here the car which was moving along in this direction with the velocity we were thinking on the card at a table here i am over on the side and uh... doctor doom was on the side and we were pushing back-and-forth on the table well i pushed it and went into this direction with the velocity v when doctor didn't want to do when in this direction the same velocity v but this is the velocity relative to the car what about the velocity relative to an observer on the ground in the picture brain well if it was pushed in this direction it's velocity to you plot being if it's in this direction it will also be used u minded b this is all very reasonable is nothing very hard to understand here the surprising thing about this expression is that it is not actor in all circumstances at very high speed by high speed i mean speeds close to the velocity of light expression breakdown at these very high speed we have to use the ideas about relative motion developed by a albert einstein in his special theory of relativity for all the speed of the we're ever likely to run into this expression you plus or minus the is completely adequate so far we've been talking about frames of reference moving at a constant velocity relative to one another now i'm going to do the experiment with the dropping ball again only that time the karte will be accelerated relative to the earth frame these weight will fall and give the cart a constant acceleration first of all out and then i'll really put the motion is very fast and i want you to watch at the point where the ball is released from the pic camera i don't know whether you thought that are not at the top of the vault with the things that was before for either prime it landed in a different spots this is because the cart kept on accelerating in this direction as the ball was falling now i'm going to let you see it again with the slow-motion camera fixed on to the car