Speed, Velocity and Weightlessness
Distance and displacement are two quantities that may seem to mean the same thing, but they have distinctly different definitions and meanings.

Distance refers to how much ground an object has covered during its motion. It has a numeric value.

Displacement refers to how far out of place an object is. It is the object’s overall change in position described with a numeric value and a direction.
Just as distance and displacement have distinctly different meanings (despite their similarities), so do speed and velocity.

Speed is how fast something is moving or how much distance is covered in a certain amount of time. There are two types of speed: instantaneous speed, which is an object’s speed at any given movement and average speed, which is the average of all instantaneous speeds. To calculate speed, you divide the total distance by the time: s = d/t.

Velocity is the same as speed but it has a direction associated with it. Speed has no specific direction.
It is possible to have a negative velocity. This means that the object is going backward. It is not possible to have a negative speed because there is no specific direction.
Vectors are units with a direction associated with them.
Free fall is objects falling unaffected by air resistance. On Earth, we can’t fall without air resistance. When things fall, they constantly accelerate until they reach terminal velocity, or the velocity at which the upward force of air resistance equals the downward force of gravity. Once you reach this velocity, you will no longer accelerate but will just fall at the same rate.
The formula for determining the distance something will fall is d = 1/2 a t^{2}, where “d” is the displacement it falls, “a” is acceleration, which on Earth is 9.8 m/s^{2}, and “t” is time. Everything accelerates at the same rate if we can neglect air resistance. Things that affect air resistance include the shape of an object (the more surface area means more air resistance.), the velocity (the faster you go, the more air resistance.) and the “thickness” of the air (there is less air resistance the higher you go up in the atmosphere.).
Weightlessness on the International Space Station has to do with the frame of reference. If everything is falling at the same rate, it appears as though an object is floating with no gravity. If you put a penny on your knee and go on an amusement park ride such as the Demon Drop at Cedar Point, the penny will “float” in front of you, at least from your frame of reference. It actually is falling at the same rate that you are. If you think about being in the space station, everything is falling with you, so you can’t tell that you are falling. 