Lesson 9: Relative Motion and Frames of Reference
A frame of reference can be thought of as any spot your doing your measurement from as long as it is not accelerating. This is called an inertial frame of reference.

Relative motion is just a way of saying that sometimes different people will say different things about the motion of the same object.

Example 1: Let’s say I am standing on the back of a pickup truck (that is motionless), and I am throwing apples forwards. I know that I can throw an apple at exactly 15m/s every time.

In each of the above examples, we are really talking about two different people having two different frames of reference while measuring the relative velocity of one object.

Frame of reference: When you are standing on the ground, that is your frame of reference. Anything that you see, watch, or measure will be compared to the reference point of the ground. If I am standing in the back of a moving truck, the truck is now my frame of reference and everything will be measured compared to it.

Relative velocity: In the above examples, each person was measuring the velocity of the apples relative to (compared to) the frame of reference that they were standing in. Relative to a person standing on the sidewalk, the apple may be moving at 10m/s, while for a person in the frame of reference of the truck, the apple is moving at 15m/s relative to him.

Example 2: Sitting at your desk, how fast are you moving?

Example 3: You might have even noticed relative velocity while sitting at a red light…

Frames of reference and relative motion is actually the reason that people get car sick. Your brain is getting two different sets of information about your body's motion that might not exactly agree with each other; information from your eyes, and information from your inner ear. Some people are more sensitive to these differences, which causes them to feel car sick as they watch the road "whiz" by. If you are prone to getting car sickness, try to look forward at a point far in the distance and stay focused on that.