape | Sides | Perimeter |
|---|---|---|
| rectangle | 5 cm, 3 cm, 5 cm, 3 cm | 16 cm |
| triangle | 4 cm, 4 cm, 4 cm | 12 cm |
| square | 6 cm on each side | 24 cm |
Try it: change the sides, watch the perimeter
Turning around a point
- A turn spins around a point, like a clock hand around the middle of the clock.
- A full turn goes all the way round, back to where it started.
- A half turn faces the opposite way. A quarter turn faces the side.
Clockwise and anticlockwise
- Clockwise turns the same way as clock hands: to the right.
- Anticlockwise turns the other way: to the left.
How far did it turn?
- A clock hand starts at 12. Turn it and see where it ends.
| Turn | Ends at |
|---|---|
| quarter turn clockwise | 3 |
| half turn clockwise | 6 |
| three-quarter turn clockwise | 9 |
| full turn | 12 |
| quarter turn anticlockwise | 9 |
Maths blocks
Common mistakes
- The perimeter of a 5 cm by 3 cm rectangle is 8 cm.That counts only two sides. A rectangle has four: 5 + 3 + 5 + 3 = 16 cm.
- A circle is a polygon.Polygons have only straight sides. A circle is curved, so it is not a polygon.
- A quarter turn clockwise and a quarter turn anticlockwise end in the same place.They go opposite ways. From 12, clockwise ends at 3 and anticlockwise ends at 9.
Your turn
1
A triangle has sides of 3 cm, 4 cm and 5 cm. What is its perimeter?
2
A square has sides of 2 cm. What is its perimeter?
3
Is a circle a polygon?
4
A clock hand points at 12. It makes a half turn clockwise. Where does it point now?
5
You face the door and make a full turn. What do you face now?
Show answer
1. 12 cm 2. 8 cm 3. No, its edge is curved 4. 6 5. the door
What to take away
- Polygons have straight sides and are closed.
- Perimeter: add every side.
- Quarter, half, three-quarter and full turns.
- Clockwise is to the right; anticlockwise is to the left.
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