# Acceleration

Acceleration is the rate of change in velocity for an object, with respect to time. The phrase "change in velocity" can mean a change in direction as well as a change in speed, because velocity is both a direction and a speed. Acceleration is due to a net force acting upon an object, as described by Newton's Second Law.

A net force causes an acceleration. For example, if I push on a block that is not locked down, it starts to pick up speed for as long as I push on the block. Furthermore, steady movement isn’t an acceleration. That’s merely constant velocity. It’s when you have more and more (or less and less) movement that you have an acceleration (a change in velocity). Also "net force" is the force outstanding among combined forces. For example, before, when I pushed the motionless block, my lone shove served as the net force. But if I push on the same block from the left and right equally, then the net force would actually be zero. This is because the two opposing forces cancel, combining to equal zero, and there is no observed acceleration.

Acceleration due to gravity comes about due to the force of weight. An object’s weight is figured by taking the acceleration due to gravity times the mass of the object. In this way, we see that gravity produces the force of weight even while the object is falling or standing motionless on some surface. Keep in mind, acceleration due to gravity is merely the stated change in speed that an object experiences when free to do so. This stated quantity doesn’t disappear just because the object has come to rest on the surface of Earth.

For example, the effects of gravity are present when an object stands motionless on a shelf. There is a force provided by gravity, called weight, and, surprisingly enough there is also an opposing force provided by the shelf called a "normal force". The force caused by the shelf is analogous to a mirror image of the one caused by gravity, just so long as both objects remain in contact with one another. These equal and opposite forces cancel each other such that we see no change in movement: just like any object being pushed equally from both sides would.

The shelf obviously prevents the object from falling. If the shelf were removed, though, the force provided by the shelf would be gone. And once that happens, there is nothing preventing gravity from doing what it wants to, and we start to see the movement imparted by gravity as the object falls. At that time, we are able to witness the acceleration due to gravity.

Until this point, however, it’s important to note that the acceleration due to gravity continues to exist as an active mathematical component of the force of weight. This is because gravity works to produce weight for objects resting on the surface of Earth. Surprisingly, when the object is in freefall, the force of weight continues to exist then too. This is because weight is calculated as the acceleration due to gravity times mass. And since both the mass and the acceleration due to gravity continue to exist, there remains a force of weight even as the object falls. It is only the sensation of the force of weight that is gone while falling. Therefore, in weightless conditions during freefall anywhere near Earth the force of weight is still present.

Obviously, the gold we’re looking at has Earth’s gravity acting upon it. It is for this reason that gold value is typically stated by weight. But, I refer to gold in terms of mass during my calculations because any scales that display grams are displaying mass. And, during the course of my calculations, it seemed easier to speak in terms of mass rather than taking the additional steps necessary to convert mass into weight, or vice versa.