hypothetical physics question

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  • atgcpaul
    Veteran Member
    • Aug 2003
    • 4055
    • Maryland
    • Grizzly 1023SLX

    hypothetical physics question

    This morning I saw a pickup carrying several sheets of plywood in the bed. They were strapped down. That got me thinking back to what my HS physics teacher told us, "Friction is your friend"--which for a classroom of mostly 15 year old boys sounds pretty funny.

    So hypothetically speaking, if friction did not exist between those boards and the strap and we're dealing with normal Earth gravity at sea level, when the pickup accelerated from a stop, would the plywood slide out of the back of the truck?

    I don't know the answer, just curious.

    Happy Monday.
  • Pappy
    The Full Monte
    • Dec 2002
    • 10453
    • San Marcos, TX, USA.
    • BT3000 (x2)

    #2
    Go back to bed, get up and try to start the day over again.....
    Don, aka Pappy,

    Wise men talk because they have something to say,
    Fools because they have to say something.
    Plato

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    • twistsol
      Veteran Member
      • Dec 2002
      • 2893
      • Cottage Grove, MN, USA.
      • Ridgid R4512, 2x ShopSmith Mark V 520, 1951 Shopsmith 10ER

      #3
      Assuming the strap is over the top of the plywood, Yes the plywood would slide out. If you accurate fast enough, the plywood will slide out. If the strap is wrapped over the ends of the plywood, then no.


      Sent from my iPhone using Tapatalk
      Chr's
      __________
      An ethical man knows the right thing to do.
      A moral man does it.

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      • Bill in Buena Park
        Veteran Member
        • Nov 2007
        • 1865
        • Buena Park, CA
        • CM 21829

        #4
        Originally posted by twistsol
        If the strap is wrapped over the ends of the plywood, then no.
        If the straps are hooked to the bed, agree. If they are tied, without friction, the straps would untie themselves...
        Bill in Buena Park

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        • LCHIEN
          Internet Fact Checker
          • Dec 2002
          • 20914
          • Katy, TX, USA.
          • BT3000 vintage 1999

          #5
          Originally posted by Pappy
          Go back to bed, get up and try to start the day over again.....
          if he goes to bed and there's no friction he will just end up on the floor.
          Loring in Katy, TX USA
          If your only tool is a hammer, you tend to treat all problems as if they were nails.
          BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

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          • radhak
            Veteran Member
            • Apr 2006
            • 3058
            • Miramar, FL
            • Right Tilt 3HP Unisaw

            #6
            : that's one way of solving all that!

            Originally posted by Pappy
            Go back to bed, get up and try to start the day over again.....
            It is the mark of an educated mind to be able to entertain a thought without accepting it.
            - Aristotle

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            • radhak
              Veteran Member
              • Apr 2006
              • 3058
              • Miramar, FL
              • Right Tilt 3HP Unisaw

              #7
              I used to love such what-if problems with science.

              If the scenario is there is no friction at all (as in, a friction-less world), then the world as we know it stops; nothing happens: the sheets of plywood keep shifting for no reason, you can't tie a knot, and even the pickup won't drive (because the wheels would just rotate in place without friction, not getting a grip for motion).

              But if we have selective loss of friction - as in, just the friction between the strap and the ply is missing, then the friction between the sheets of plywood might still be good enough to hold them together (some of the nastiest fights I have had with lumber were to try and separate out a full sized sheet of ply from its buddies!). So if you are a gentle driver, you could transport without the strap at all.

              On the other hand if we are talking sheets of slick glass with no friction between them, that's a scenario that a hollywood stunt director would love to think up :-)!
              It is the mark of an educated mind to be able to entertain a thought without accepting it.
              - Aristotle

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              • cwsmith
                Veteran Member
                • Dec 2005
                • 2737
                • NY Southern Tier, USA.
                • BT3100-1

                #8
                I think Radhak has the best answer yet... simply if there is no friction, then the truck cannot excellerate, because the tires would just spin. However, if you're just asking about the plywood itself, then there may be other complications. First I'd have to ask about the plywood and whether it was perfectly smooth and if so, then the lack of friction may well leave the plywood in it's geographical position as the truck pulls away. But, if there are any snags, like wood grain sticking out that would hook onto the straps then they be enough to restrain the mass of the wood? I would not consider hooks as being "friction"... as they are more like retainers, and especially so if they are in the tens or hundreds.

                As I envision it, a "hook" does not retain an object because of friction, it does so as mechanical restriction in that the strength of the hook or hooks is such that their total strength is more than the mass that they are restricting against either gravitational forces or inertia. For example, a lifting hook does not rely on friction, it is the material strength and mechanical design against the weight being lifted. Should the crane accelerate to rapidly, the mass of the object being lifted could overcome the material strength of the hook and it could well snap, leaving the mass in place or worse allowing it to fall. Likewise would be be the plywood, if there are not enough wood fibers to snag on the strap or they are materially weak. It's would be a matter of material strength vs the mass of the object as opposed to just friction.

                Of course one has to define friction too... is it just the force of gravity and/or force against an aerodynamic object with little to no drag on it's surface or is greater drag on an increased surface enough to account for the object to be restrained and thus accelerated with the vehicle? The strap would have mechanical restriction of the object against lifting if forces were to attempt to lift the plywood upwards. The tightness of the strap would hold the plywood against it slipping only if it could possible snag on the plywood's edges. I question whether there would be enough friction by itself to overcome the forces of acceleraton without the mechanics of some other mechanical restraints, like the many fibers that would hook onto the strap.

                Now I think I'll go back to sleep,

                CWS
                Think it Through Before You Do!

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                • seahawk
                  Forum Newbie
                  • Oct 2012
                  • 54
                  • Maryland
                  • BT 3000 x 2!

                  #9
                  They would definitely slide out..even WITH friction, they can slide out...don't ask me how I know.... ;-)

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