forces

A student in my office temporarily confused the words centripetal and centrifugal. This started me thinking about these two words. They mean different things, but do sound and look similar. I have previously talked about the difference between fake and non-fake forces, but let me quickly define these two: Centripetal: This is the force needed to make something move in a circle. The force could actually be a number of things such as: friction, gravity, tension in a rope or any combination. Centripetal force is a name for a real force that has the role of making something move in a circle…
I have already indirectly talked about this before, but I see it more and more. Say you have a show. It has to be more than entertaining, it has to educational. How do you make things educational? Describe the physics behind the concepts - right? Fine. How do you explain stuff? It's simple. Just make some diagrams with arrows. Be sure to use words like "force", "velocity", "acceleration". You know, physicsy words. Everything will be fine. Everything will not be fine. First, I love MythBusters. They are awesome. I know they are not scientists, and that is maybe why I like them.…
I am so pumped up that MythBusters is back on. Not only do I like the show, but it offers so many blogging opportunities. Their latest show featured car crashing myths. One of the myths from the episode was a redo of the myth where two trucks crash head on simultaneously crushing a smaller car in the middle. The first test was very similar to the previous time they tested this, but faster. They towed two 18 wheelers to crash together around 50 mph and smash a stationary car. The results were impressive. However, they did not have the result of the car completely contained in the…
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…
Tom and Jerry, or Fetch! with Ruff Ruffman? Answer: Tom and Jerry. What? Yes. The reason: Tom and Jerry has bad physics but does not pretend like it has good physics. I know this is probably going to "type-cast" me as "that guy that keeps attacking Ruff Ruffman." Soon to be followed by "Leave Ruff ALONE!" I am sorry, I can't help it. Here is my problem. If you are going be a show, do whatever you like (I might still make some comments). If you are going to be a show that attempts to teach some stuff, don't you think you should get it correct or at least not reinforce bad ideas? The…
I have always wanted to ask a question like this - but never found the right test for it. Perfect for a blog. Question: Suppose you are in your car at a stop light behind a truck. The truck is pulling a trailer with a rollable tractor on it. When the light turns green, the truck starts to accelerate. However, the cable holding the tractor to the trailer breaks and the tractor starts to roll off the trailer. You are right behind the trailer. What do you do? Do you have an answer yet? You should probably come up with one quickly. That tractor is rolling off now. I will put the answer…
First, I think we should all be using smaller lighter cars. They get better gas mileage, they are better for the environment and maybe even reduce traffic? (ok, not sure about that last one - but it is possible). In Buzz Out Loud 918, they were discussing Toyota working on new car bodies built from sea-weed based plastics (from a wired article). One point that was brought is the comparison between old plastic bodies (the pontiac fiero comes to my mind) and these new plastics that should be much stronger. Well, here is the problem. Light strong bodied cars can be dangerous. Really? Yes…
This is one of my favorite demos. Easy to do and doesn't really require any equipment. The basic idea is that you support a meter stick horizontally with two fingers. Slide your fingers in and they will both meet at the center of mass. Here is a video. Meterstick friction demo from Rhett Allain on Vimeo So, how does it work? To explain this, you need to understand friction and equilibrium. Well, nobody really understands friction - but anyway.. First, what is equilibrium? Equilibrium means that an object's motion does not change. For this case that means that both it's velocity and it…
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…
People say I am picky. Ok, sometimes I am. But somebody has to stand up for what is right and just. Maybe I am that person. Please stop using the word force if you don't know what it is. There. I said it. You can attack me now. It wasn't just one thing that got me fired up. It was two things. First, I read this article on physics and football (Physics of 'The Hit' from the NY Times). If it was just this article, I would have let it go and moved on. But no. One of my kids just happened to be watching MythBusters (We all love MythBusters) and there was a discussion that used the term…
I said I would come back to this, and I am. I am a man of my word. Yesterday, I posted a link to a video of this really cool water rocket thingy. How does this work? What is the physics going on here? I think this can be best explained with the momentum principle. Let me start by pretending like I have some object that shoots out a piece of water (or really it could be anything). Also, let me pretend like this is in space ore something where there are no external forces. Above is a before and after picture. Initially, there is something (the box) with water inside. Through some process…
