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Homework Help: Finding velocity with friction?

  1. Sep 18, 2014 #1
    1. The problem statement, all variables and given/known data

    A m = 0.500 kg block is started at velocity vo and slides along a rough table (μk = 0.300) for 1 m before flying horizontally off the end of the table.(Figure 1) The surface of the table is 1.2 m up in the air, and the block hits the ground 3.1 meters away from the base of the table.

    What is the velocity with which the block leaves the table?

    What is the acceleration of the block (assume the +x direction is positive) while it is sliding along the table?

    What was the initial velocity v0 of the block?

    I feel like I should also explain that the real problem is that I just don't get anything we've talked about in class. I've been out of school for over a decade, and I've never had any physics before. I'm completely overwhelmed and I'm getting frustrated because I know my problem is comprehending what information I have to work with. For example, I look at this problem and wonder how I'm supposed to use a kinematic equation when all the variables I have apply to mass?!

    2. Relevant equations
    I have no clue. I know I don't have initial or final velocity, or time. I do have the acceleration of gravity (9.81m/s^2), and a distance of 1m before the block leaves the table. So maybe, if I can consider initial velocity 0:
    so V^2=19.6m/s

    3. The attempt at a solution
    The above equation would be:
    so V^2=19.6m/s
    Which I tried, and is wrong. I don't want this solved or anything I really just want to know how I'm looking at the variables wrong. This is probably a good question to ask that about since that's my problem with every question, and this one seems like I'm starting with no variables other than gravity. Any other tips on beginning to understand physics would be appreciated too.
  2. jcsd
  3. Sep 18, 2014 #2

    Simon Bridge

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    You should list what you do know, not what you don't know.
    i.e. you know the forces on the block and the distance that it moves while it is on the table.
    you also know the forces and distances when the block is not on the table - and you have a bunch of notes about the motion of falling objects.
  4. Sep 18, 2014 #3
    The value for g is often ommited, but its crucial.
    Id be working this problem backwards, starting with the time it takes the block to fall to the floor.
  5. Sep 18, 2014 #4


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    Have you done any other projectile problems?

    One thing beginners miss is that the TIME taken for the projectile to move horizontally must be the same as vertically.
  6. Sep 18, 2014 #5


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    Hi dba80,

    It sounds like you may be confusing yourself by trying to solve this problem all at once, when it would actually make sense to split this up into two separate stages:

    (1) A projectile motion problem - forget about the sliding portion of the problem at first and just focus on this block flying off the side of the table.

    Draw a diagram of this and label your variables. Then work towards figuring out the velocity in which the block leaves the table using some kinematics equations.

    Here's an example that might be useful to study if you get stuck.

    (2) A force + 1D kinematics problem - once you've completed part (1) and found the velocity at the edge, now work on solving for the initial velocity.

    Again draw a diagram, this time of the block sliding across the table. Label your variables.

    Then use a free-body-diagram to label your forces and use some force balance equations to find your (negative) acceleration due to friction.

    Once you've done that, then you can use some 1D kinematics to find your initial velocity.

    Try this and post back if you have questions or get stuck.

    Good luck!
  7. Sep 18, 2014 #6
    As other people above have mentioned, maybe starting from the point the block falls off the table until it hits the ground.


    d = falling distance
    g = gravity (my school always used [itex]9.81 \frac{m}{s^2}[/itex])
    t = falling time

    Once you have the time from the point where the block leaves the table and hits the ground, you should be able to find the velocity with which the block left the table.
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