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Correction to force of friction keeping the block stationary

AP.PHYS:
INT‑3.B.2 (EK)
Correction to Force of Friction Keeping the Block Stationary. Created by Sal Khan.

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  • piceratops ultimate style avatar for user Mrbobthebuilder6
    Isn't the additional force of 1 N pretty arbitrary? Wouldn't the budging force be just over 49, so the coefficient of friction would be 49/49sqrt3=1/sqrt3?
    (8 votes)
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    • starky ultimate style avatar for user JJ Arnold
      This was completely confusing me as well: how did Sal come up with 1N?

      All he's saying is that with the hypothetical materials of the block and plane (which are completely unknown really) he began applying force and at 0.5N it didn't move, at 0.99N it didn't move, but finally at exactly 1N the block began to move. So in this case the coefficient of static friction was 49N at rest + the 1N extra force to finally make it move = 50N (or budging force) and divide that by the normal force to get coef of static friction of 0.59 for this scenario's materials.

      In other words, he could've just as easily said it took 10N to finally move the object. He tried 5N, then 7N, 9.99N and finally at 10N the object moved. in this case the budging force would've been 59N (the 49N of parallel force + the extra 10N of force to finally get the block to move) and the coef would've been 59N / 49*sqrt3N = mu(static) or 0.695.
      (15 votes)
  • leaf green style avatar for user Morwarid Najafizada
    what is coefficient? what is it's usage in this question? thanks
    (2 votes)
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    • male robot hal style avatar for user George Arrington
      This is the "coefficient of static friction". It is a dimensionless measure of how much frictional force exists between an object and a surface. It indicates the force resisting the object to get started moving. The coefficient of static friction is typically larger than the coefficient of kinetic friction, which indicates the friction force that exists while the object is moving.
      (7 votes)
  • leafers tree style avatar for user Xihai Luo
    Wait, isn't the coefficient of static friction the tan of theta? That is what my textbook says. So shouldn't the coefficient of static friction be tan(theta) = tan(30) = 0.577 ?
    (2 votes)
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    • male robot hal style avatar for user Satwik Pasani
      No. What your textbook probably says, that an angle theta for which a block will begin to move spontaneously under its own weight on an inclined plane, must be related to the coefficient of friction between block and the incline, mu, as mu=tan(theta). That is called the angle of repose. mu is a property of the nature of surfaces in contact only.
      (6 votes)
  • leaf green style avatar for user Awsaf Alam Anindo
    why is energy a scalar quantity
    (4 votes)
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    • leaf green style avatar for user azuro010
      though energy is a vector quantity but still from experience we can say that it works in a definite direction. But it is scalar quantity because the final product of Mass*square of velocity is always a positive quantity. Hence, by mathematical calculations , energy need not be scalar.
      (0 votes)
  • aqualine ultimate style avatar for user Julia
    Why did he add additional 1N? Can't he add .5N (or any other number) because that is still greater than 40N?
    (2 votes)
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  • piceratops seed style avatar for user Christian LaDon
    I'm still very confused on the "budging force" example. Normally when asked to find the coefficient for static friction you are only told so much information. how do we come up with the "budging force" given limited information. 1N seemed pretty arbitrary. Please help!
    (1 vote)
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    • leaf green style avatar for user samimorin654
      Is static friction a constant? I am confused as to why it was 1N when it could have been 0.1. Wouldn't that change the static friction? Is the static friction different for different materials, and how would that be calculated or would it have to be given?
      (2 votes)
  • leaf green style avatar for user Harvey  G
    What is the difference between static and kinetic friction
    (2 votes)
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    • leaf green style avatar for user N.Fajardo28
      Static Friction is the friction present in a stationary object. In order to set a object in motion, the applied force must be greater than the force of static friction. Kinetic friction on the other hand is the force of friction present when a object is moving. Static friction will become kinetic friction once the object has been set in motion.
      (1 vote)
  • blobby green style avatar for user Wizzy.Wig16
    what are the units of the coefficient of static friction?
    (1 vote)
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  • starky tree style avatar for user Rafea Taji
    I think Sal used 1 N additional force because he wanted to just overcome the static friction. Had it been a greater additional force, coefficient of Kinetic friction would have come into action. Correct me if I'm wrong.
    (2 votes)
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  • piceratops ultimate style avatar for user dawood.aijaz97
    how to calculate gravitational potential energy of a mass on inclined plane of height H?
    (1 vote)
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Video transcript

Just realized that I typed in the wrong numbers at the end of the last video when I was trying to put them in the calculator. We were trying to determine the coefficient of static friction, and it's 50 newtons divided by 49 square roots of 3 newtons. And in the last video, by accident, in the calculator, instead of doing 49 square roots of 3, I wrote 40 square roots of 3. So let's actually calculate it again. 50 divided by 49 times-- let me write it this way, make sure we get-- 49 times the square root of 3 gives us a coefficient of static friction of, if we round it, 0.59. So this is equal to 0.59. And so that's it. I just forgot to-- I did the calculation wrong on the calculator in the last video.