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Is mechanical energy conserved ?

  1. Nov 15, 2006 #1

    An object has a mass of M ... it explodes up into two pieces.. one piece has a mass of M/3 and the other piece has a mass of 2M/3.

    1. which object has larger l momentum l ? ...... both have same
    2. which object has larger speed ? .... smaller mass has larger speed
    3. which object has larger kinetic energy ? .... smaller mass has larger KE
    4. Is mechanical enrgy conserved ? ..... No

    I understood the part (ii) and (iii) ... but i am not getting that why they same momentum ? why is mechanical energy not conserved ?

    please help me .
  2. jcsd
  3. Nov 15, 2006 #2


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    They do not have the same momentum, but they do have the same magnitude of momentum. If the object is at rest before exploding, it has zero energy and zero momentum. Momentum is a vector. Equal and opposite momenta add to zero. Energy is a scalar; the separate energies of the two masses simply add and cannot cancel. Chemical energy is converted to mechanical energy (and heat and light and sound) in the explosion.
  4. Nov 15, 2006 #3
    A bomb just sits there. 0 KE. Then it explodes... KE all over the place! Mechanical energy is clearly not conserved... Where did all this energy come from?
  5. Nov 15, 2006 #4
    so you mean to say that the two objects must have the equal and opposite momentum so that they can add up to zero. In this way, we have to follow this rule

    Intial momentum before explosion = Final momentum after explosion

    the initial momentum of the bomb was zero, the final momentum must add up to zero too.
    rite ?

    and for the energy u mean, that it was saved in the form of chemical energy and after explosion, it is lost in the form of heat and sound and therefore it is NOT conserved.

    am I thinking rite?
  6. Nov 15, 2006 #5
    hi , I need to ask ... is that tangential speed and tangential velocity mean the same thing or there is a difference between two of them ?
  7. Nov 15, 2006 #6


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    Yes. You have the momentum part exactly right. The energy question was asking if mechanical energy is conserved. The answer is no because some of the chemical energy was converted into kinetic energy. If you examined all of the kinds of energy there are, you would conclude that total energy is conserved. The amount of chemical energy lost would equal the sum of all the forms of energy created.
  8. Nov 15, 2006 #7


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    Speed is a scalar. An object in a circular orbit has constant speed; it moves the same distance in the same amount of time. Velocity is a vector that has both magnitude (speed) and direction. The direction of the tangential velocity of an object in a circular orbit is constantly changing, so the tangential velocity is constanly changing even though the speed remains constant.
  9. Nov 15, 2006 #8
    Thanks a lot
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