These 3 pictures were all taken of the ruler and the wall scale I used when I was firing eleastic bands into the air. I didn't move the ruler at all - I just changed the angle I was looking at it. If I look at it from a bad angle, I will get bad readings. This problem is called parallax error. The problem is caused by the gap between the ruler and the wall. To solve the problem, we have to find ways to make sure we are looking at things from the correct angle, which is 90 degrees to the scale.
