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Homework Help: Momentum and Collisions

  1. Feb 7, 2009 #1
    The problem statement, all variables and given/known data
    Ok, I have a couple problems that I'm having trouble with. Bear with me, my physics teacher can be a little creative...lol.

    1. I am an expert swan diver. If I am jumping off a 3m diving board, what is the velocity with which I strike the water? If the water brings me to rest in 0.3 seconds, what is the force the water applies to me?

    2. Driving along in your 850 N Mini Cooper you pass by a hemp clothing yard sale and slam on your brakes. If you were traveling 20 m/s and your brakes apply a force of 8000 N in the opposite direction, how long does it take you to stop? How far do you skid?

    3. Super Mario, who weighs 5 kg, is standing on a platform, which weighs 25 kg. Seeing Bowser in the distance he jumps off the platform with a speed of 3 m/s to the right. What is the final velocity of the platform?

    Relevant equations
    Vi = √2ghi
    F∆t = -mVi/∆t
    F∆t = mVf - mVi
    m1V1,i + m2V2,i = m1V1,f + m2V2,f

    I'm not sure if there are any others that I need, but I have a feeling that I'm missing something.

    The attempt at a solution
    For the first problem, I used Vi = √2ghi and found the man to be going 7.7 m/s. Then I tried to use the equation F∆t = -mVi/∆t, but I don't have the mass. Is there another equation to use or did my teacher just forget to include mass? (he does that a lot)

    For the second one I think it has something to do with the impulse momentum theorem, but how would I find the skid distance with that?

    For the third one, I'm pretty sure I need to use m1V1,i + m2V2,i = m1V1,f + m2V2,f. But I don't really know where to start.

    EDIT: I just tried to solve number 3 and I got -0.6 m/s. Is this correct? I'm still not too sure about the other problems though.
    Last edited: Feb 7, 2009
  2. jcsd
  3. Feb 7, 2009 #2
    All three of these questions can be solved by application of a relevant conservation law (energy or momentum). What you could do in each case is select a conserved quantity and then think about what is happening as the events in the question proceed.

    You've used energy conservation to find the velocity of the diver just before they hit the water in question one. Try thinking about the physics behind that calculation and ponder what is going to happen next to the motion of the diver and their kinetic energy.

    Likewise, in question two there is kinetic energy at the start and then later on....? Also, since you know the resultant force on the car and its mass you should be able to calculate the acceleration.

    You may find it helpful to look up how to calculate the work done by a force (and what this term means!) for these first two problems.

    You have the correct answer to question 3.
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