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Force needed to move a crate

  1. Oct 3, 2011 #1
    1. The problem statement, all variables and given/known data

    Someone pushes a crate with a force F at 21° below the horizontal. The crate weights 38 kg, and the coefficient of static friction between the crate and the ground is .57. Find the amount of force needed to move the crate.

    2. Relevant equations

    f=ma
    Fs=([itex]\mu[/itex]s)(Fn)

    3. The attempt at a solution

    First i tried to find Fn, so i multiplied the mass by 10, and got 380N. Then i added Fsin21 to find Fn.....thats about as far as i got haha i dont know what to do after that. i know i need F, but i dont know how to find what it is.
     
  2. jcsd
  3. Oct 3, 2011 #2
    So you know the value of the frictional force.
    What force must just be a little bit greater than this frictional force?
     
  4. Oct 3, 2011 #3
    If you haven't done so, I recommend drawing a free body diagram (FBD). It will help you to visualize what you need.

    Also, make a list of what you know and what you don't know.

    Can you construct the same number of equations as your unknowns?
     
  5. Oct 3, 2011 #4
    the force you need to push with?
     
  6. Oct 3, 2011 #5
    im not exactly sure what i need to know to solve this haha
     
  7. Oct 3, 2011 #6
    Like I mentioned above, draw out a FBD with your forces. It will help you to see what you need to solve for.
     
  8. Oct 3, 2011 #7
    already had it drawn out, the only things i seem to be missing are F, Fk, and Fn
     
  9. Oct 3, 2011 #8
    The force you push is at 21 below horizontal. But I asked for the force just to balance the friction and the if two forces are to balance each other they have to be opposite in dir.
    Follow advice given and draw the FBD.
     
  10. Oct 3, 2011 #9
    i did, to the best of my ability
     
  11. Oct 3, 2011 #10
    Alright. Remember that there is no acceleration up or down (since the ground is solid). What does this mean to the forces in the vertical direction? Can you write out an equation showing this relation?

    What are you looking for in the horizontal direction? Can you write out a relation with the forces you know to create this?

    Once you have these 2 equations, solve them simultaneously.
     
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