About this video
Sal rotates a line segment by negative 90 degrees around the origin using a rotate tool. A negative angle means turning clockwise, so the arrow that straddled the x-axis ends up straddling the negative y-axis. The image is still a line segment, with the same length as the original.
You'll be able to
- explain that a negative rotation turns clockwise.
- rotate a line segment 90 degrees about the origin.
- explain why a rotated segment has the same length.
Key words
- rotation
- Turning a figure around a fixed point by a given angle.
- origin
- The point (0, 0) where the x- and y-axes cross.
- clockwise
- The same direction that clock hands turn; negative angles go this way.
- line segment
- Part of a line with two end points.
Transcript
We're asked to rotate the line segment negative 90 degrees around the origin. So this is our line segment. We want to rotate it around the origin. So let's use our Rotate tool. So let's define our point of rotation. So we're going to rotate around the origin so that's why I put it there.
And I'm going to rotate it negative 90 degrees. So I'm going to rotate it in this direction. So let's see if I can pull this off. So actually, let me undo what I just did. Let me get my Rotate tool back here. So negative 90 degrees. So these arrows right over here, right now they're straddling the x-axis.
If I rotate negative 90 degrees, these arrows, this double-headed, curvy arrow should exactly straddle the negative y-axis. So let's make sure I do that. So this is the negative y-axis. There we go. So I have rotated this line segment negative 90 degrees about the origin.
And now we've got to answer some questions. When a line segment is rotated, the image created is a, well, it's, of course, a line segment. We know this is true because the rotated image has two end points. The rotated image is the same length as the original image.
We haven't changed the length of this, we just rotated it around this dot.
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