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Homework Help: Magnitudes of forces

  1. Oct 12, 2007 #1
    1. The problem statement, all variables and given/known data

    A block is on a ramp. What is happening to the block for each of the following situations?:
    1. fk(down the ramp)
    2. fs(up the ramp) < w,x(down)
    3. Tension(up) > fk(down) + w,x(down)
    4. fk(up) < w,x(down)..FBD also looks like fk(up) magnitude is equal to fs(up) from 2.

    A) a block is sliding down a ramp at increasing speed.
    B) a block is being dragged up a ramp.
    C) a block is sliding down a ramp at decreasing speed.
    D) a block is sliding up a ramp.
    E) a block is stationary on a ramp.
    F) a block is momentarily at rest; it is changing directions-after sliding up a ramp, it is about to slide back down.

    2. Relevant equations

    Newton's second: Fnet=ma

    3. The attempt at a solution

    This is what i think. Please correct me if I'm wrong at any point. Thanks.

    1.A - because both weight and friction pulls it down
    2.E - because fs is acting.
    3.B - Since tension is greater than the sum of fk and weight that is pulling it down. But
    what is the difference between sliding up and being dragged up? Since tension is
    making it go up, should it be dragged up?
    4.F - momentarily at rest because magnitude of fk is equal to fs from 2. But changes to
    downwards because given that fk and not fs is applied.
    Last edited: Oct 12, 2007
  2. jcsd
  3. Oct 12, 2007 #2


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    but friction always acts opposite the direction of motion.

    I agree. So is there always a tension involved? Because if fs<w,x the block would be sliding unless there were other forces involved.

    Yeah, both dragged up and sliding up seem right. I don't understand the difference.


    I don't see why... if the object is momentarily at rest, then kinetic friction will not be acting.
  4. Oct 12, 2007 #3
    tension is only involved in 3. These are the given FBD:
    1. http://www.learning.physics.dal.ca/library/Graphics/Gtype10/blockfbd1.gif [Broken]
    2. http://www.learning.physics.dal.ca/library/Graphics/Gtype10/blockfbd6.gif [Broken]
    3. http://www.learning.physics.dal.ca/library/Graphics/Gtype10/blockfbd2.gif [Broken]
    4. http://www.learning.physics.dal.ca/library/Graphics/Gtype10/blockfbd3.gif [Broken]
    Last edited by a moderator: May 3, 2017
  5. Oct 12, 2007 #4
    For 1, as you see in the FBD, only fk is drawn. I don't know if ALL forces are drawn in but without a pushing force, the fk and gravity will bring it down right? Im not sure for which cases but i remember that friction doesn't ALWAYS oppose motion. Im confused.

    For 4, the fk is drawn so that it is equal to fs from #2. Um.. it's not possible unless there is other forces not drawn that are applied to the block, right?
  6. Oct 12, 2007 #5


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    Not necessarily. It could be sliding upwards and decelerating.

    Hmmm... as far as I know, static friction prevents motion, and kinetic friction acts against the direction of motion.

    In 4, it seems to me it is sliding downwards since kinetic friction is acting upwards.
  7. Oct 13, 2007 #6
    Ok. Im convinced that 1. is sliding up the ramp.
    If 4 is sliding downwards, would it be going down at increasing speed because w,x is greater in magnitude than fk so Fnet is positive (downwards) so that acceleration is also positive(downwards)?
  8. Oct 13, 2007 #7


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