Category physics

Fetch with Ruff Ruffman uses a laser to measure temperature?

rhettallain Leave a comment

I am sorry to point this out, but I can’t help it. My kids watch this show “Fetch with Ruff Ruffman”. It’s mostly an ok kids show. However, there was a problem. In one episode, some kids were in the desert and measuring temperature with (they said it several times and it was even a quiz question at the end) – a LASER. Here is the device they used:

![Images](http://blog.dotphys.net/wp-content/uploads/2008/10/images.jpg)

This is an infrared thermometer with a LASER aiming system. The laser is only there to help you aim. The temperature is determined by measuring the infrared radiation from the object. You don’t even NEED the laser. Those ear thermometers work the same way, but they don’t have lasers.

I guess when a laser is involved, it just makes it so cool that it must be the most important thing. I am going to put a laser pointer on my computer – that way people can say “look, that computer runs on LASERS.”

A note about numerical calculations

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[In a previous post, I talked about numerical calculations](http://blog.dotphys.net/2008/10/basics-numerical-calculations/). The basic idea is to use the momentum principle and the following “recipe”:

  • Update the position of the particle
  • Update the momentum of the particle
  • Update the force on the particle

Looks great, right? Well, it mostly is great. I want to give a couple of pointers about the last step, update the force on the particle. How and when can you do this? Really, in numerical calculations, you will see two types of forces:

  • Forces that you can calculate: That looks strange, but it’s true. Maybe you are thinking, can’t you calculate all the forces? – the answer is no. Yes, you can calculate the gravitational force and the electromagnetic force. Also, really all forces you are likely to see are one of those two. You can also calculate the force due to a spring(depends on position), the air resistance force (depends on velocity). These types of forces work well in the above numerical recipe.
  • Forces that you CAN NOT calculate: These are all the other forces. Typically, these are forces of constraint. Suppose a block slides down a plane. Yes, you can calculate the force the plane exerts on the block, but it depends on things other than just the position of the block. The force the plane exerts on the block is such as to keep the block on the plane. You can not calculate this in the same way as the previous category of forces. Yes, technically the force the plane exerts on the block IS the electromagnetic force. If you want to calculate this force between all the atoms in the two objects, I encourage that.

So what does this all mean? This means that you can not use the above “recipe” for whatever you want. Sorry.

(I have a trick I will show you later)

Word Police: Use of the word pressure

rhettallain 2 Comments

I know I should just let go, but this is what makes me, me. I understand that there are terms in physics (like for instance ‘pressure’) that are used in all sorts of ways in common language. The problem is when someone tries to explain something scientifically and misuses a word. Pressure means something. It is the average force per area due to collisions of a gas or liquid on a surface. Really, you can see a good animation of this, I have a link and explanation when [I talked about MythBuster’s Lead Balloon](http://blog.dotphys.net/2008/09/mythbusters-how-small-could-a-lead-balloon-be/).

So, what is my problem? I was reading this interesting article on [Scientific American.com about solar refrigerators](http://www.sciam.com/article.cfm?id=solar-refrigeration). Here is a small quote:

*The key is the energy exchanged when liquids turn to vapor and vice versa—the process that cools you when you sweat. By far the most common approach, the one used by the refrigerator in your house, uses an electric motor to compress a refrigerant—say, Freon—turning it into liquid. When the pressure created by the compressor is released, the liquid evaporates, absorbing heat and lowering the temperature.*

“Pressure created by the compressor” isn’t too bad (although it might be interpreted by some that pressure is some substance). My biggest problem is the “When the pressure created by the compressor is released” part. How do you release pressure? Yes, you could allow the particles in the gas to stop colliding with the wall and thus “release” them. But aren’t you releasing the particles not the pressure?

I don’t mean to pick on this particular case because it’s not that bad. There are many other cases where the word pressure is used in a really incorrect manner.

On a side note, about the above article, I find cooling to an extremely interesting problem. It is funny how easy it is to heat something up, but so difficult to cool it off.

**PS:** I also don’t like how the article says “absorb heat”. This also implies that heat is a substance. I don’t really like the word “heat”.

Basics: Work Energy

rhettallain Leave a comment

**Pre Reqs:** [What is a Force](http://blog.dotphys.net/2008/09/basics-what-is-a-force/)

[Previously, I talked about the momentum principle](http://blog.dotphys.net/2008/10/basics-forces-and-the-momentum-principle/). Very useful and very fundamental idea. The other big (and useful) idea in introductory physics is the work-energy theorem. Really, with work-energy and momentum principle, you will be like a Jedi with a lightsaber and The Force – extremely powerful.

