Finding Perimeter of Classroom Objects in Small Groups
About this lesson
In this 45-minute Grade 4 lesson, children work in groups of three to estimate, measure and add up the side lengths of rectangular things around the room, like desk tops and notebooks. They learn to find perimeter in centimetres, check answers using equal opposite sides, and compare results with another group. You'll need rulers or metre sticks, paper, pencils, chart paper and markers.
- Grade
- 4
- Subject
- Math
- Time
- 45 min
Objective
Students will apply addition and measurement skills to calculate the perimeter of rectangular classroom objects in centimetres and compare their results with a partner group.
Materials
- Rulers (30 cm) or metre sticks
- Pencils
- Blank paper or recording sheets drawn on plain paper
- Chart paper and markers
- Whiteboard and markers
Hook
Hold up a book and ask, 'If an ant walked all the way around the edge of this cover, how far would it travel?' Have pairs whisper an estimate in centimetres to each other. Record several estimates on the whiteboard, then explain that today groups will find the exact answer for objects around the room.
Main Activity
Model how to find perimeter on the whiteboard: measure each side of a book to the nearest centimetre, label the sides on a sketch, and add them together. Point out that opposite sides of a rectangle are equal, so students can check their work. Place students in groups of three and assign each member a role: measurer, recorder, and checker, rotating roles for each object. Each group chooses four rectangular objects (such as a desk top, notebook, tissue box face, or door window), estimates the perimeter, measures and labels each side on a sketch, and calculates the actual perimeter. Two groups then pair up to compare results for any objects they both measured and talk through any differences until they agree. Finally, each group writes its most interesting object, estimate, and actual perimeter on a shared class chart paper.
Discussion Questions
- Why did some groups get slightly different perimeters for the same object, and how did you decide which answer was correct?
- How did knowing that opposite sides of a rectangle are equal help your group work faster or check your answer?
- Which of your estimates was closest to the real perimeter, and what helped you make a good estimate?
- When might someone in real life need to know the perimeter of something?
Exit Ticket
Draw a rectangle that is 12 cm long and 5 cm wide. Label all four sides, find its perimeter, and write one sentence explaining how you found it.
Differentiation
Support: Give struggling learners a pre-drawn rectangle template on which they write each measured side length in labelled boxes, followed by an addition sentence with four blanks (___ + ___ + ___ + ___ = ___). Pair them with a supportive checker, and have them begin with small objects that measure to whole centimetres.
Extension: Challenge advanced learners to find the perimeter of an L-shaped or irregular object or area, such as two desks pushed together in an L shape. They can also find two different rectangles that share the same perimeter and explain how this is possible.
