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Problems involving Newton's law

  1. Nov 11, 2006 #1
    these were the questions:

    1.Heather can apply a force of 72 N to a wagon in which sits her little brother, Bryce. The combined mass of Bryce and the wagon is 48 kg. If starting from rest, how much speed will the wagon pick up after Heather has pushed the wagon for 3s?

    2.When the wagon in the previous problem reaches a speed of 8 m/s, Heather lets go and Bryce puts on the brakes. If the wagon comes to rest 6s later, how much force did the breaks apply?

    for 1 i thought you had to first convert bryce's mass into Newtons, so i did 48kg/10 and i got 4.8 N is the weight of the wagon and Bryce. But after that i got lost. Do i have to find a resultant force? I really have no clue what to do in these problems besides what i just told u
     
  2. jcsd
  3. Nov 11, 2006 #2

    Hootenanny

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    For the first question all you need to consider is Newton's second law.
     
  4. Nov 11, 2006 #3
    Check the units of (48 kg) / (10 m/s^2). Compare this units to Newtons.

    Also, why are you converting mass to weight? :smile:
     
  5. Nov 11, 2006 #4

    radou

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    You can use the impulse law, too.
     
  6. Nov 11, 2006 #5
    i put this as an answer:

    Since we’re trying to find the acceleration I used the formula A= f/m A=72/48. Therefore the acceleration is 1.5 m/s2. So after three seconds (1.5*3) they would have been going at 4.5 m/s2.
     
  7. Nov 11, 2006 #6
    is that right?..and also the second question i need help with
     
  8. Nov 11, 2006 #7

    radou

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    Yes, that is perfectly right. Show us some work on the second question, I'm sure you can at least try to solve it.
     
  9. Nov 11, 2006 #8

    Hootenanny

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    Just a slight correction. Their velocity would be in m/s not m/s2
     
  10. Nov 11, 2006 #9
    thanks but heres the question with the second problem, how do i know the friction? is there a formula i could use for it? and i dont undersatnd how would i could find the force that the breaks applied...could an1 explain this concept to me please?
     
  11. Nov 11, 2006 #10

    radou

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    There is no friction force in the problem, there's only the force of the break. The second part of the problem is just kinematics, and an application of Newton's 2nd law in the end. So, what equations cross your mind?
     
    Last edited: Nov 11, 2006
  12. Nov 11, 2006 #11
    oh lol, i basically had no clue what to do for the second problem so i was guessing friction was involved
     
  13. Nov 11, 2006 #12

    radou

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    Simply use the equation for velocity in the case of uniform acceleration (i.e. deceleration in this case): v(t) = Vo - at. You know what the velocity must equal at t = 6 seconds, and you know the initial velocity Vo, so you can calculate the acceleration easily.
     
    Last edited: Nov 11, 2006
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