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Homework Help: Wood and bullet Friction Question

  1. Jan 19, 2009 #1
    1. The problem statement, all variables and given/known data
    2 kg block of wood rests on a long tabletop. A 5g bullet moving horizontally with a speed of 150m/s is shot into the block and sticks into it. The block then slides 270cm along the table and stops
    A) find the speed of the block ( answer: 0.374 m/s)
    B)*** find the frictional force between block and table ( answer: )

    2. Relevant equations

    Ff = mew * Fn
    Ft = mv

    3. The attempt at a solution

    after trying to find the solution from relevant momentum formulas, i am still completely stumped as to how i would find the frictional force between table and block

    much appreciated if you read this over
  2. jcsd
  3. Jan 19, 2009 #2


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    Well, think about what happens. A bullet with a certain kinetic energy comes along and embeds itself in a block. The combined block and bullet slide across the table. Where does that initial kinetic energy go? If you are allowed to neglect the heat produced in the block during the collision, don't you suppose it goes into work to overcome friction as the block slides to a stop? If you are not allowed to neglect the heat generated, then you need the velocity of the block plus bullet right after the collision that gives you the kinetic energy of the pair at the start and then that energy disappears the same way as before. Think work = force times distance. That should get you started with the second part. However...

    The first part makes use of momentum conservation. It would be a useful exercise for you to calculate the answer to the second part both ways (from the KE of the bullet and from the KE of the bullet plus block right after collision.) Do you find a difference?
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