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Homework Help: Two force problems

  1. Sep 18, 2005 #1
    3. [SFHS99 4.P.22.] An 1680 kg car is moving to the right at a constant velocity of 2.94 m/s.

    (a) What is the net force on the car?
    wrong check mark N

    (b) What would be the net force on the car if it were moving to the left?
    in Newtons = ??




    4. [SFHS99 4.P.23.] A freight train has a mass of 5.5 107 kg. If the locomotive can exert a constant pull of 15 105 N, how long would it take to increase the speed of the train from rest to 85 km/h?
    in seconds =??

    thanks alot
     
  2. jcsd
  3. Sep 18, 2005 #2
    For your first question.

    The net force will be zero if there is no acceleration! Newtons first law of motion.

    same would apply to part b if it moves at constant velocity towards left.

    For second question.

    we know the mass and we know the force it can apply.
    [tex]F=ma[/tex]

    [tex]\frac{F}{m}=a[/tex]

    [tex]\frac{15105}{5.5*10^7}=a[/tex]

    [tex]a=2.75*10^-^4[/tex]

    [tex]v=u+at[/tex]

    velocity is 85KM/h which is 23.61 m/s

    [tex]23.61=0+2.75*10^-^4t[/tex]

    [tex]t=\frac{23.61}{2.75*10^-^4}[/tex]

    [tex]t=85972 s[/tex]

    23 hours 52 mins 52 seconds!

    if the force applied by engine is 15105N.

    if it is [tex]15*10^5[/tex] then the answer is 865.7 seconds
     
  4. Sep 18, 2005 #3
    thanks alot they are correct, your explanations helped alot
     
    Last edited: Sep 18, 2005
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