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19 December, 04:58

A father racing his son has 1/3 the kinetic energy of the son, who has 1/2 the mass of the father. The father speeds up by 1.4 m/s and then has the same kinetic energy as the son. What are the original speeds of (a) the father and (b) the son

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Answers (2)
  1. 19 December, 05:08
    0
    Vf = 1.91m/s. Vs = 4.67m/s

    Explanation:

    Given that that Ms = 1/2 * Mf and that

    (K. E) f = 1/3 * (K. E) s

    Then

    1/2 * Mf * Vf² = 1/3 * Ms*Vs²

    1/2 * Mf*Vf² = 1/3 * 1/2 * Mf*Vs²

    Dividing out common coefficients

    Vf² = Vs²/6

    Vs² = 6Vf²

    The father increases his speed by 1.4m/s

    Thereby having the same kinetic energy as the son. That is

    1/2*Mf * (Vf + 1.4) ² = 1/2*Ms*Vs²

    Dividing through by 1/2 and sybstituting for Ms and Vs

    Mf * (Vf² + 2.8Vf + 1.96) = 1/2*Mf*6Vf²

    Dividing through by Mf

    Vf² + 2.8Vf + 1.96 = 3Vf²

    Rearranging,

    Vf² - 2.8Vf - 1.96 = 0

    Solving the equation above by a calculator

    Vf = 1.91m/s.

    Substituting this value into the the equation for Vs,

    Vs² = 6Vf² = 6 * 1.91² = 21.89

    Taking the square root

    Vs = 4.67m/s
  2. 19 December, 07:09
    0
    Answer: The initial velocity of the father is 1.91m/s

    and the one of the son is 4.68 m/s

    Explanation:

    First, the kinetic energy is written as:

    K = (m/2) * v^2

    where m is mass and v is velocity.

    The information that we have is that:

    If we use tabs (M, K, V) as the data for the father and (m, k, v) as the data for the son:

    K = k/3

    m = M/2

    now, let's proced with these two equations:

    the first one says:

    M*V^2 = (m*v^2) / 3

    (where i cancelled the 1/2 in both sides)

    now, we can replace the second equation into this and get:

    M*V^2 = ((M/2) * v^2) / 3 = (M*v^2) / 6

    Here we can divide by M in both sides, and we get:

    V^2 = (v^2) / 6

    We apply the square root to both sides, and we get:

    V = v / (√6)

    Afther that, the father increases his velcity by 1.4m/s and then the kinetic energys are equal, so now we have:

    M * (V + 1.4) ^2 = m*v^2

    again, using that m = M/2

    M * (V + 1.4m/s) ^2 = (M/2) * v^2

    We divide by M in both sides:

    (V + 1.4m/s) ^2 = (v^2) / 2

    Now we apply the square root to both sides:

    V + 1.4m/s = v/√2

    Now we have two equations and two variables:

    V + 1.4m/s = v/√2

    V = v / (√6)

    We can replace the second equation in the first one and obtain the velocity of the son:

    v / (√6) + 1.4m/s = v/√2

    v (1/√6 - 1/√2) = - 1.4m/s

    v = 1.4m/s / (-1/√6 + 1/√2) = 1.4m/s*0.299 = 4.68m/s

    Now, knowing the velocity of the son, we can find the velocity of the father using:

    V = v / (√6) = (4.68m/s) / √6 = 1.91m/s
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