High school geometry
Positive rotation angles mean we turn counterclockwise. Negative angles are clockwise. We can think of a 60 degree turn as 1/3 of a 180 degree turn. A 90 degree turn is 1/4 of the way around a full circle. The angle goes from the center to first point, then from the center to the image of the point.
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- So just to confirm what Sal briefly explained in the video, a positive number of degrees means you rotate counterclockwise, while a negative number of degrees means you rotate clockwise, correct?(86 votes)
- Seems a bit un-intuitive but since the 1st quadrant starts from right and goes to left, adding positive number of degree means you go towards left i.e. you rotate counter-clockwise and vice versa.(47 votes)
- Are there formulas to do 90,180,270, rotations around a point that is not the origin?(13 votes)
- Four years late, but here it is.
Yes, there are. I was learning about this to help with a diagnostic. All you have to do is make the point you´re rotating around the new origin. You might have to tilt the whole coordinate plane, or even chop off parts of it and make new areas.(7 votes)
- Is rotating basically just eyeballing the correct angle?(13 votes)
- For these exercises, it is my impression that you are best served by eyeballing.
If you want to get more precise, you would use an instrument that measures angles (the most common example is a protractor) and verify that your point-to-point mappings satisfy the rotation angle requirement. You would also want to make sure that distances from the point of rotation are the same. Most protractors also have a limited ruler along the edge.(9 votes)
- Why do you have to rotate counterclockwise if the angle is
positive, and clockwise if the angle is negative?(10 votes)
- It goes back to the unit circle where 0 degrees is along the positive x axis. All our angles greater than 0 and less than 90 (positive y axis) are in the first quadrant (positive x, positive y), so this is that counterclockwise rotation.(14 votes)
- Why is that when he was talking about going to the positive direction, he went left, and when he was talking about the negative direction he when right. Shouldn't it be the opposite way around?(6 votes)
- A positive rotation is in the counterclockwise direction. With unit circle theory, the positive x axis is 0 degrees, so rotating into the first quadrant gives positive values for sin and cos which make best sense for angles between 0 and 90.(9 votes)
- How do we know whether to go counter-clockwise or clockwise as it didn't say so in this question...
EDIT: Saw the video again and am now confused on why positive degrees means counter-clockwise and negative degrees mean clockwise! I mean it feels more satisfactory for the positive degrees to be clockwise and the negative degrees to be counter-clockwise!(9 votes)
- Anti-Clockwise for positive degree. Clockwise for negative degree.
For your second question, it is mainly a conventional that mathematicians determined a long time ago for easier calculation in various aspects such as vectors. Product of unit vector in X direction with that in the Y direction has to be the unit vector in the Z direction (coming towards us from the origin). 
There is also a system where positive degree is clockwise and negative degree anti-clockwise, but it isn't widely used.
There are many different explains, but above is what I searched for and I believe should be the answer to your question. I included some other materials so you can also check it out.
- Why would POSITIVE 60 mean that your going counterclockwise?(5 votes)
- If you think about the positioning of the quadrants, it goes counterclockwise. That is probably why it goes counterclockwise with a positive. I DO NOT KNOW IF THIS IS RIGHT, so please ignore this post(6 votes)
- am i the only future eight grader doing this just so I can go ahead.(6 votes)
- Sal: Makes an explainer video to explain rotation
Sal: Doesn't explain rotation.
Positive rotation is going in the negative/counter-clockwise direction. You forgot to actually explain that, haha.(7 votes)
- [Instructor] We're told that point P was rotated about the origin (0,0) by 60 degrees. Which point is the image of P? Pause this video and see if you can figure that out. All right, now let's think about it. This is point P. It's being rotated around the origin (0,0) by 60 degrees. So if originally point P is right over here and we're rotating by positive 60 degrees, so that means we go counter clockwise by 60 degrees. So this looks like about 60 degrees right over here. One way to think about 60 degrees, is that that's 1/3 of 180 degrees. So does this look like 1/3 of 180 degrees? Remember, 180 degrees would be almost a full line. So that indeed does look like 1/3 of 180 degrees, 60 degrees, it gets us to point C. And it looks like it's the same distance from the origin. We have just rotated by 60 degrees. Point D looks like it's more than 60 degree rotation, so I won't go with that one. All right, let's do one more of these. So we're told point P was rotated by negative 90 degrees. The center of rotation is indicated. Which point is the image of P? So once again, pause this video and try to think about it. All right, so we have our center of rotation, this is our point P, and we're rotating by negative 90 degrees. So this means we are going clockwise. So we're going in that direction. And 90 degrees is easy to spot. It's a right angle. And so it would look like that and it looks like it is getting us right to point A. So this is a negative 90 degree rotation right over here. Gets us to point A.