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Homework Help: Momenrum problems

  1. Nov 21, 2004 #1
    momentum problems

    it's be really cool if someone could help me out with the following two momentum problems. nothing much just enough to get me started. i have ideas but i'm not quite sure about what's the right way to go about doing them. also wanna say hi to all the people here. must say the whole homework help thing is a very neat concept. :biggrin:

    problem 1- A 25 kg child is in a 10 kg sled that travels 1m/s east on ice. The child throws a 3 kg snowball at 40m/s at an angle of 50 degrees north of east. Find the velocity of the sled after the snowball leaves the child's hand.

    problem 2- A 2kg block and an 8-kg block are both attached to an ideal spring (for which k=200 N/m) and both are initially at rest on a horizontal frictionless surface.

    In an initial experiment, a 0.1 kg ball of clay is thrown at the 2kg block. The clay is moving horizontally with speed v when it hits and sticks to the block. the 8-kg block is held still by a removable stop. As a result, the spring compresses a maximum distance of 0.4 meters. Find:

    a) the energy stored in the spring at max compression.

    b) speed of the clat ball and 2-kg block immediately after the clay sticks to the block but before the spring compresses significantly.

    c) the initial speed v of the clay.
    Last edited: Nov 21, 2004
  2. jcsd
  3. Nov 22, 2004 #2


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    Use conservation of momentum. I think you have to assume the 3 kg snowball was part of the sled+child system before the kid threw it.
    The initial momentum is (25+10)1=35 kg m/s in the (say) x-direction.
    The final momentum is equal to this.
    Find the momentum of the snowball and the sled+kid which now weighs 32 kg together.
  4. Nov 22, 2004 #3
    problem 2

    a) it's just E = 1/2kx^2 = 1/2k(0.4)^2 J
    b) & c) I couldn't really understand the question but I think it has something to do with the Kinetic Energy being transformed into Elastic Energy which means that 1/2kx^2 = 1/2mv^2
  5. Nov 22, 2004 #4
    thanks a lot for the replies, Galileo and futb0l. it was due today and i got it done(eventually) but i had to ask my friend to help me out in lunch.
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