Well, what is work? What is energy? How are they related? In [another post, I talked about energy.](http://blog.dotphys.net/2008/10/what-is-energy/) I think it is interesting to look at how most textbooks define energy:

*Energy is the ability to do work*

This is really a stupid definition. Kind of circular logic, if you ask me. In the post I mentioned earlier, I claim there are two kinds of energy, particle energy and field energy. At low speeds (not near the speed of light), particle energy can be written as:

![Screenshot 53](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-53.jpg)

Where *m* is the mass of the particle, *c* is the speed of light. So, if you just look at a particle, that is it for the energy. Now, what about the “work” portion? Work is defined as:

![Screenshot 54](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-54.jpg)

Where *F* is the net force on the particle, ?r is the vector displacement of the particle. The “dot” in between F and ?r represents the “dot product” operation between vectors (also known as the scalar product). In a [previous post](http://blog.dotphys.net/2008/09/basics-vectors-and-vector-addition/) I showed that you could multiply a scalar quantity by a vector quantity. Here I need to do “something” with two vectors. You can’t multiply two vectors in the same sense that you multiply scalars. A general definition of the dot product for two vectors:

![Screenshot 55](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-55.jpg)

That looks a little more messy than I wanted, but it can not be helped. Really, it is not that complicated. The dot product is simply the projection of one vector on the other. Let me explain in terms of work.

Continue reading “Basics: Work Energy” →

Spring Motion and Numerical Calculations

rhettallain 2 Comments

Maybe you know I like numerical calculations, well I do. I think they are swell. [VPython](http://vpython.org) is my tool of choice. In the post [Basics: Numerical Calculations](http://blog.dotphys.net/2008/10/basics-numerical-calculations/) I used vpython and excel to do something simple. I will do that again today (in that this problem could also be solved analytically). However, there is one big difference. This problem has a non-constant forces. Suppose I have a mass that is connected by a spring to a wall. This mass-spring is sitting on a table with no friction.

![Screenshot 27](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-272.jpg)

There is a very interesting property of springs. The more you stretch them, the greater the force they exert (in the usual model of springs). This model works very well.

![Screenshot 28](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-282.jpg)

This is known as Hooke’s law. I have written it as a scalar for simplicity. The “k” is called the spring constant. It is a measure of how “stiff” the spring is. The value “s” is the amount the spring is stretched. Typically, there is a minus sign in front of the ks to indicate that the force is in the opposite direction that the spring is stretched. Really, in a scalar equation this is rather silly to include (but everyone does anyway).

**Question: What will the motion of the mass be like if I pull it back and then let go?**

Although this can be determined analytically, I am going to first calculate this with vpython. I will try to show all the details so that you can reproduce this also. If you have not already installed [vpython](http://vpython.org), do that now (don’t cost nothing).

Continue reading “Spring Motion and Numerical Calculations” →

Basics: Numerical Calculations

rhettallain 4 Comments

**Pre Reqs:** [Kinematics](http://blog.dotphys.net/2008/09/basics-kinematics/), [Momentum Principle](http://blog.dotphys.net/2008/10/basics-forces-and-the-momentum-principle/)

What are “numerical calculations”? Why are they in the “basics”? I will give you really brief answer and then a more detailed answer. Numerical calculations (also called many other things – like computational physics) takes a problem and breaks into a WHOLE bunch of smaller easier problems. This is great for computers ([or a whole bunch of 8th graders](http://blog.dotphys.net/2008/09/computational-physics-and-a-group-of-1000-8th-graders/)) because computers don’t mind doing lots of little problems. Why are they “basic”? Well, most text would say they are not basic. I disagree. I think this is a legitimate method for solving problems. In particular, this is a great way of solving problems that can not be solved analytically (meaning solving one hard problem).

**Numerical Calculations are Theoretical Calculations**

Let me just get this out of the way. Numerical calculations and analytical calculations are really in the same “class”. Often people will lump numerical in with “computational experiment” but that is a really bad thing to do. Some others will claim that there are three different “paths” to discover stuff in science: theory, experiment, and simulations. Simulations are the same thing as numerical calculations which are the same as theory. ([I wrote a letter about this in the American Journal of Physics](http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000076000009000797000001&idtype=cvips&gifs=yes))

**Example Problem**

Let me start with a problem that can be solved analytically. Suppose I have a ball of mass 0.5 kg and I throw this straight up with a speed of 10 m/s. How high will it go?

