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:

Yes, maybe someone would add gravity and the chair pushing up, but this shows the important points. What is this force of acceleration? What causes this? This is EXACTLY the same thing as centrifugal force. If you think centrifugal force is real, this also should be real. I think this is enough discussion to show that this force (and centrifugal) is not real, but I will continue. There is another mystery: why does it feel like there is a force pushing you back when you accelerate? (if you have read all my blog posts, you may have a hint to the answer).

Let me replace the person with a model of a person. Here is my model (very simplistic)

In this model of a person, there are 4 masses each connect to the adjacent “atoms” with a spring (I represent the springs as rectangles because of my laziness). Now suppose I push on this model from both sides with equal forces.

I put these big bars on the side to make it clear the force was applied to both “atoms” on that side. So, when these two forces are applied, 1) the object stays at rest and 2) the horizontal springs are compressed.

Now what if I just apply 1 of these forces:

Notice that the compression is EXACTLY the same before (Eye-dentical). Hey wait! How do I know that this one force would compress this exactly the same? Well, you or I could easily model this and in fact I have done so for a previous article (weightlessness and gravity)

If the above model looks the same, it means a person would feel the same. The only difference is that this person would be accelerating. The point of this story is that when a person accelerates, it FEELS like a force is pushing on you in the opposite way. One note: when you accelerate, it doesn’t feel exactly the same as if someone was pushing on you. When someone pushes on you, they are exerting a force on just part of you. When you accelerate, it feels like something is pulling on ALL of you.

Ok, now on to circular motion and centrifugal force. In the above case, what if I took a “picture” of the velocity vector after 1 second? The two vectors would look like this:

And using the definition of acceleration:

I can find the direction of the acceleration by finding the change in the two velocity vectors:

Ok, so maybe we are all happy with this? (I am happy) Let me move to circular motion. I will once again “take a picture” of the velocity vectors for an object moving in a circle.

Now, I can do the same thing as before to find the direction of the acceleration. (it is ok to move a vector as long as you don’t change its direction or length)

Key points: 1) the velocity did change (although only in direction and not in magnitude). 2) This change in velocity means the object accelerated. 3) in this case, the acceleration is towards the center of the circle.

This would make it “feel” like a force is pushing outwards. It is this force that people call centrifugal force.

Whenever one is thinking about forces, it is important to realize that forces are an interaction between two objects and there are only a few real forces. They are:

• Gravity - an interaction between objects with mass
• Electromagnetic - an interaction between objects with electrical charge
• Strong nuclear - an interaction between hadrons (protons and neutrons are two examples of hadrons)
• Weak nuclear - an interaction between quarks and leptons

Anything that is a real force should be one of these. Gravity is an easy one to pick out. What about me pushing on a book? That would be the electromagnetic force because the atoms in my hand are interacting with the atoms in the book (and that is what prevents my hand from going through the book).
What about centrifugal force? What are the objects that are interacting? (hmmmm.....) Which of the fundamental forces is it? (hmmmm.....). Well, it must not be a real force.

Don’t get me wrong, sometimes the idea of a centrifugal force is useful, but that does not make it real.

Tags: astronomy, forces, circular motion

So, here is a video (from break.com - so you know it is likely fake).

Extreme Catapulting - Watch more free videos
If for some reason, you can not view this video, here is the plot.
1) Guy wears parachute and brings a portable thing like a see-saw.
2) Guy approaches large crane dropping a large mass repeatedly (I assume to flatten a dirt road)
3) 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.
4) Mass drops, guy shoots up and parachutes down. Read More...

Tags: forces, video, fake, energy

You see this all the time in textbooks:

This is often described as “the acceleration due to gravity”. Is this really the best thing to call this? No. A better name would be “the local gravitational field” and list it in units of:

Read More...

Tags: gravity, kinematics, forces

I like demos. They are kind of like magic tricks, but you get to explain something also. I recorded this demo and I was going to use it for my astronomy lectures, but it just didin’t fit in anywhere. I recorded this demo, but my kids were making a lot of noise, so I re-recorded the audio. Didn’t quite turn out like I wanted. Here is the video:

Inertial demo from Rhett Allain on Vimeo. Read More...

Tags: forces

So, you knew I was going to make some mistakes on the physics of scooting - right? I don't think these mistakes are major and they don't take away from the main idea. Read More...

Tags: forces, kinematics, modeling, momentum

There you are, in your rolling chair. You need that extra pencil, it is just a few feet away but out of reach. For some reason, you don't want to touch your feet to the floor. How do you get that pencil? You scoot. Still don't get it? Here is an example:

Why do people scoot? I don't know. My kids do it too. When does a human learn to scoot? How does a scoot work? Really, I am just going to answer that last question, but the others are interesting also. Read More...

Tags: forces, kinematics, modeling, momentum

Suppose I want to have a lab activity with a dropping tennis ball. Do I need to worry about air resistance or can I assume there is none? This is a that was recently asked of me. Below is my answer: Read More...

Tags: forces, kinematics, numerical , computer, air resistance

Some time ago, I posted an analysis of the Ford F-150 commercial where the truck is shown to stop a landing C-123 Airplane. Apparently, this was quite popular and I received some useful and interesting feedback. From this, I have a few updates. Read More...

Tags: video, kinematics, forces, momentum

The normal force is a force exerted by a surface onto an object. There is some really cool things about this force, almost like it is magic. Read More...

Tags: forces

First, I am not really sure why Newton got to call this HIS law. Second, its not really a law. What happens if you don't obey this law? Would I go to physics jail? Ok, now for the good stuff. I am going to try to give a brief summary of Newton's 2nd law. Read More...

Tags: kinematics, momentum, forces

Clearly, I find some joy in analyzing these Ford commercials. In the latest release, a F150 is spun around in a homemade centrifuge. Yes, this is an awesome opportunity to discuss physics, but is that Ford's intent? Ford is clearly trying to get a message out with this. The following screen shot shows the real message:

See that at the bottom? Closed centrifuge. Do not attempt. Damn you Ford! Why must you ruin my fun? Why do you think I have a centrifuge (an open centrifuge, btw)? Well, I guess all that is left is for me to analyze this commercial. (you can see the whole behind the scenes here - http://www.fordvehicles.com/f150behindthescenes/) Read More...

Tags: video, circular motion, forces, momentum

Aristotle: A constant force makes an object move at a constant speed.

Newton: A constant force makes an object move at a constant acceleration (changing velocity)
Read More...

Tags: forces, kinematics

On this week's 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? Read More...

Tags: mythbusters, forces

Ford, why are you the way you are? Do you actually have people look at your commercials? Or do you just assume that you can say and show whatever you want and I will buy a truck? I am sorry, I shouldn't say that. I do enjoy analyzing your commercials - so, please continue.
Ford's latest F150 commercial shows a big thing dropping from a helicopter to the bed of an F150. The claim is that Ford is awesome. 30,000 lbs awesome. Let's check this out.
Read More...

Tags: video, modeling, momentum, forces, kinematics

Line Rider. Yes, its a game. Yes, I know its not real life. Nonetheless, I analyze what I see. How is the physics in line rider? Does it agree with Newtonian mechanics? What is the scale of the line rider? What other questions are there? What other questions are there? Read More...

Tags: forces, kinematics, momentum, energy

I am sure you have seen this commercial:

They say its a real demonstration, but it has seemed odd to me. (they also say this is a professional driver on a closed course and don't try this at home - damn! I was SO going to do that this weekend). Here is my analysis: Read More...

Tags: momentum, forces, kinematics