From: "Jonathan Johnson" > to my current understanding(possibly incorrect as I've never gone to great > depths of the physics of gravity), but, basically gravity is a force > relative to an objects mass and its speed of motion, force creates > acceleration which gives a > speed....which brings me to....how do you get a speed of a force? You have a speed of a force field propagating out from an object. The force is caused by the interaction of this field and a mass. > acceleration due to gravity on earth is 9.8m/s/s correct? > but that is due to > earths mass and is thus different on diff planets, moons, asteroids etc. > drop an object in the gravitational field and it will accelerate towards the > body creating the gravitational field (in this case earth) at a rate in this > case equal to 9.8 m/s/s until it hits terminal velocity? tell me I'm not > wrong......please?: ) The accelleration due to gravity at the earth's surface is about 9.8m/S**2. An object dropped in this field will speed up by 9.8m/S every second, assuming no other forces on that object. Terminal velocity comes about due to one of those other forces, in this case friction with the atmosphere. The friction force increases with speed, so at some speed the upwards frictional force balances the downward gravitational force and the object ceases to accellerate. That is terminal velocity. Note also that 9.8m/S**2 is only at the earth's surface. It decreases as you get further from the planet. It would also decrease if you could move into the planet's interior because some of the mass would be above you producing a gravity field in the opposite direction. Gravity would be zero in the center of the planet because the gravity fields from all the mass particles of the planet would add (vector sum, of course) to zero. So, what would happen if you propelled something straight up (ignoring atmosphere friction)? It would decellerate at 9.8m/S**2. Eventually its speed would go to zero, then it would accellerate downwards, then come crashing down at the same speed it was launched upward at. However, this assumes that the accelleration due to gravity is constant. This is a good approximation when dealing with human-scale distances from the planet's surface. Now consider launching a projectile upwards fast enough so that it gets high enough (far enough away from the planet's surface) to experience the drop in accelleration due to gravity. It would go higher than what would be calculated with a constant 9.8m/S**2. So, can you launch a projectile so fast that it keeps going far enough to get to where the gravity is significantly smaller, so that it can keep going, which makes the gravity even smaller, etc, etc, even though the gravity field is infinite? Yes, that's what escape velocity is. If launched at escape velocity, the projectile would reach zero speed and have converted all its kinetic energy to potential energy at inifinity. Conversely, and object starting at a great distance to earth and falling due to gravity would hit the surface at escape velocity. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.