Checking Your References Relative Motion

How would Homer know that he is hurtling through interstellar space if his speed were constant? Without a window, he wouldn’t! All of the Laws of Motion apply within his FRAME of REFERENCE

Do you feel like you are motionless right now? ALL Motion is RELATIVE ! The only way to define motion is by changing position… The question is changing position relative to WHAT?!? You are moving at about 1000 miles per hour relative to the center of the Earth! The Earth is hurtling around the Sun at over 66,000 miles per hour! MORE MOTION!!!

Example #1 A train is moving east at 25 meters per second. A man on the train gets up and walks toward the front at 2 meters per second. What is his velocity? –Depends on what we want to relate his speed to!!! +2 m/s (relative to a fixed point on the train) +27 m/s (relative to a fixed point on the Earth) v train = +25 m/sv person = +2 m/s

Example #2 A passenger on a 747 that is traveling east at 230 meters per second walks toward the lavatory at the rear of the airplane at 1.5 meters per second. What is the passenger’s velocity? –Again, depends on how you look at it! -1.5 m/s (relative to a fixed point in the 747) m/s (relative to a fixed point on the Earth)

Non-Parallel Vectors What happens to the aircraft’s forward speed when the wind changes direction? v thrust No wind – plane moves with velocity that comes from engines v wind Wind in same direction as plane – adds to overall velocity! Wind is still giving the plane extra speed, but is also pushing it SOUTH. Wind is now NOT having any effect on forward movement, but pushes plane SOUTH. Wind is now slowing the plane somewhat AND pushing it SOUTH. Wind is now working against the aircraft thrust, slowing it down, but causing no drift.

Perpendicular Kinematics Critical variable in multi dimensional problems is TIME. We must consider each dimension SEPARATELY, using TIME as the only crossover VARIABLE.

Example A swimmer moving at 0.5 meters per second swims across a 200 meter wide river. 200 m v s = 0.5 m/s How long will it take the swimmer to get across? t =0 The time to cross is unaffected! The swimmer still arrives on the other bank in 400 seconds. What IS different? Now, assume that as the swimmer moves ACROSS the river, a current pushes him DOWNSTREAM at 0.1 meter per second. v c = 0.1 m/s The arrival POINT will be shifted DOWNSTREAM!