
Words used in this website that need to be defined Speed and velocity: Imagine that you own a frictionless tape measures and a digital watch. It's an otherwise simple tape measure in that it's only marked in feet, and your digital watch beeps once every second. Now imagine that you're holding the body of this tape measure and one of your friends (Mike) has a hold of the free end of this tape measure. You ask Mike to start running and you look down at the part of the tape measure where the tape meets the body. You notice that every time your watch beeps that twelve more feet have been pulled out of this tape measure. So, Mike's speed, with respect to you, is 12 feet per second. Now you hand the watch and the body of the tape measure to another friend (Donna). Then you ask Donna to start walking in the direction that Mike is running. Donna starts walking and then screams out that she only sees seven feet of tape being pulled out for every second that beeps on the watch. So, Mike's speed, with respect to Donna, is 7 feet per second. It is obvious what speed is  it is change of distance for a given amount of elapsed time. It is very important to realize that speed MUST be measured from one object to another object. There is no such thing as "absolute" speed in our universe. Velocity is the same as speed except that a direction is implied when one uses the word velocity  whereas when you just say speed its object's direction (with respect to the other object) is not specified. Mass: Mass is a property that is intrinsic to some objects and is a measure of object's with mass resistance to a change in speed due to an applied force. One examples of an object that has mass is a human being. A person's mass is constant throughout the universe (at any given time in a person's life). Even though a person's weight might show up differently on a bathroom scale if they use the scale on Earth and the Moon, that person's mass is the same everywhere in the universe. Something with mass is sometimes referred to as "a mass". Force: Since this one of the target entities of discussion in this website, any definition here will appear to be self serving. I am very aware of this and will attempt to do my best. Sir Isaac Newton described force as being equal to the change in an object's mass times its velocity  both with respect to a change in time. Written as a differential equation, F=d(mv)/dt  where F is the applied force (which includes its direction), m is the mass of the object that's being forced, v is the velocity of that object with respect to something else (which includes its direction), and t is time. We will see that this definition is still valid, even after 300+ years, but it must be taken in the proper context. However, since this website doesn't deal with math (only concepts) then here is a conceptual definition: Force is something that a human being can feel if it's being applied to them. For example, if someone pushes you on the shoulder then you feel a force on your shoulder and the pusher feels a force (in the opposite direction) on their hand. Photon: A photon is an example of an object that has zero mass, and because of this they can and do travel at the speed of light. Visible light is made of photons. The stuff we call "radio waves" are made of photons. Radiated heat is made of photons. Xrays are photons. Gamma rays are photons. Another name for photons is electromagnetic radiation. Spacetime: Space can be thought of as having three separate dimensions:  left/right  forward/backward  up/down However something else is tied to space and that is time itself. You can remain fixed in space (from your point of view), but time is always progressing. For this reason, when discussing our universe in scientific terms, time has been incorporated as a fourth dimension. Space and time, as an entity, are referred to as "spacetime". However, for the purposes of this website, one can just think of space when the word spacetime is used. Geodesic: A geodesic is a mathematical name for a curve that follows a locally straight line through a curved space. For example, on the surface of a sphere, geodesics are great circles  that is, they're the shortest path you can find between two points on that sphere. As they apply to the discussion of gravity, in spacetime geodesics can be thought of as a "natural" path, or a path of "least resistance". There will be more discussion of this in the website because the concept of a geodesic is so important in the discussion of gravity. 
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