Rigid motion — Grade 8 Math, what it means and where it is taught
A move that keeps every length and every angle. Slides, flips and turns are all rigid.
Taught in Grade 8 Math.
Where it is taught
Practise this idea
3 questions in the course bank use Rigid motion, in Translations, Reflections & Rotations, Congruence & Sequences of Transformations.
Problem 1
A figure is moved so that every point travels 6 units right and 0 units vertically. Which is true of the image?
- It is congruent to the original
- It is larger than the original
- It is a mirror image of the original
- It has different angle measures
Worked answer
It is congruent to the original
A translation changes position only
Lengths and angles are unchanged
So the image is congruent
Problem 2
A square is rotated 45°. What is its new side length if the original was 6 cm?
Worked answer
6
Rotation changes position, not size
The side is still 6 cm
Problem 3
Which sequence guarantees the image is congruent to the original?
- Reflect, then translate
- Dilate by 2, then translate
- Reflect, then dilate by 3
- Dilate by 12, then rotate
Worked answer
Reflect, then translate
Reflections and translations are both rigid
Dilations are not, unless k = 1
So the first sequence is the only one