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WAEC 2013 · Paper 2 · Q7

  1. (a)

    The initial velocity of a particle of mass 0.1 kg0.1\text{ kg} is 40 m s−140\text{ m s}^{-1} in the direction of the unit vector j\mathbf j. The velocity of the particle changed to 30 m s−130\text{ m s}^{-1} in the direction of the unit vector i\mathbf i. Find the change in momentum.

Worked solution (try it first)
  1. Change in momentum =mv−mu= m\mathbf v - m\mathbf u.
  2. =0.1(30i)−0.1(40j)= 0.1(30\mathbf i) - 0.1(40\mathbf j)
    =3i−4j= 3\mathbf i - 4\mathbf j.
  3. Its magnitude is 9+16=5 kg m s−1\sqrt{9 + 16} = 5\text{ kg m s}^{-1}.

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