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2 simple question help please ?

  1. Dec 23, 2011 #1
    1)- An object is moving with constant velocity. Wich of the following statements is true?

    A) There are no forces acting on the object.
    B) There is no frictional force acting on the object.
    C) The net force on the object is zero N.
    D) A constant force is being applied in the direction of motion.


    2)- The ratio of the mass of object A to the mass of object B is 2:1 and the ratio of their speeds is 1:2 in the same oreder. What is the ratio of their kinetic energies?

    A) 1:3
    B) 2:1
    C) 1:2
    D) 3:1


    * Please help me by giving me the answers for these 2 questions and explain the answer, thanks.
  2. jcsd
  3. Dec 23, 2011 #2
    I won't give you the answer but I'll help you think it through.


    For Q1, think of Newton's second law.

    For Q2, think of the formula for kinetic energy and compare the two situations.
  4. Dec 23, 2011 #3
    For q1, i get the answer C, because the object is moving with constant velocity.. so the net force on the object is zero .. right? im not sure
  5. Dec 23, 2011 #4
    Yes you're right ...
  6. Dec 23, 2011 #5
    Ditto previous post. You are correct.
  7. Dec 23, 2011 #6
    Hint for Q2:

    Write the ratio of kinetic energies.
  8. Dec 23, 2011 #7
    i didn't understand this question anymore, its complicated!
    can i have some help please?
  9. Dec 23, 2011 #8
    As LawrenceC already said, write the ratio of the kinetic energies.

    [tex]KE = \frac{1}{2}mv^{2}[/tex]
    Last edited: Dec 23, 2011
  10. Dec 23, 2011 #9
    1/2 x 2:1 x (1:2)2

    and then? how can i multiply 1/2 and 2:1 .. ?
  11. Dec 23, 2011 #10

    Object A mass : Object B mass → 2:1
    Object A velocity : Object B velocity → 1:2

    For example, let's say ##m_A = 6##, then this forces ##m_B = 3##.
    Again, we can choose ##v_A = 3##, then this forces ##v_A = 6##.

    Now apply the formula for kinetic energy and compare their values.

    $$KE_A : KE_B$$

    What do you get?
  12. Dec 23, 2011 #11
    if you have two quantites A,B what is their ratio?

    A:B= A/B
    So eg for masses 1:2 means M_A/M_B =1/2 or M_B=2M_A

    so writting KE of A and KE of B KE_A and KE_B their ratio would be
    KE_A/KE_B (or KE_B/KE_A above is the one you want to compare with the one below).

    writing their = will give you the answer.
  13. Dec 23, 2011 #12
    i've got the answer C.. thank you guys for help
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