# Pseudoforces or fictitious forces

#### logearav

Joined Aug 19, 2011
243
Revered members,

#### Wendy

Joined Mar 24, 2008
22,368
Just curious, do you know what the Coriolis force is? It is closely tied with centrifugal force.

I've never heard the term pseudo forces use with these before, but I understand why they would be called that.

Acceleration takes power, as in rocket. Simply falling doesn't apply, although you are converting virtual energy into real energy during the duration of the fall.

The acceleration caused by centrifugal force only uses the energy need to speed up a wheel. After you have input that energy on the wheel and stop, the acceleration inside the wheel still exists, even thought no more energy is used or required. If the wheel was already spinning it will slow down a little as you move to the edge to compensate for your inertia, but that is it.

#### Markd77

Joined Sep 7, 2009
2,806

Joined Jul 7, 2009
1,583
Start here. If you're still confused, consult an intermediate mechanics book like Symon's "Mechanics" which derives the equations Mr. Bond was too lazy to do.

#### Papabravo

Joined Feb 24, 2006
15,779
Pseudo forces arise in the context of pseudovectors. A pseudovector happens when you take the vector cross product of two polar vectors.

#### russ_hensel

Joined Jan 11, 2009
825
No to both Bill_Marsden and Papabravo. A pseudo force is not a force in the usual inertial frame of reference where newtons laws hold true. In an accelerating frame of reference objects do not move as described by newton. This can often be fixed up by adding a force that does not exist. For example you are standing in a car that surges forward, you precieve yourself as thrown back by some force. Actually you are standing still and the car surges toward you. You tend to believe in the force because you are using the car as a frame of reference. Rotating frames are also accelerating. It takes now power ( or energy ) to accelerate as long as the acceleration is perpendicular to the motion. Of course any moving system will have kinetic energy. Pseodovectors are real, but something quite different.

Joined Jul 7, 2009
1,583
Russ is exactly right -- these fictitious forces are added in to the equations to make Newton's second law hold in the accelerated (e.g. rotated) reference frame. That's useful to those of us on the surface of the Earth for obvious reasons.

#### Wendy

Joined Mar 24, 2008
22,368
Hmm, I didn't use da fancy words, but I do believe I said the same thing in layman's terms. Given I hadn't encountered the terms I think I did pretty well.

#### logearav

Joined Aug 19, 2011
243
Thanks for the link. It says " The surface of the earth is a rotating frame of reference". We live in the planet earth. So when i sit on a railway platform(say), does it mean , i am in rotating frame of reference, aka, non inertial frame of reference. Don't feel offended by my doubt, still as i am a learner, i need some clarification with expert members. Thats why this query.

#### Wendy

Joined Mar 24, 2008
22,368
All the forces we have talked about exist on earth, but are so small compared to the basic forces of gravity that that are unnoticeable. Tidal forces from the moon are the second most prevalent force on earth, right after the earths gravity itself.

Joined Jul 7, 2009
1,583
Thanks for the link. It says " The surface of the earth is a rotating frame of reference". We live in the planet earth. So when i sit on a railway platform(say), does it mean , i am in rotating frame of reference, aka, non inertial frame of reference.
Yes, exactly that. It's impossible for us humans to detect with our senses that, sitting on the Earth's surface, we're in a non-inertial reference frame -- we need indirect means to detect it (e.g., run an experiment). Therefore its outside of our normal everyday experiences. It would be a great deal of fun to be on a large rotating space station, as that would afford one with the ability to do experiments that showed one was in a non-inertial frame of reference. For example, if you were to throw a ball, it would appear to you in the space station that its trajectory was curved. You'd account for this with a force because you'd still want Newton's second law of motion to be satisfied so you could use it to make predictions. However, if you were to view the same event from an external non-rotating frame of reference, you'd see that the ball traveled along a straight line, just as would be predicted by Newton's second law saying $$m \ddot a = 0$$. In the inertial frame, you as the observer would see that in fact it was the observer (the "first" you) that actually rotated under the ball. And that, of course, also takes a force to accomplish; we call it the centripetal force and it's definitely not a fictitious force.

As Bill mentions, these resulting "fictitious" forces are small. However, they have to be accounted for in some cases. As an example, when firing artillery over e.g. naval gunnery ranges from the battleship era, you'd miss your target entirely if you didn't take into account the Coriolis force. The Coriolis force is also important when looking at the motion of large air masses, as it causes the characteristic circular movement of storms.

#### Wendy

Joined Mar 24, 2008
22,368
If you were to throw a ball inside a space station you would note that something wasn't quite right with the trajectory. I asked earlier if logarav knew what the Coriolis Force was, and didn't get an answer. In a space station you could see it in action. From what I've read, it also causes a lot of inner ear disturbance, to the point of making people sick just like with sea sickness.