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Momentum Concepts [Checking my Understanding]

  1. Feb 11, 2007 #1
    1. The problem statement, all variables and given/known data
    What are elastic and inelastic collisions? What is an explosion?

    2. Relevant equations

    3. The attempt at a solution
    In elastic collisions, the colliding objects bounce off of each other to some degree, and kinetic energy is conserved. Inelastic collisions involve the transformation of some initial kinetic energy into other forms, such as heat or potential energy. Objects that collide in a completely inelastic way stick together. Explosions are a form of inelastic collisions, for kinetic energy is not conserved.

    How can I elaborate upon this? I'm still slightly confused about momentum in general, and I don't understand explosions whatsoever. My notes state that kinetic energy is conserved for explosions, but other online sources state the opposite. That's never a good thing.
    Last edited: Feb 11, 2007
  2. jcsd
  3. Feb 11, 2007 #2


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    Staff Emeritus
    Science Advisor

    What specifically about momentum are you confused about? Momentum is the product of mass and velocity, and is conserved in all collisions.

    Do you think that kinetic energy is conserved in an explosion? Think about it; what actually happens in an explosion?
  4. Feb 11, 2007 #3
    According to Newton's 3rd law, the forsces exert on each other are always equal in magnitude and opposite in direction. thus we define the total momentum of a system between two blocks of masses m1,m2 and v1,v2 as momentum=mv1+mv2
    hence we arrive at the conservation of momentum,this states when no resultant force acts upon a system of bodies, and the total momentum of the stystem remains unchanges
    total momentum before= total momentum after

    elastic collisions are when the interaction forces between two bodies are conserved, the Ek of the system is the same after the collison as before the collison. therefore Ek is conserved

    Inelastic collisions is a collision in which the total Ek of the system after the collision is less than the Ek before the collision. in one kind of inelastic collision, the colliding bodies stick together and move as one unit after the collison.

    To determine wheather or not a collision is elastic or inelastic find the Ek before and the Ek after.If it is not conserved it is inelastic if it is it is elastic

    Problem solving stategy for elastic collison
    a) find the total momentum/Ek before collison
    b)find the total momentum/Ek after
    c) remember to consider a) = b) in problem solving

    problem solving strategy for inelastic collision
    Consider two masses Ma and Mb right before a collison then
    a)find the total momentum before collision
    i) MaVa1+MbVb1
    b) find the total momentum after the collision
    c) remember to consider a) = b)
    iii) remember they stick together in an inelastic collision for part b) and a) not equal to b), in the case of their kinetic energies

    if the masses and velocities are known then we can compute the final velocity. suppose we have a body of mass Ma and initial velocity V1 and it collides inelastically with a body of mass Mb initially at rest

    *After the collision the two bodies have a common velocity V2

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