frame of reference

Pouring tea in a plane - upside down

Let me start with the video. Here is a guy flying a plane in a barrel roll and pouring some tea at the same time. Talk about multitasking. How can he pour upside down? Well, there are two ways to look at this. First, I can look at this in the frame of the plane. For this case, I can invoke the fake force - centrifugal force. Oh yes, I am going to do it. You probably remember all your physics instructors warning you to never ever do this. Well, they say that because they are afraid you will do something bad with it. Here, I will only use the centrifugal force for good. What is the…

Hey, look kids! There's Big Ben.

And there's parliament. Ok - sorry, I had to make a "Tom (Swans on Tea)" title for this one. Tom, forgive me. Here are two great circular motion videos. First, this one is from Dale Basler. He made himself a fine little floater-type accelerometer. Better than just make it, he made a video of the accelerometer in his car going around a round about. Check it out. Bobber Meets Roundabout from Dale Basler on Vimeo. So, if he is driving at a constant 10 mph, how big is the round about (traffic circle)? Next video - more silly kids First, I saw this one on ZapperZ's Physics and Physicists who…

MythBusters - velocity is relative

So, I complained about MythBuster's explanation of relative velocity. How would I explain this? I would start by saying that velocity is relative. Here is the definition for velocity: I put the "avg" in there because it is more true. If the acceleration is zero, I could drop this. For the rest of this post, I am going to assume zero acceleration. Ok. But what is the r vector? It is simply a vector from the origin to the object. Here is a picture. Simple, right? And so the velocity tells how this vector r changes. But wait. Who says that I used the correct origin? How do you…

Tides. Why are they so hard?

Every introductory astronomy text and most intro physics texts talk about tides. The usual explanation is something along the lines of: The moon exerts a gravitational force on the Earth and all the stuff on the Earth. This force decreases with distance (1/r2). Thus the moon pulls greater on one side of the Earth than the other This doesn't matter except for oceans which can move. BOOM. Two tides a day due to a bulge on the side close to the moon and the opposite side. Oh, the Earth is slowing down. Really, that is what almost all intro texts say. Go check for yourselves. Yes, the tides…

Fake Forces - sometimes they are Fantastic

What is a fake force? A fake force is one of those forces that introductory texts tell you aren't real - like centrifugal force. They aren't real in the sense that they are due to one of the fundamental interactions. Basically, introductory texts (and even blogs like this one - not a bad summary of real vs. fake forces) attack the centrifugal force. This is because it is so common for students to want to use these faux forces in the wrong way. Better to just not use them at all. Anyway, there are times when faux forces are awesome. Just to be clear, a faux force is needed to use normal…

Centripetal Force, Centrifugal Force - what's the deal?

A couple of commenters expressed concern over the use of centrifugal force after my rant on the use of the word force. So, what is the deal with these two terms? Are they ok to use? Are they real forces? First, are they real forces? It depends on what you mean by real. What is a force? Here is a quick overview of what a force is. I previously talked about real vs. non-real forces. For me, I say that if the force is essentially one of the 4 fundamental forces then it is "real". With this definition, centripetal force would be real and centrifugal not real. Centripetal Force Centripetal…

Fake vs. Real Forces

In this post, I am going to talk about real and not real forces as well as the fake centrifugal force (if you don't like the word "fake" you could replace that with "fictitious") First, an example: suppose you are in a car at rest and press the gas pedal all the way down causing the car to accelerate. What does this feel like? If I weren't skilled in the art of physics, I might draw a diagram something like this: ![Screenshot 20](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/10/screensho…) Yes, maybe someone would add gravity and the chair pushing up, but this shows the…

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…