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Center of mass and Newton's laws of motion

  1. Aug 20, 2015 #1
    1. The problem statement, all variables and given/known data
    two particles of 2 kg and 3 kg are at rest .
    a 6 N force is exerted on the 3 kg particle at t=0.
    what is the speed of the center of mass at t=5s?

    2. Relevant equations

    3. The attempt at a solution
    first i applied f=ma to the 3 kg object and got 2 as the accn. And applied v= u+ at and got10 as the velocity but the answer is 6. Can someone show me where I went wrong please?
  2. jcsd
  3. Aug 20, 2015 #2


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    What is u?
    What is the velocity of the common centre of mass of two particles?
  4. Aug 20, 2015 #3
    U is initial velocity and is zero. Therefore I took the initial velocity of the center of mass as zero.
  5. Aug 20, 2015 #4
    Can we consider the 6N force is applied to both the particles when considering them as one system eventhough they are not connected?
  6. Aug 20, 2015 #5


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    If you have two footballs (not connected) and you kick one, do they both move?

    Regarding the original question, is here a diagram to show the configuration of the masses and the applied force?
  7. Aug 20, 2015 #6
    ⭕2kg ⭕➡6N.
    . 3kg
  8. Aug 20, 2015 #7
    One particle is 2 kg while the other is 3 kg and 6 N is applied on 3 kg one only.
  9. Aug 20, 2015 #8


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    So, you calculated the speed of the 3kg object after 5 seconds and got 10 m/s. That's clearly not the speed of the centre of mass. So, let's think this through.

    IF both masses were moving in the same direction at 10 m/s, THEN the speed of the centre of mass would be 10m/s. But, the first mass isn't moving. So, how could you analyse the motion of the centre of mass in this case?
  10. Aug 20, 2015 #9
    What is your understanding of the definition of "center of mass?" Can you provide a general equation for the location of the center of mass of a system of particles?

  11. Aug 20, 2015 #10
    I can't come up with one... Can you please help me with that sir?
  12. Aug 20, 2015 #11


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    It's appropriate to search for that sort of thing on the web rather than ask on a forum. That said, some of the definitions I see out there are not very good. This one is ok: https://en.m.wikipedia.org/wiki/Center_of_mass
  13. Aug 20, 2015 #12
    How did you think you were going to solve a problem involving center of mass if you don't even know it's definition?
  14. Aug 20, 2015 #13
    I know what the center of mass is but I don't know any equation to find the center of mass. If I am going to find the center of mass I would take the torques and find it.
  15. Aug 20, 2015 #14
    Can I equal the momentum of the moving object to the momentum of the whole system?
  16. Aug 20, 2015 #15


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    Momentum is additive. The momentum of the whole system is equal to the sum of the momenta of its component parts. So yes, if there are two pieces and one is stationary (zero momentum) then the momentum of the combined system is equal to the momentum of the other piece alone.

    [In classical mechanics], the momentum of the combined system is equal to its total mass multiplied by the velocity of its center of mass. So yes, you could use this line of reasoning to solve the problem.
  17. Aug 21, 2015 #16
    Thank you very much jbriggs444!
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