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Finding the force to pull a block up a frictionless slope?

  1. Mar 11, 2016 #1
    1. The problem statement, all variables and given/known data
    "A 68 kg crate is dragged across a floor by pulling on a rope attached to the crate and inclined 15° above the horizontal. (a) If the coefficient of static friction is 0.50, what minimum force magnitude is required from the rope to start the crate moving? (b) If μk = 0.35, what is the magnitude of the initial acceleration of the crate?"

    2. Relevant equations
    fs, max = μsFN
    fk = μkFN
    ##m=68kg##
    ##θ=255°##
    ##μ_s=0.5##
    ##μ_k=0.35##

    woZpNBK.png

    Answer according to book: (a) 300 N; (b) 1.3 m/s2

    3. The attempt at a solution
    ##F_g=(cos255°)(9.8\frac{m}{s^2})(68kg)=-172.48N##
    ##F_N=(sin255°)(9.8\frac{m}{s^2})(68kg)=-643.69N##
    ##f_s=(-643.69N)(0.5)=-321.84N##

    But I know this isn't all that is needed to make the block go up. I have to add the difference between the gravitational force and friction, plus the friction, going up, to yield the force to pull the block up. But when I do this, the numbers do not match.
     
  2. jcsd
  3. Mar 11, 2016 #2

    haruspex

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    Why does your diagram show friction going both ways?

    Edit: as in the other thread, you have created an unknown force called Fg but not defined it. We cannot check whether your equations are right if you do not state what the variables represent.
     
  4. Mar 11, 2016 #3
    Because there's friction when the block goes up and goes down.

    It's gravity, relative to the block's axes.
     
  5. Mar 11, 2016 #4

    haruspex

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    It will not do both at once. Draw each diagram for a specific context. In this case, I don't see any part of the question where the block will be sliding down.
    Ok, but that's still ambiguous. Do you mean parallel to the slope or normal to it?
     
  6. Mar 11, 2016 #5
    It's 255° from the direction of its travel.

    aFkecMS.png
     
  7. Mar 11, 2016 #6
    Never mind; I'm just so bad at this. No one needs to help with this thread anymore.
     
  8. Mar 11, 2016 #7

    haruspex

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    I just reread the question and realised your diagram is quite wrong.
    The floor is level. The rope is at 15 degrees.
     
  9. Mar 11, 2016 #8
    I'm not doing the problem. I'm too damn tired of this.
     
  10. Mar 12, 2016 #9
    Thank you for this insight. Let F = magnitude of the force pulling the block at a 15° angle. I know someone already did this, but it got deleted. Anyway, I want to show my process. This will be the last physics problem I do until the fall. Self-studying just isn't working for me. I get too frustrated and it messes up my mood. It's not good for my health. Thanks, guys, for being so patient with me when I've been in such a pissy mood. Anyway, here's how I did the problem. I expressed ##f_s## in terms of ##-(a)(68kg)(cos15°) = F_x## since the latter opposes the former, and vice versa. Then I express friction as such:

    ##f_s=(68kg)((a)(sin15°)-9.8\frac{m}{s^2})(0.5)##
    ##-(a)(68kg)(cos15°)=(34kg)((a)(sin15°)-9.8\frac{m}{s^2})##
    ##-(a)(68kg)(cos15°)=(34kg)(a)(sin15°)-333.2N##
    ##-(a)(68kg)(cos15°)-(a)(34kg)(sin15°)=-333.2N##
    ##-a((68kg)(cos15°)+(34kg)(sin15°))=-333.2N##
    ##a=\frac{333.2N}{(68kg)(cos15°)+(34kg)(sin15°)}##
    ##a=4.474\frac{m}{s^2}##
    ##F=(68kg)(4.474\frac{m}{s^2})=304.2N##
    ##F_x=(68kg)(cos15°)(4.474\frac{m}{s^2})=293.87N##

    ##f_k=(68kg)((4.474\frac{m}{s^2})(sin15°)-9.8\frac{m}{s^2})(0.35)=-205.7N##
    ##F_x-f_k=88.17N##
    ##a_x=\frac{88.17N}{68kg}=1.297\frac{m}{s^2}##

    Again, thank you, everyone. I haven't been acting very grateful recently, I'm sorry to say.
     
  11. Mar 12, 2016 #10
    I worked the problem out earlier and got the same results

    Good job and perseverance
     
  12. Mar 12, 2016 #11

    haruspex

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    Thanks for returning to this and not letting my efforts go to waste.
    I found your method a little strange in that you seem to have defined a as tension/mass, not an obvious thing to do. But it works.
     
  13. Mar 12, 2016 #12
    a defines acceleration of the upward 15° force, and 68 kg defines the mass. That makes a force, which is opposite of friction.
     
  14. Mar 13, 2016 #13
    Thanks for the tip, by the way; really helped me out.
     
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