forces
rallain | October 6, 2008Popular demand (from a few people - you know who you are) requested another level for the force game. I am open to naming this game. Ideas for levels have been suggested also. There is a new rule change. You must be stopped (or almost stopped) in the red circle.
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Update:
I guess I didn't include instructions on how to play the game. Just click on the applet window to make sure it is active. Press the space bar to start the game. Once the game starts the objective is to use rocket thrusters (the arrow keys) to move the rocket (the black rectangle) into the…
rallain | October 6, 2008**Pre reqs:** [Free body diagrams](http://scienceblogs.com/dotphysics/2008/09/basics-free-body-diagrams.php)
Friction is an interaction between two objects in contact that opposes relative motion of those two objects. It is not something fundamental (like gravity, or electromagnetic force), but it comes up enough that it will be worthwhile to talk about it. Let me start with a simple example. Suppose I have a book on a table. Here is the free body diagram for the book:
Simple enough - right? There…
rallain | October 6, 2008I already said I like [scratch from MIT](http://scratch.mit.edu). After building a simple rocket model, the kids said it should be a game. I caved. Here it is:
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To play, press the space bar. The arrow keys are rocket thrusts. The goal is to get to the red circle in as little time. If you hit the wall or the sides, you start back at the green circle. Please forgive me masters of scratch (I know who you are - you find something to complain about in my program).
rallain | October 1, 2008I actually have some important things to do. However, I thought I would make another scratch program. Yes, this will all lead to something useful (that is what I told my wife). Anyway, in this program, I made a spaceship with 4 thrusters. The great thing about this is it show what forces do to the motion of an object. I already had a suggestion to make it into a game with a score.
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Use the arrow keys to turn on the 4 thrusters.
rallain | September 30, 2008**Pre Reqs:** [Intro to Forces](http://scienceblogs.com/dotphysics/2008/09/basics-what-is-a-force.php), [Vectors](http://scienceblogs.com/dotphysics/2008/09/basics-vectors-and-vector-ad…)
Hopefully now you have an idea of what a force is and what it isn't. What do you do with them? The useful thing to do with forces is to determine the total force acting on an object. At the beginning of the introductory physics course, you will likely look at cases where the total force is the zero vector. This is called equilibrium. Even if you are looking at cases where the forces don't add up to the…
rallain | September 26, 2008On a previous episode of The MythBusters, Adam and Jamie made a lead balloon float. I was impressed. Anyway, I decided to give a more detailed explanation on how this happens. Using the thickness of foil they had, what is the smallest balloon that would float? If the one they created were filled all the way, how much could it lift?
First, how does stuff float at all? There are many levels that this question could be answered. I could start with the nature of pressure, but maybe I will save that for another day. So, let me start with pressure. The reason a balloon floats is because the air…
rallain | September 26, 2008**Pre-reqs:** None.
I intend to talk about forces and force diagrams, but there is a more fundamental question to address first. What is a force? Most texts define it as a push or a pull. That really isn't a bad definition. Maybe a better (or maybe worse) definition would be "forces are things that change the motion of an object" (change being the key word). If I had to choose one definition of force, it would be something like this:
**Force:** *A force is an interaction between two objects. There are 4 known forces:*
Gravitational force: An attractive long range force between objects…
rallain | September 23, 2008In 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…
rallain | September 14, 2008Friction in Line Rider
Is there friction in Line Rider? Does it function as physics would expect? To test this, I set up a simple track:
Basically, a slope with a flat part to start with and to end with. Let me show you something simple before further analysis:
This is the x-position vs. time for the line rider on the first horizontal portion of the track (before he or she goes down the incline). This shows the rider…
rallain | September 13, 2008There is no air resistance in line rider. Sorry to spoil the suspense.
To test for the presence of an air resistance force, a track was created that let the rider fall.
(note the markers on the side. These are used to keep track of how the origin is moving).
Below is the y position of the rider as a function of time:
In this situation, the rider falls about 100 meters. A quadratic line is fit to the data…
rallain | September 11, 2008The MythBusters aren't really doing it wrong, but they give me a chance to talk about some physics. In the latest show, they tested the myth that two phone books with their pages alternating were indestructible. To test this, they put the two phone books together and then pulled them apart in a sort of tug of war. Here is a diagram:
Looks great, what is wrong with this? The problem is that by pulling this way, the MythBusters produces 320 pounds of force on the book - but they could have done twice that. This…
rallain | August 26, 2008I know the olympics are basically over. Really, I should have posted this earlier. Anyway, the gymnastics feat that always impresses me is the Iron Cross (I think that is what it is called). I know you have seen this, but here is a picture from wikipedia:
(http://en.wikipedia.org/wiki/Rings_(gymnastics))
Why is this so impressive? Why is this so difficult? Let me start with something completely different that is exactly the same (in some ways).
Here is a heavy box hung from a rope that has…