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Homework Help: Kinetic energy & speed in a pursuit - Ek possible to find?

  1. Jan 30, 2015 #1
    1. The problem statement, all variables and given/known data
    A cat is chasing a mouse in a straight line. The cat's mass, mc, = 5.0kg. The mouse's mass, mm, is 0.035kg.

    Problem 1. If the kinetic energy of the cat is 100 times the kinetic energy of the mouse can the cat catch the mouse?

    Problem 2. If the cat and the mouse have the same speed, what is the kinetic energy of the cat?

    2. Relevant equations
    Conservation of energy. Kinetic energy = (1/2)mv2

    3. The attempt at a solution
    Solution for Problem 1.

    Ekc = (100)Ekm
    (1/2)mcvc2 = (100)(1/2)mmvm2
    mcvc2 = (100)mmvm2
    vc2 = [(100)mmvm2] / mc
    vc2 = [ (100)(0.035kg)vm2 ] / 5.0kg
    vc2 = (7/10) vm2
    vc = (7/10)(1/2) vm
    vc ≅ 0.83 vm

    Therefore, the cat cannot catch the mouse. The cat's speed is 83% of the mouse's speed

    {Is the result correct? Is my therefore statement correct? Is this the proper interpretation of the result?}

    Solution for Problem 2. - If the speed of the cat is the speed of the mouse what is the cat's kinetic energy?

    vc = vm
    [(2Ekc)/mc](1/2) = [(2Ekm)/mm](1/2)
    Ekc/mc = Ekm/mm
    Ekc = mc Ekm / mm
    Ekc = {mc (1/2mmvm2)} / mm
    Ekc = (1/2) mcvm2
    Ekc = 2.5kg vm2
    Ekc = How can I find this value? All of the known problem info. is stated.

    The correct answer according the answer page of the text is "140 times."

    What approach should I take to solving this problem.

    Many thanks!
  2. jcsd
  3. Jan 30, 2015 #2
    For the second problem why don't you express the velocities as the same variable and then solve for Ec
  4. Jan 30, 2015 #3
    Is your suggestion different from how I started the solution? vc = vm I do not understand your suggestion.
  5. Jan 30, 2015 #4
    You have the answer here! Just plug in the values for m and solve for E cat!
  6. Jan 30, 2015 #5
    Ekc is 2.5 * "speed of mouse" squared. What do i use for the speed of the mouse?
  7. Jan 31, 2015 #6
    Look at the last line of the expression I just quoted, that you generated. Do you see velocity there?
  8. Jan 31, 2015 #7
    The kinetic energy of the mouse, Ekm, is unknown! The velocity of the mouse is unknown! Only the mass of each body is known!

    Clearly you're far too brilliant for me! What is the value of Ekc?
  9. Jan 31, 2015 #8
    Ek_cat = 140xEk_mouse
  10. Feb 1, 2015 #9
    So, like I said, "how do I find the value?"

    Thanks for your effort. In the future, save both of our time by confirming my solution. Ek cat is unknown! It might be expressed in terms of Ek mouse, but Ek cat's value is, like I asked, not possible to be found.
  11. Feb 1, 2015 #10
    Wow. I wasn't expecting that response. I wasn't trying to save you time. I was trying to help you get to the answer yourself, which is infinitely more valuable than having me just solve the question myself or just confirming your answer. The solution that you yourself indicated is not in itself a value but rather a multiplication of another one. You said the answer was 140 times! 140 times what? Well, 140 times the kinetic energy of the mouse. It is impossible to find the value of the cat's KE given the information of the question.

    In the future, don't lambast people who are trying to help you.
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