Gravity, Weightlessness, and Apparent Weight

In my classes, I like to bring up the question:

*Why do astronauts float around in space?*

The most common response to this question is that they float around because there is no gravity in space. Some people take this a small step further and say that there is no gravity in space because there is no air in space. This is why they claim there is no gravity on the moon (even though there is – more on this later).

I like to start off with the concept of gravity. Gravity is an attractive force between any two objects with mass. Your pencil and your dog both have mass so there is a force pulling your dog and your pencil (that is if you have a pencil) together. This force turns out to be extremely small. So small that you would never notice it. However, if one of the masses is very large, it is noticeable. An expression for the gravitational force was first determined by Newton. He came up with the following (turning off vector notation for simplicity).

![Page 25 1](

Where G is the universal gravitational constant, m1 and m2 are the two masses in the interaction and r is the distance between their centers. This force (as are all forces) is really a vector that points from one mass to the other.

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Unit for gravitational potential – the Golt

Today I was talking about electric potential. My favorite analogy for electric potential energy is gravitational potential energy. But electric potential is something different. Electric potential (commonly called potential) can be defined as:


So, V is the electric potential in units of Joules per Coulomb or Volts.

What about gravitational potential? I am sure some astrophysicist use gravitational potential. Maybe they even have some units for it, but I have never seen it. My students asked me if there was such a thing as gravitational potential. I said, sure. Here it is:


I picked the letter Y, why? Why not? The units for gravitational potential would be Joules per kilogram. I think this should be called the “golt”.

Amazing Blob Jump Launch Video Analysis

Can you believe it? Have you seen this video?

Are you thinking what I am thinking? WOW. How could these people not follow my rules for cool internet video. Once again, here they are:
1 Keep the camera stationary. This way I don’t have to keep moving the origin in the movie.
2 Don’t Zoom. Same reason, this video followed that rule.
3 Include a clear and obvious calibration object. A meter stick would work, or even a Kobe Bryant (I can look up his height). Maybe it could be a Ford F-150 that has a known length. Something!
4 Include the mass and height of all people involved.
5 Use high quality video.
6 Don’t talk about fight club – oh wait, wrong list.

Despite failure to follow all these rules, I have managed to analyze this video. Really when I saw it, I said “wow” – was that real? It looked real, but who would get shot up that high? (it is on, so fake is a possibility).

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Physics of Linerider Part II: Scale

Scale of the Line Rider

First, we assume that the line rider is on Earth and for low speeds will have a free-falling acceleration of 9.8 m/s2. Next, an arbitrary distance is selected. In this case the length of the sled is chosen to be 1 LU (Linerider Unit).

![line rider](

The goal will be to put the linerider in a free fall (where air resistance should be able to be ignored) and determine his (it could be a she, it is difficult to tell) acceleration in LU/s2. Then we can determine the conversion factor from LU/s2 to m/s2.

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How about a massive catapult to replace the space shuttle

I recently saw a comment on a blog somewhere about putting satellites into space (I think it was from a post about a rocket that blew up). The poster suggested using a giant catapult to put things in space instead of rockets. Maybe he or she was kidding, or maybe not. But I have heard this idea before. Would it work?

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Mythbusters Moon Stuff and Me

Clearly the MythBusters did this before I did (it’s just air tonight though). I just wanted to say that I posted some videos of the Apollo “jump salute” video analysis and also sped it up to “Earth-like” accelerations. I then made a video of my daughter doing the same thing on Earth and slowed it down. Yes, the MythBusters did it better, but I just wanted to say “me too”. My analysis is here:
[Undoing Fake Moon Landing Videos](

P.S. I also slowed down a video of Kobe Bryant Jumping so he has an acceleration of that on the moon.