Here is the Ninth conversation I had with money manager Andrew Horowitz…. new insights for anyone who invests in anything. What to do? This chat is not produced and is presented as-is for anyone who wants to listen in.

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  1. Mac Guy says:

    Did the maker of this graphic not study physics? Acceleration should always point to the center.

  2. kap says:

    Just wanted to say that these conversations are valuable and interesting, I started with eight but now I’m going to go back and listen to them all. Keep up the good work.

  3. clubstoic says:

    Is there an address to subscribe to these casts?

  4. bobbo says:

    I didn’t study physics but I thought acceleration would be in a straight line and its the curve of the track making the cart go in a circle?

    I think you are confusing something===so what is it “that always goes to the center?” All I can think of is the piece of string you hold when demonstrating “centripetal” force.

  5. Mac Guy says:

    #4 – No, the straight line you’re thinking of is momentum.

    See NASA’s explanation here…

    Personally, I think NASA is a fairly good source.

  6. Owemgreergy says:

    Don’t speak if you can’t say out something better than the silence is.

  7. pcsmith says:

    Besides offering the locking gas cap to stop people from stealing gas in the 1970’s, they also changed the filler tube design, which also prevents you from getting gas out of your own vehicle.

  8. bobbo says:

    Thanks MacGuy. You are less inaccurate than I was. From your link, I would say we were both thinking of “angular velocity” and not acceleration at all?

    It is all “close” though. Reminds me of Einstein saying you can’t tell the difference between acceleration and increase in gravity?==or the introduction of angular velocity which in my elevator world is starting to “feel” like some sort of acceleration as well?

    This is what happens when you try to “feel” physics.


    ((PS–does angular velocity mean acceleration towards the middle? That makes sense and is just what you said. But if you accelerate towards something, why don’t you “get there?” Seems like angular velocity would be acceleration AWAY from the center. As stated–I’m no physicist.))

  9. bb says:

    Bobbo, think of it the other way around. Without acceleration (or equivalently, without an force) objects continue on a straight line at a constant speed.

    To move in a circle, “something” must push an object towards the center of that circle as it moves around. That “something” is a force and per F=MA (Force=Mass*Acceleration) that is exactly proportional to the acceleration.

    No, I’m not a physicist, I’m a Engineer. Unlike physicists, we have to get things to work.

    And yes, the diagram is still wrong.

  10. Mac Guy says:

    #7 – Because your velocity at each moment is enough to keep from getting pulled in towards the center. It’s been 15 years since my last physics class, but I’m trying to find the correct term for this concept.

  11. mcosmi says:

    #3, yes there is a way to subscribe to it as a podcast here:

    you can look at dvoraks shows and The Random Space channel has some icons under it so you can click on the one that corresponds to the app you use to get podcasts. Also, you can go directly to the The Random Space channel page here:

    but you should subscribe to all of johns channels..i use itunes and i just subscribe to all of em.

  12. Erm……. get a copy of Principia and read the page with Sir Isaac’s three laws???

    This is demonstrating a “centripetal force”.

    1. “gravity (weight)” arrow

    This always points down, with almost exactly the same force at every point around the loop.

    2. “Acceleration force” arrow

    Wow – so wrong! The only other force on the object other than gravity is the reaction force from the track against the cart. This will always point to the centre, in order to make the loop a perfect circle.

    However, this size of this force (and direction as you go round the circle) will be constantly changing. It will be the largest at the very bottom, as it has to combat gravity, whilst at the top the force will be the smallest as gravity is adding to the net force.

    3. “Apparent weight” arrow

    Weight is the reaction force on an object due to gravity. “Apparent” is referring to the fact that you can pretend your in a gravitational field by having a reaction force on you, such as being in a rocket in space and firing the thrusters. Thus what you are drawing is the reaction force on the cart from the track, so this arrow should be pointing towards the centre at all points around the loop.

    I didn’t do Physics at Cambridge for nothing.

  13. #8 – angular velocity when talking about centripetal forces is the velocity in the direction perpendicular to the centripetal force. If this remains constant, and so does the net centripetal force, the object will move in a circle.

    So the cart has a constant speed (angular velocity), but has a constantly changing velocity in a set direction (as F=ma, and the track is applying a force, there must be an acceleration, and thus a change in velocity)

  14. bobbo says:

    #9–bb==thanks, that made sense to me. After the first 3 simplest Newtonian laws, most concepts in physics get beyond my comprehension.

    That said, I still don’t conceptualize how there can be a “slingshot effect” using planets to increase the speed of spaceship on its way thru the solar system. Bubba Ray a while back said it was because of “relative speeds” but that still doesn’t compute==seems any gain made in approaching the planet would be removed when going away from the planet.

    Thats why I love science. Test it and see.

  15. Mac Guy says:

    Wow. I totally hijacked this thread, didn’t I?

  16. brian t says:

    Agree with the some of above – the “normal” acceleration due to circular motion always points towards the center of the circle.

    However… the main problem with a diagram like this is the confusion between the forces caused BY you, and the forces that act ON you. that centripetal force (towards centre of circle) acts ON you, but your weight is caused BY you. To be consistent, it should show your weight as always pointing UPwards, since the force that acts ON you is the force created by the Earth that stops you falling downwards!

    That could be doubly confusing to readers, so if you look at it from the other side, the forces you exert, then you exert a force downwards due to your weight, and a force outwards from the circle, your body’s _reaction_ to the inwards centripetal force.

    Halfway up the circle, the resultant of those two would point at an outwards angle downwards. That “apparent weight” should be the vector sum of the other two forces, and scaled appropriately – not the same size as each component as in that diagram.


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