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Dynamics ProblemGrade 11 Urgent Help Needed

  1. Nov 15, 2009 #1
    Dynamics Problem..Grade 11 Urgent Help Needed!!

    1. The problem statement, all variables and given/known data

    Paul and Mintu are attempting to push a heavy fridge across the kitchen floor. When Paul pushes it with a muscular force of 550N, the fridge accelerates from rest to 28.0cm/s in 3.5s. When Pintu pushes it with a muscular force of 560N, the fridge accelerates from rest to 36cm/s in 3.0s. Calculate the force due to friction and mass of the fridge.

    2. Relevant equations
    v1=0.0cm/s
    v2=28.0cm/s
    t=3.5s
    F=550.0N
    F_2=?
    a=?



    3. The attempt at a solution

    Paul
    v1=0.0cm/s
    v2=28.0cm/s
    t=3.5s
    F=550.0N
    F_2=?
    a=?

    Step 1
    a=(v_2-v_1)/t
    a=(28-0)/3.5
    a=28/3.5
    a=8m/s^2
    Step 2
    Fnet=ma
    F_net=F+F_2
    ma=F+F_2
    Type equation here.



    That is what I've done so far but after I got to the end of that I realized mass wasn't given and now I'm totally confused as the teacher didn't give us any notes on how to deal with this! Please help me solve this I am completly confused
    Thanks
     
  2. jcsd
  3. Nov 15, 2009 #2

    rl.bhat

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    Homework Helper

    Re: Dynamics Problem..Grade 11 Urgent Help Needed!!

    You have two equations. One due to Paul and the other due to Mintu. In both the case F_2 is the same. Solve the two simultaneous equations to find m and F_2.
     
  4. Nov 15, 2009 #3
    Re: Dynamics Problem..Grade 11 Urgent Help Needed!!

    I'm sorry, I do not follow I realize that there are 2 equations and F2 should be the same but I do not understand how I use those 2 F1's to get F2 and mass. Would it be possbile to please explain further/more detail...I am completly clueless
     
  5. Nov 15, 2009 #4
    Re: Dynamics Problem..Grade 11 Urgent Help Needed!!

    can anyone please help me with this its 11pm here and this is due first thing in the morning and right now I'm clueless on how to do it
     
  6. Nov 15, 2009 #5

    rl.bhat

    User Avatar
    Homework Helper

    Re: Dynamics Problem..Grade 11 Urgent Help Needed!!

    OK.
    For Paul
    m*a1 = 550 N + F_2 -----(1)
    Fro Pintu
    m*a2 = 560 N + F_2 -----(2)
    You have found a1. Now find a2. Substitute these values in eq.1 aand 2. Subtract eq.2 from eq.1 and find m. Then using any one of the equation finf F_2.
     
  7. Nov 15, 2009 #6
    Re: Dynamics Problem..Grade 11 Urgent Help Needed!!

    ohhh we have to use substitution to find m...and then find f2 using the rest? Thanks! I think i just kind of went mind blank there for a moment
     
  8. Nov 15, 2009 #7
    Re: Dynamics Problem..Grade 11 Urgent Help Needed!!

    ok im kind of stuck again...after I got f2=-530N, the mass makes no sense...after solving it seems that mass =2.5....would it be 2.5 tonnes? kg? g? or is there something i did wrong?
    Step 1 (Paul)
    v1=0.0cm/s
    v2=28.0cm/s
    t=3.5s
    F =550.0N
    F_2=?
    a=?

    a=(v_2-v_1)/t
    a=(28-0)/3.5
    a=28/3.5
    a=8m/s^2




    Step 2 (Mintu)
    v1=0.0cm/s
    v2=36.0cm/s
    t=3.0s
    F =560.0N
    F_2=?
    a=?

    a=(v_2-v_1)/t
    a=(36-0)/0
    a=36/3
    a=12m/s^2
    Step 3 (Substitution)
    Fnet=ma
    Fnet=F+F_2


    m*8=550+F_2
    m=(550+F_2)/8


    m*12=560+F_2
    m=(560+F_2)/12

    Substitution
    m=(560+F_2)/12→m=(550+F_2)/8
    (560+F_2)/12=(550+F_2)/8
    560+F_2=(550+F_2)/8*12
    560+F_2*8=550+F_2*12
    4480+8F_2=6600+12F_2
    8F_2-12F_2=6600-4480
    -4F_2=2120
    F_2=-530N
    Sub for equation 1
    m=(550+F_2)/8
    m=(550+(-530))/8
    m=20/8
    m=2.5
     
  9. Nov 15, 2009 #8
    Re: Dynamics Problem..Grade 11 Urgent Help Needed!!

    NVM I realised what i was doing wrong...i was using 8 (cm/s/s) instead of 0.08 (m/s/s)
     
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