It's amazing how long it takes to complete something you are not working on. - McGee's First Law -

TO TALK ABOUT MOTION, WE HAVE TO DEFINE POSITION. POSITION REFERS TO THE LOCATION OF AN OBJECT. TO DESCRIBE THE POSITION OF AN OBJECT, WE NEED A REFERENCE POINT. FOR EXAMPLE, WE MIGHT SAY JAKE LIVES IN THE THIRD HOUSE ON THE LEFT PAST THE STOP LIGHT. IN THIS CASE, WE ARE USING THE STOP LIGHT AS A REFERENCE POINT. FOR WIDER AREAS, WE MIGHT USE THE COORDINATES OF A MAP – LONGITUDE AND LATITUDE.

LONGITUDE USES THE PRIME MEREDIAN AS A REFERENCE, AND LATITUDE USES THE EQUATOR. IF WE ARE PLOTTING POINTS ON A GRAPH, WE USE X,Y COORDINATES, AND THE ORIGIN IS OUR REFERENCE. AS PEOPLE STARTED VENTURING INTO SPACE, ASTRONOMERS STARTED WONDERING IF THEY COULD COME UP WITH AN ABSOLUTE REFERENCE POINT. THINK ABOUT IT. WHAT WOULD YOU USE?

THE EARTH IS ROTATING ON ITS AXIS – AT THE EQUATOR, THE SURFACE OF THE EARTH IS MOVING AT 1070 mi/hr OR 1670 km/hr. THE EARTH IS ORBITING THE SUN AT 108,000 km/hr OR 70,OOO mi/hr. THEN, THE SOLAR SYSTEM IS MOVING ABOUT THE CENTER OF THE GALAXY AT 720,000 km/hr OR 450,000 mi/hr. TO COMPLICATE THINGS FURTHER, SPACE IS EXPANDING. THE FURTHER OUT YOU GO, THE FASTER IT IS EXPANDING – 74.2 km/sec/mpsec. (about 3 million light years).

FRAME OF REFERENCE GSklU

IF AN OBJECT IS CHANGING POSITION RELATIVE TO US OR TO ANOTHER REFERENCE, WE SAY IT IS IN MOTION. WE CAN DESCRIBE THAT MOTION USING THE FUNDAMENTAL UNITS OF LENGTH AND TIME. TIME RATE OF CHANGE OF POSITION – EITHER SPEED OR VELOCITY. WE SOMETIMES USE THEM INTERCHANGEABLY, BUT THEY HAVE DIFFERENT MEANINGS.

SPEED IS A SCALAR QUANTITY – IT ONLY DESCRIBES MAGNITUDE, E.G., km/hr, km/sec, mi/hr – YOU HAVE ONLY A NUMERICAL VALUE (magnitude) AND A UNIT. VELOCITY IS A VECTOR QUANTITY – IT INCLUDES MAGNITUDE (numerical value and a unit) AS WELL AS DIRECTION. VELOCITY CAN BE REPRESENTED GRAPHICALLY USING ARROWS WITH THE LENGTH OF THE ARROW REPRESENTING SPEED AND THE DIRECTION OF THE ARROW REPRESENTING DIRECTION.

AVERAGE SPEED IS THE TOTAL DISTANCE TRAVELED DIVIDED BY THE TIME SPENT TRAVELING IT. YOU CAN THINK OF DISTANCE AS THE ACTUAL PATH TRAVELED. speed = distance/time =  d/  t INSTANTANEOUS SPEED WOULD BE THE ACTUAL SPEED AT ANY GIVEN MOMENT (THE TIME INTERVAL WOULD BE VERY SMALL). THINK OF IT AS THE SPEED YOU WOULD READ OFF A SPEEDOMETER.

A B DISPLACEMENT DISTANCE DISPLACEMENT IS THE DIFFERENCE IN POSITION BETWEEN INITIAL AND FINAL POSITIONS. DISTANCE IS THE ACTUAL PATH TRAVELED BETWEEN INITIAL AND FINAL POSITIONS.

VELOCITY IS SIMILAR TO SPEED, BUT A DIRECTION IS INVOLVED. AVERAGE VELOCITY IS THE DISPLACEMENT DIVIDED BY THE TOTAL TRAVEL TIME. ALSO, A DIRECTION MUST BE INCLUDED. v =  d/  t Note: the bar over the v means average. IN THE PREVIOUS DRAWING, THE DISTANCE FROM A TO B MIGHT BE 35 mi. IF THIS WERE TRAVELED IN 1 HOUR, THE SPEED WOULD BE 35 mph. THE DISPLACEMENT MIGHT BE 25 mi, SO THE VELOCITY WOULD BE 25 mph northeast.

DISPLACEMENT VERSUS DISTANCE 1PXyY DR. SKATEBOARD’S VIDEOS ON MOTION ZEeHEA5CJmhOMKdnG5MnFT8L1UV36Gc

PROBLEM: THE SPEED OF LIGHT IN A VACUUM IS 3.00 X 10 8 m/s. HOW MANY SECONDS DOES IT TAKE LIGHT FROM THE SUN TO REACH THE EARTH IF THE DISTANCE IS 1.50 X 10 8 km (average)? speed = c = d/t