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Grade 2 · Geometry

Bus-stop counters: trace all four boundary lengths

A rectangular plan for a transport model measures 10 units across and 3 units high. Change the controls and explain what the animation reveals.

Predict what will happen, press Play, then pause and explain what changed. Every control also works without playback.

Watch the relationship

Paused
Bus-stop counters: trace all four boundary lengths. Boundary traced: 0. Perimeter in length units: 26. Area in square units: 30Perimeter 26 · area 30103Boundary traced: 0 length units
The quantities are invented for this investigation. Controls change the model rather than real-world records. Displayed decimals are rounded; whole-number counts are exact.

Starts paused. Play once, pause anywhere, or use Step to inspect the mathematics. Playback stops when this panel leaves the screen.

Make it your experiment

Change one value. Notice what follows.

The controls adjust the model. Numbers below describe the current frame. Decimals are rounded.

Boundary traced
0
Perimeter in length units
26
Area in square units
30

HD animation studio

From experiment to screen.

Present a crisp Canvas scene, save a full-HD image, or capture your model as a silent video.

Understand what you are seeing

The idea behind the motion.

A rectangular plan for a transport model measures 10 units across and 3 units high. Perimeter is the complete boundary distance. Opposite sides of a rectangle are equal, so add both widths and both heights. Area instead measures the interior covering. The full boundary is 26 length units, while the area is 30 square units. These quantities answer different questions.

A relationship to keep

P = 2(10 + 3) = 26; A = 30

Read the symbols alongside the explanation. A diagram shows the relationship; the assumptions tell you when it applies.

  1. STEP 1

    Read the starting situation

    A rectangular plan for a transport model measures 10 units across and 3 units high. Identify what each number measures before changing a control; the worked example refers to these starting values.

  2. STEP 2

    Follow the mathematical change

    Trace the boundary without jumping across the interior. Pause at a corner and identify which lengths have already been counted. Pause on an intermediate frame and compare the picture with the live readouts.

  3. STEP 3

    Explain and test the relationship

    The full boundary is 26 length units, while the area is 30 square units. These quantities answer different questions. Change one input, predict its effect, and test whether the same relationship still works.

Your turn to explain

Make a prediction. Test your reasoning.

Bus-stop counters: use the original situation. A rectangular plan for a transport model measures 10 units across and 3 units high. Increase the width by one unit while keeping the height fixed. How much does the perimeter increase?

Use the values specified in the question. Reset restores the initial values for this investigation.

Compare your explanation

It increases by 2 units because both horizontal sides grow by one. The new perimeter is 28.

Work through a full lesson

Connect the animation to a worked example and practice questions.

Grade 2 · Geometry

Measure all the way around

Add boundary side lengths to find a polygon's perimeter.