Lets say you are in a car going 60 mph and ahead of you is a flat bed truck (that you can drive on top) that is going 50 mph. There is someone standing along the road that can measure your speed (like with a laser gun, say.) Now, as soon as you drive up on the truck, how fast are you going relative to the truck and to the ground? Your tires should immeditely "see" the truck as the ground and since the tires are going around at 60 mph at that instant, wouldn't it follow that you would go flying of th truck at 60 mph? If that is the case, wouldn't the person on the ground suddenly measure your speed at 120 mph? Obviously this can't happen, but nontheless, the tires ARE going 60 mph at the instant it touches the truck and the truck is stationary as far as those tires are concerned, because how do the tires "know" what the truck speed is relative to the road? But lets say that as soon as you go on the truck your speed is 10 mph because the truck was going at 50 relative to the road. What, then, does your speedometer say at that very instant? Does it immediately decelerate to 10 mph? If this is the case, does the driver immediately experience a decelertion feeling from 60 to 10 mph? If there is no deceleration, and the speedometer still says 60 but you are really only going 10 relative to the truck, then that implies that the car is still communicating with the ROAD frame of reference (because there is no deceleration). In other words, when you go from the road to the truck, you are instantly changing the frame of reference for the car. SO: which is it? If the car's frame of reference is now the truck, you should decelerate to 10 mph and the speedometer shold reflect that OR f the frame of reference is still the road you you hould fly off the truck at 120 mph! Can someone help to resolve this apparent pardox?