Continue reading “Basics: Numerical Calculations” →

Fun things to do with a green laser pointer

rhettallain 1 Comment

I went home for lunch today. While waiting, I saw this lizard. We have many lizards in Louisiana, I like them, really I do. So, this is what I did:

No lizards were harmed in the filming of this video.

I was really surprised. I thought for sure that I had tried this before and nothing happened. Maybe it was because of the white surface the lizard was on. Maybe it was because it was an older and wiser lizard (it was larger than normal). Maybe this is already a well known fact about lizards and laser pointers. Needless to say, this was quite entertaining and completely justifies the cost of my laser pointer.

‘Weapon Masters’ doesn’t understand floating

rhettallain 2 Comments

There is this show “Weapon Masters” – I think it comes on the discovery channel. It is not a bad show. The basic idea is that they have this history guy talk about the historical aspect of some type of weapon and this other guy tries to make an improved version. Last night the goal was to recreate the original flame thrower mounted on a boat.

They found a boat and they needed to test it’s sea worthiness. The builder guy (sorry, I don’t know his name) estimated that they would have 1000 lbs of equipment in the boat. To simulate this weight, they put 4 guys and two barrels of water in the boat and motored around.

After a little bit, the history guy noticed one of the barrels was leaking water. *”Quick! Put your finger in the hole before we all sink!”* he said. This is where he doesn’t understand floating. I think I can explain his error with two simple pictures:

![Screenshot 05](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-051.jpg)

The first picture has the boat with the water in the barrel. The second picture has some water that has leaked out. Either way, the total mass of water in the boat is the same. If you would like to talk more about floating, [here is my post about MythBuster’s lead ballon](http://blog.dotphys.net/2008/09/mythbusters-how-small-could-a-lead-balloon-be/) – same idea.

I think the weapon master history guy should have said:

*”Quick! Plug the hole before my shoes get wet! But, we won’t sink because the mass of water is the same no matter if it is in the barrel or in the boat.”*

Basics: Forces and the momentum principle

rhettallain Leave a comment

**Pre reqs:** [Free Body Diagrams](http://blog.dotphys.net/2008/09/basics-free-body-diagrams/), [Force](http://blog.dotphys.net/2008/09/basics-what-is-a-force/), [Kinematics](http://blog.dotphys.net/2008/09/basics-kinematics/)

The time has come to look at things that are NOT in equilibrium. The most basic question to ask yourself is: *”What do forces do to an object”*? Aristotle would say that forces make things move. Constant forces make things move constantly. Actually, Aristotle said there were two types of motion:

  • Natural motions: These motions don’t need anything to happen, they just do. Example: a rock falling. You don’t need to do anything to it. Example: fire rising. It just rises. (there was more to it than that, but you get the idea).
  • Violent motions: These motions are due to some interaction that forces them from their natural state. The natural state of a cart is to be at rest. If someone pushes on it, it will move. When you stop pushing (stop the violent motion) it returns to its natural state – at rest

I am talking about Aristotle, because these basic ideas are what most people think. If you push something it moves. If you stop pushing, it stops. And these people are correct. The problem is that there is always this extra force that no one thinks about – friction. Without friction, the rules change.

**New Rules (Newtonian ideas)**

If you push something with one force, it changes velocity. If you stop pushing, it stays at a constant velocity.

If you want to test your feelings for force, [try this force game I made on Scratch](http://scratch.mit.edu/projects/rhettallain/285748). The idea is that you need to move the box to the red circle. The arrow keys exert a **force** on the object.

Continue reading “Basics: Forces and the momentum principle” →

MythBusters – a tree is 90% air

rhettallain 1 Comment

Dear Mythbusters. I hope you know that I think you are awesome. I know you are not scientists, but rather master robot builders. I respect that. I envy your robot-building abilities. Please forgive me for pointing this out – but even if a tree is 90% air, that does not mean a ball has a 90% probability of passing through it.

For those of you who are unaware, in the last episode of MythBusters, they explored the idea that a golf ball should pass through a tree 90% of the time. What if they were to test following alternative myth:

*A golf ball has a 90% chance of passing through something that is 90% air.*

Here is my test object:

![Screenshot 05](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-05.jpg)

It is a steel box with air inside. I did not give the dimensions, but you can imagine it would not be difficult to make this 10% steel and 90% air. What would happen if I hit a golf ball at this? I think there were would be a 0.000000000000000000001% percent chance of it going through (smaller than that – but I think you get the idea).

What is really important is the scattering cross-section, not the density.