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> Not centrifugal force (a fictional force) but centripetal force So-called "fictional forces" are used all the time to simplify calculations. Furthermore, if

> Not centrifugal force (a fictional force) but centripetal force So-called "fictional forces" are used all the time to simplify calculations. Furthermore, if you are going to be pedantic, in GR centrifugal force is just as real as gravitational force. Of course, in GR, gravitational force is also a fictional force. But one makes few friends by constantly pointing this out whenever anyone says "gravitational force".

lutusp 14y ago
> So-called "fictional forces" are used all the time to simplify calculations. Consider the context -- someone wants to soft-land on the moon at 2700 miles per hour. Under those circumstances, I suggest avoiding any other gratuitous fictions until more has been learned. > But one makes few friends by constantly pointing this out whenever anyone says "gravitational force". Yes, unless you're instructing a student. Someone who plans to slide into home base at 1.23 km/s is a student and should receive the pedantic version.
- nessus42 14y ago
  > Consider the context In this context, I'm sure that determining the "centrifugal force" at each point on the inside of the ball to calculate the pressure on the outside wall, and comparing that to the tensile strength of the outside wall, should work just fine. At least for that one aspect of the problem.
  - lutusp 14y ago
    > In this context, I'm sure that determining the "centrifugal force" at each point on the inside of the ball to calculate the pressure on the outside wall ... That's a centripetal force -- the unified wall's resistance to its component parts' tendency to move away in a straight line. Source: http://en.wikipedia.org/wiki/Centripetal_force http://en.wikipedia.org/wiki/Centripetal_force Quote: "Centripetal force (from Latin centrum "center" and petere "to seek") is a force that makes a body follow a curved path: its direction is always orthogonal to the velocity of the body, toward the fixed point of the instantaneous center of curvature of the path. Centripetal force is generally the cause of circular motion." Perhaps in this case, argument for argument's sake isn't constructive. > At least for that one aspect of the problem. As is often the case when quoting fictitious forces, it can only confuse the student. Maybe as a side effect, it might make him think he can land an intact spacecraft on the moon by sliding to a stop.
    - nessus42 14y ago
      > That's a centripetal force It's also centrifugal force, if you are working in the rotating reference frame of the ball, which in the case of calculating the internal pressure, is probably a bit easier to do. > it can only confuse the student. I'm not confused.
      - lutusp 14y ago
        >> it can only confuse the student. > I'm not confused. You're not a student, the OP is.
        nessus42 14y ago
        When I first learned Newtonian mechanics, I found it much easier to solve certain problems thinking in terms of centrifugal force, rather than centripetal force, and thinking so did me none the worse, as long as I kept in mind the caveats required for this approach. E.g., the classic problem of determining at what point you will fall off when skiing straight down a giant frictionless spherical snow ball. For me this is much easier to analyze by asking yourself at what point the centrifugal force pushing you off of the snowball will counteract the vector of gravity that is pushing you onto the snowball's tangent. The so-called "right" way of thinking about this problem is at what point the centripetal vector of gravity is no longer sufficient to constrain your movement to a circle. That's much harder to visualize for me. And would be even more so for non-spherical shapes. The real right way to think about this, is to think about it both ways and then to convince yourself that they are equivalent. Not to pretend that there is no such thing as centrifugal force.

lutusp 14y ago

> So-called "fictional forces" are used all the time to simplify calculations. Consider the context -- someone wants to soft-land on the moon at 2700 miles per hour. Under those circumstances, I suggest avoiding any other gratuitous fictions until more has been learned. > But one makes few friends by constantly pointing this out whenever anyone says "gravitational force". Yes, unless you're instructing a student. Someone who plans to slide into home base at 1.23 km/s is a student and should receive the pedantic version.

nessus42 14y ago
> Consider the context In this context, I'm sure that determining the "centrifugal force" at each point on the inside of the ball to calculate the pressure on the outside wall, and comparing that to the tensile strength of the outside wall, should work just fine. At least for that one aspect of the problem.
- lutusp 14y ago
  > In this context, I'm sure that determining the "centrifugal force" at each point on the inside of the ball to calculate the pressure on the outside wall ... That's a centripetal force -- the unified wall's resistance to its component parts' tendency to move away in a straight line. Source: http://en.wikipedia.org/wiki/Centripetal_force http://en.wikipedia.org/wiki/Centripetal_force Quote: "Centripetal force (from Latin centrum "center" and petere "to seek") is a force that makes a body follow a curved path: its direction is always orthogonal to the velocity of the body, toward the fixed point of the instantaneous center of curvature of the path. Centripetal force is generally the cause of circular motion." Perhaps in this case, argument for argument's sake isn't constructive. > At least for that one aspect of the problem. As is often the case when quoting fictitious forces, it can only confuse the student. Maybe as a side effect, it might make him think he can land an intact spacecraft on the moon by sliding to a stop.
  - nessus42 14y ago
    > That's a centripetal force It's also centrifugal force, if you are working in the rotating reference frame of the ball, which in the case of calculating the internal pressure, is probably a bit easier to do. > it can only confuse the student. I'm not confused.
    - lutusp 14y ago
      >> it can only confuse the student. > I'm not confused. You're not a student, the OP is.
      - nessus42 14y ago
        When I first learned Newtonian mechanics, I found it much easier to solve certain problems thinking in terms of centrifugal force, rather than centripetal force, and thinking so did me none the worse, as long as I kept in mind the caveats required for this approach. E.g., the classic problem of determining at what point you will fall off when skiing straight down a giant frictionless spherical snow ball. For me this is much easier to analyze by asking yourself at what point the centrifugal force pushing you off of the snowball will counteract the vector of gravity that is pushing you onto the snowball's tangent. The so-called "right" way of thinking about this problem is at what point the centripetal vector of gravity is no longer sufficient to constrain your movement to a circle. That's much harder to visualize for me. And would be even more so for non-spherical shapes. The real right way to think about this, is to think about it both ways and then to convince yourself that they are equivalent. Not to pretend that there is no such thing as centrifugal force.