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### S1. Frame of reference

“In order to describe the motion of moving bodies, we need to state where the object is at any given time. But to state where an object is, we need to measure its position relative to something else, right? So we need a reference point from which to define the position of objects. Once we have chosen such a point, which is called the origin, we can specify the position of the object by saying, for instance, that the object is distance x to the east, distance y to the north, and distance z up from the origin. We also need a clock so that we can specify at what time t the object was at the given position.1

When we have the origin and the directions in which to measure the distance from the origin set up, and a clock to measure the time, we say that we have a frame of reference or simply a frame.

One interesting Question regarding frame of reference is to consider the following.

Sitting at your desk, how fast are you moving???
• Relative to the ground: Zero. You’re not moving relative to the frame of reference of the ground.
• Relative to the sun:  $2.97\times10^{4}$ m/s! That’s a pretty big difference, but since the Earth is orbiting the sun at this speed, an observer standing on the sun (ouch!) would say that you are moving at $2.97\times10^{4}$ m/s.
• who's correct?
• ans: Both of these answers are correct but in their own frame of reference.
“A frame of reference can be thought of as any spot you are doing your measurement from as long as it is not accelerating.”

Solve the problem in this website for fun.