Buzz Out Loud is one of my favorite tech podcasts. Unfortunately, I am a little behind in the episodes. However, in the episode 900 there was an email from a listener regarding LHC and black holes. The emailer claims that a massive black hole would mess up Earth's orbit. This is not quite true. I was getting ready to make a lengthy post about how black holes work, but did a way more awesomer job than I could do. Stefan and Bee describe some very important points: First, black holes created in the Large Hadron Collider are based on a theory that has not really been verified. So, it might…
It has been windy here lately. Sometimes I think that is an ok thing. You see, when I ride my bike to work I am probably going to have the wind at my back for one of directions. It is great feeling like Lance Armstrong because of the boost you get from the wind. With a good wind at my back, I can almost keep up with the traffic (I would keep up if they went the 25 mph speed limit). Of course, with a great boost comes a great drag. When I ride into the wind, I feel weak. I pedal as fast as I can and cars just whiz right by like I am standing still. When you are in a car, you don't…
So, here is a video (from break.com - so you know it is likely fake). null - Watch more free videos If for some reason, you can not view this video, here is the plot. Guy wears parachute and brings a portable thing like a see-saw. Guy approaches large crane dropping a large mass repeatedly (I assume to flatten a dirt road) Guy sneaks up an puts the see-saw under the area that the mass drops on and then stands on the other end of the see-saw. Mass drops, guy shoots up and parachutes down. So, why is it fake? I think the best thing is to give an analysis of the see-saw. To start this analysis…
I finally saw the movie Iron Man. It was good. I feel that I am qualified to evaluate the movie. When I was in high school, I was totally into comic books. Mostly Spider-man, but I still have a significant collection of Iron Man comics. Ok, now you know I am not an Iron Man attacker. I will now attack the movie. Sorry, it's what I do (remember, I already said I liked it). There are several things I could comment on, in fact I recall some other blog talking about the physics of Iron Man. My attack will center on the scene where Tony Stark (Iron Man) escapes from captivity with his home…
Just to be clear, Newton probably didn't have a portable video camera. I do. I have one of these Flip Mino pocket cameras. Very nice. You can put it in your pocket. However, there is a problem with cameras like these (think of video from camera phones). The problem is the mass, it is too small. Video from a camera phone or small portable camera looks too jerky (unless you are careful). Sometimes it is jerky to the point that it makes me feel barfy. The reason this happens is that with a small mass, a force can cause a significant acceleration. Let me draw a picture (you know I like…
In part I of this post, I talked about the basics of projectile motion with no air resistance. Also in that post, I showed that (without air resistance) the angle to throw a ball for maximum range is 45 degrees. When throwing a football, there is some air resistance this means that 45 degree is not necessarily the angle for the greatest range. Well, can't I just do the same thing as before? It turns out that it is a significantly different problem when air resistance is added. Without air resistance, the acceleration was constant. Not so now, my friend. The problem is that air…
Clearly, I am not a professional blogger. I am an amateur. This is because I was under the impression that only amateur bloggers could compete in the blogging olympics. When did they change these rules? Anyway, Adam Weiner did a physics-based analysis of the latest Star Trek movie trailer. Here is the trailer: In the trailer (oh, spoiler alert) a young Kirk jumps out of a car before it goes over a cliff. It does look odd, and that is why I had intended to analyze it. In Adam's analysis, at PopSci.com the basic approach was: Take the initial velocity of the car (from the clip) Assume the…
First, don't play with fire. Fire is dangerous. You can get hurt. Here is a video of two guys trying to (presumably) make the Miami Heat flaming basketball. Note: the above is just a picture link to the actual movie. Embedding the movie was doing some weird stuff. In the movie, two guys pour gasoline on a basketball and then light it on fire. One of the guys then gives the ball a good kick. The result is that some of the flaming gas stays in place and some goes with the ball (and some gets on the guy's foot). This could be a good example of the momentum principle (also know as Newton…
I am sure I have mentioned the Discovery show Time Warp before. The basic idea of the show is to find how many cool things they can look at with an ultra slow motion video camera (btw - this is on my christmas list). So, I have seen this Samurai guy on the show twice. Yes, he has a big sword. Yes, he could utterly destroy me if he wished. However, I can not handle him talking about physics. Mr. Samurai guy, please stop using the words "kinetic energy" until you know what they mean (I said please). Here is my first problem. Samurai guy is showing how to chop people down. He plans to…