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Does mass impact speed on a ramp?

  1. Jan 14, 2015 #1
    Mentors Note: Posts from a duplicate thread have been merged into this one.

    Hello, please explain the following in detail. I am a learning assistant (LA) at a major university but am stuck explaining a concept to my students.

    Does mass have an impact on an inclined plane? A cart with wheels or a block of wood sliding down? I say, no. And, I can mathematically show this. However, my students showed me a Java applet in which a mass is put into a toy truck, and, upon its release off of a ramp, the travels farther! Is this true?

    When I substantiate my claim, I continually get the masses to cancel.

    acceleration = g * (-mu_k * cos(theta) + sin(theta) ). (equation 1)
    simple model, I get

    mu_k = a/g (equation 2)

    Where mu_k is the coefficient of kinetic friction.

    Mass never appears.

    The students said that on Olympic bobsled on Wikipedia, the claim is that heavier sledders sleigh down faster. On YouTube videos, heavier skate boarders skate down faster.

    I keep claiming that, using conservation of energy,

    v = sqrt(2gh) (equation 3).

    Am I right or are they right? I tutor an entire class of university students and I am angry at myself for getting entangled in something I should know.

    Thank you.
     
    Last edited by a moderator: Jan 14, 2015
  2. jcsd
  3. Jan 14, 2015 #2

    A.T.

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    If you consider air resistance, then yes. Just like during falling through the atmosphere.
     
  4. Jan 14, 2015 #3

    Bystander

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    "Farther" I can see. If you meant "faster," take a look at the moment of inertia of the wheels on the cart/truck. It should be insignificant, but depending on magnitude, might have an effect.
    Sounds like "Cool Runnings" and John Candy are tough to argue with in students' minds. Coefficient of friction for the two different sled masses on same runners ----- tough to say constant, and tough to say what might affect it. Film thickness if we subscribe to the change in m.p. temperature with pressure idea, and any number of folklore ideas if we listen to sportscasters, script writers, or real competitors.

    Help any?
     
  5. Jan 14, 2015 #4
    What is the skateboarders and bobsleigh practitioners are shaped exactly the same? What is instead I roll to identical balls of different masses?
     
  6. Jan 14, 2015 #5
    Hello! Thank you for response. Oh, actually, I think I am mixing farther and faster.

    The heavier travels farther, but not faster, is that right? It does not travel faster is my claim. I see your point about the misconceptions of sports casters. If you go onto Wikipedia "Bobsleigh" the Olympic section immediately posts that weight matters.

    So, is it physically correct to say that, given the same roughness index of surface area of two people on identical sleds, they will sled down at the same speed and cross the finish line at the same time?
     
  7. Jan 14, 2015 #6

    A.T.

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    Same shape & size makes the more massive object faster.

    Rolling makes the speed also dependent in the moment of inertia (mass distribution).
     
  8. Jan 14, 2015 #7

    Yes, yes, I see. Now, two identical block of different masses? They must travel at same speed and cross line at same time, correct?
     
  9. Jan 14, 2015 #8
    There was some lame response I saw on a forum by a student on a national laboratory and, the writer said that heavier objects on ramps can overcome friction whereas lighter ones cannot. I think that is categorically wrong. Friction is proportional to mass and there is no way to "overcome" friction. Is that right?
     
  10. Jan 14, 2015 #9
    Sorry to inundate you with this. So, a student of mine claims that when his group does the blocks-on-ramp experiment, the heavier block slides off first (i.e., at a lower angle). I said this cannot be possible. This goes against everything I know. They taped heavy washers onto one block but not the other. This cannot possibly be true, can it? When I derive the equation from first principles, the masses always cancel!

    I can only see this situation: heavy block travels farther because of the effects of inertia. But, does not travel faster. And, does not have a lower angle-of-slip.
     
  11. Jan 14, 2015 #10

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    Same mass, they should. The coefficient of friction on ice being constant no matter what the load --- I've listened to too many hockey games and no longer can convince myself I've got a clue. I'm reasonably certain the "pressure melting" (it's only a few hundredths of a degree for loads on ice skates) can't be it, but at the same time, ice skates and bobsled runners are "hollow ground" and the loading area and pressure may be variable --- that's actually your out on that item, come to think of it.

    As "far" as the cart with two different masses is concerned, the heavier cart will furnish more torque to accelerate the rotational motion of the wheels, and does accelerate more rapidly. As I said, most carts I've seen have very low mass wooden wheels, and it should be a miniscule difference.
     
  12. Jan 14, 2015 #11

    Right, yes, thank you.
     
  13. Jan 14, 2015 #12

    A.T.

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    Not if air resistance is considered, as I already said.

    That is the most simple friction model. Not sure if it's still accurate on real ice, for bobsleds and skaters.
     
  14. Jan 14, 2015 #13

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    Blocks on ramp: this is static friction? Tip them up and see at what angle they start to slide? Taping weights on top of the block and tipping is NOT generally going to be loading the contact area between the block and ramp uniformly, at which point, I don't want to be the guy trying to integrate normal force over contact area to say what's going to happen. For a lab I'd want a block with a hole drilled through the center of mass from one side (not end) to the other that I could insert a close fitting brass, iron, lead weight into for such an experiment.
     
  15. Jan 14, 2015 #14
    Yes, this is a static friction tipping experiment. Students claim heavier block slips at a lower angle. I will discuss tomorrow with the laboratory manager about your second recommendation, then replicate the experiment myself. Thanks. Seems like reality is not so simple as theory.
     
  16. Jan 14, 2015 #15

    CWatters

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    Indeed, Google suggests that it's not "pressure causing ice to melt" that makes it slippery...

    http://www.exploratorium.com/hockey/ice2.html [Broken]

     
    Last edited by a moderator: May 7, 2017
  17. Jan 14, 2015 #16

    jtbell

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    The frictional force equation Ff = μFN, with constant μ, is only an approximation for a limited range of FN. For example, if the mass is heavy enough to "dig into" the surface it is resting on, that will make a difference. Also, in a typical student lab setup, it's hard to make the surface conditions exactly reproducible from one trial to the next.
     
  18. Jan 14, 2015 #17

    A.T.

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    Right, but his students reported the heavier block as moving faster, which could be an effect air resistance. The same air resistance slows a light object more than a heavy object.
     
  19. Jan 14, 2015 #18

    CWatters

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    I guess we must remember that this was a java applet not a real world experiment. Quite possible that whoever wrote the code got it wrong.

    Any chance of a URL?
     
  20. Jan 14, 2015 #19

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    Regarding quality of ice, coefficients of friction, myths and folklore, the following link may be interesting.
    http://www.covers.com/articles/articles.aspx?theArt=169900
    Last winter olympics, there was a fair amount of time devoted to details of surface preparation for curling, including temperature, "pebbling" of the surface, and other esoteric details. None of this is reference quality data, but may serve to illustrate the scope of the problems to be dealt with when discussing coefficient of friction and ice surfaces.
     
  21. Jan 14, 2015 #20
    Heavier sledders are less affected by air resistance, since the force of gravity scales with their mass, while the force of drag scales with their cross sectional area. Generally mass increases faster than area, so at larger scales, gravity wins out. (For similar shapes, area scales as length^2, and mass scales as length^3. Generally, viscous drag becomes less important for larger objects. Research "Reynolds number" for related info.)

    Snow is not quite a fluid, but probably takes on some of its characteristics at high relative velocity.
     
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