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Delta, usually represented by the Greek letter of the same name in mathematical equations, refers to change; thus, delta-V represents a change in the quantity of velocity. Delta-V is a popular quantity used in orbital mechanics, for it takes a specific change in velocity to get from one place in space to another. Delta-V is not an idea it is a manifestation of Classical Newtonian physics. It is tied directly to the inertial properties of a mass. Let’s think of you on a train, facing the front of the train. If you were to throw a baseball at 25 mph while the train is at rest, not moving, it would move at 25 mph; however, if you were to throw that same baseball in the same direction while the train is moving, the speed (or velocity) of the ball would be that of the train plus that of your throw. For example, if you throw a ball at 25 mph, and the train is moving at 50 mph; the ball is moving at 75 mph. The Earth has the same effect on a rocket. If you think of the launch pad as a point on the surface of the Earth, then that point has a velocity equal to the distance it travels in 24 hours (the period of the Earth) divided by 24 hours (remember, velocity is distance divided by time). Since a point on the equator must move a greater distance than a point north or south of the equator (i.e., the distance traveled by a point offset from the equator is shorter), it follows that a point on the equator must move faster. We know that the Earth rotates eastward; thus, if we launch in an easterly direction we get an additional velocity equal to the rotation of the earth at the launch site. This velocity is free since the Earth imparts it without the need to put energy into the system. This velocity is not put on the vehicle at any specific time during flight since it is an initial condition in the launch equation. The rocket, launch vehicle, starts with this given amount of inertia. Thus, the rocket launching in the direction of the Earth’s rotation is the same type of motion that the baseball on the train follows. Any further questions, feel free to ask! Dennis DKVanGemert@alumni.usc.edu www.madsci.org home.att.net/~dkvangemert

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