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Homework Help: Impulse and Momentum

  1. Nov 7, 2006 #1
    Conceptual Question

    If two objects have the same momentum do they necessarily have the same kinetic energy? Give your reason.

    I would say no basically from just looking at formulas.

    P=mv for momentum

    K.E.= 1/2mv^2 for kinetic energy
  2. jcsd
  3. Nov 7, 2006 #2


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    Homework Helper

    Of course momentum is not the same as kinetic energy. But momentum and kinetic energy both depend on two variables; the mass and the velocity. Also, note that momentum is a vector quantity, while kinetic energy is a scalar quantity.

    Edit: look at this example. Let [tex]\vec{p}_{1} = m \vec{v}_{1}[/tex], and [tex]\vec{p}_{2} = m \vec{v}_{2}[/tex], where [tex]\vec{v}_{1} = 3 \vec{i}[/tex], [tex]\vec{v}_{2} = 3 \vec{j}[/tex]. The momentums are not the same (the magnitudes are, but the directions are not), and the kinetic energies are the same.

    Or, you can look at s non-vector example (assume both momentums are in the same direction): let the momentum be equal, so that m1v1 = m2v2. For example, you can take m1 = 3, and v1 = 4, and m2 = 4, and v2 = 3. Now try to calculate the kinetic energies and see in what relation they are.
    Last edited: Nov 7, 2006
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