 Now, we throw around the word microgravity, quite a bit, microgravity, sort of a catchphrase term used by the National Aeronautics and Space Administration used by NASA, NASA depending on where you live, a lot, but what does it mean microgravity, is there just a little bit of gravity there, so from time to time we're going to have these bonus physics lectures, so let's have a look at this. We know from Newton that force is equal to gm1, m1, m2 divided by r squared, so what does it say, that's the force by gravity, and we know that's proportional to the product of the two masses, that's the mass of the Earth and the mass of the astronaut or space station or whatever, divided by the square of the distance between them. So if we have the Earth, there's the center of the Earth, from the center of the Earth to the surface of the Earth, that is the radius of the Earth and it is a staggering 6,371 kilometers from the center of the Earth to the surface of the Earth, and the international space station is another tiny little, very tiny little, 370 kilometers, 370 kilometers above that. So if we have our little astronaut stand on the surface, we're going to calculate the force of gravity on him, force of gravity on him by multiplying the gravitational constant with his mass or her mass, the mass of the Earth divided by this 6,371 kilometers squared. Okay, now let's use kilometers here, remember g is in meters cubed, okay, so we just have to convert this to meters, you can't work in kilometers and meters, but it's not what this is about, this is just a bit, now I'm going to increase this distance a little bit in orbit, yeah, and now he's on the international space station, so he's going to feel a different force of gravity, but he's still going to feel or she is still going to feel the force of gravity, it doesn't disappear, it's still there. Now you can do very simple algebra, you can say the force of gravity on the ISS versus the force of gravity while the astronaut is on the face of the Earth, now what is that going to be very simply, well, it's still going to be g, the masses of the individual and the Earth divided by, now it's the square that's going to be the radius of the Earth plus that height of the ISS, we're going to square that divided by still, this is still the same, gm1m2 divided by the radius of the Earth, because now he's just on the surface or she's just on the surface of the Earth squared, so all of these are going to cancel out, and what are we left with, well that's the radius just on the Earth squared, this is going to now be in the numerator and that in the denominator and the radius of the Earth plus the height of the ISS, if you just square that, and you see it's going to be about 0.89 somewhere there, the ratio of the force of gravity that the astronaut feels there versus there, so he's still going to feel this gravity but so why do we say there's zero g, why do we say there's zero g, well it's very simple, the ISS and the astronaut are in orbit, and orbit has different components to it, and very plainly there's a lot more physics to it, it is falling, it is falling it's falling at a rate that it keeps on missing the Earth, so it's falling towards the center of the Earth but it has a velocity tangential to the surface that all the time, so in its fall it's moving so fast that it always misses the Earth and it's more complicated than that, so if you were in an elevator very high up in the building, and lo and behold the cables break and you fall, you and the elevator are going to fall initially at least at the same velocity, well there will be a different bit of inertial mass difference there, so you're going to fall and you're going to experience weightlessness and you're going to float inside of this elevator and it's going to feel as if you are weightless, so it is that fall, that fall that creates this effect of zero gravity, certainly there is still a massive force, almost a full force of gravity on the International Space Station as they, but because you and the Space Station astronaut say Space Station is falling at the same velocity towards the center of the Earth, always missing it because there's more but it's free fall and there is no way that you can distinguish in the absence of your senses the difference between zero gravity and free fall, and that is why it's called zero gravity and that's where the catchphrase comes from, just to distinguish that it's called micro gravity probably means, well there's still gravity, there's still almost full gravity there, you haven't escaped the gravitational pull of the Earth, you can never ever do that, even if you approach the outer limits of the known universe there's still going to be some radius between the Earth, by the way you're still going to have mass, there's still going to be the gravitational constants, there's still going to be some minute gravitational force, anyway call it microgravity because in effect you are in free fall and we know from Einstein's thought experiments there's no way that you can distinguish free fall from being absolutely impossibly far away from any gravitational forces on you, that's quite phenomenal. Okay so zero G microgravity but there's obviously some physics behind it.