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Newtons 2.law help

  1. Oct 24, 2005 #1
    A car drags a trailer like the illustration below shows. [​IMG]


    a)The speed is so low that the friction has nothing to do with the movement of the car.
    i)What is the acceleration when the tires are dragging the car with the power of 1700 N.
    ii)What is the power from the car to the trailer?



    b)After quite a long time the speed gets great. And now the air resistance is alot bigger. The power of the resistance is now 1000N. But only 2/3 of it works on the car. Now what is the power from the car to the trailer? (The power from the tires are still the same as in excerisise before (1700 N)).

    I do not know the answers yet.
    But I got 1 M/S^2 on A)i
    And on A)ii i got 500 N
    and on B) I got 303N
     
    Last edited: Oct 24, 2005
  2. jcsd
  3. Oct 24, 2005 #2

    Diane_

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    Homework Helper

    We need to know what you've tried so far before we can point you in the right direction. So - what do you think?
     
  4. Oct 24, 2005 #3
    Diane_ sorry my fault. I've edited my post and at the bottom you'll se what answers I got. But in my next post I'll try to cover how I found that answer. Just wait couple of minutes.
     
  5. Oct 24, 2005 #4
    Sorry I am not from an English-speaking country so I'll try to translate the words correctly. But..

    On A)i:
    F = mass * acceleration
    F/mass = acceleration
    1700N / 500kg+1200kg = acceleration
    1700N / 1700KG = 1 m/s^2




    On A )ii:
    I don't know what I am doing but this seems to be ok.
    1700 N / 500 KG + 1700 KG = 1 N/KG
    1700 N - (1200 KG * 1N) = 500 N which is the answer that I got.




    On B):
    This is a question I don't know how I did.
     
  6. Oct 24, 2005 #5

    Diane_

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    I understand you, luv. We have a couple of problems:

    a) i) This looks fine. A native-speaker would more likely say "the tires are pushing the car", but that's more English than physics. :)

    ii) Here we have a problem. First off, you're asked for power. The units should be in Watts, or joules/second, or something along those lines. Secondly, in the work you've shown, you're mixing units in ways that won't work. In essence, you're adding apples to oranges. All of the terms in your equations need to be in the same dimensions. Go back and check your textbook again for similar problems and see if you can find an example that relates to this. If not, post again and we'll try it again.
     
  7. Oct 24, 2005 #6
    No, we have about newton laws and FORCE IN N (newton) and not watt or joule.
    Yeah I know but yet I came to the answer that I compared with my classmates and they've the same!

    But actually the question I am my knees to get answer on is B :( I am very confused, many have got different answers.
     
  8. Oct 24, 2005 #7

    Doc Al

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    Staff: Mentor

    Although you use the word "power" in the problem statement, what you really are looking for is the force. (Power has a specific meaning in physics: it is not force. This was Diane_'s point.)
    To solve B, first find the new acceleration. Then apply Newton's 2nd law to the trailer to find the force that the car exerts on the trailer.
     
  9. Oct 24, 2005 #8
    Doc Al can please solve that question (B).
    I've now tried 1-2 hrs, I just get 303 N.

    Actually the problem lays in the air resistance. It says it has a total air res. of 1000 N, but only 2/3 of this works on the car. So one of my mates said the trick of this question was that 2/3 had nothing to do with it. 2/3 was just to confuse us, is that true?
     
  10. Oct 24, 2005 #9

    Doc Al

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    Staff: Mentor

    Don't give up! Take the hints I give below and give it another try. As I said in my last post, the first step is to find the acceleration.
    Hint: If 2/3 of the total air resistance acts on the car, how much acts on the trailer?
    It's not true at all. It gives you a vital piece of information about forces acting on the trailer.
     
  11. Oct 25, 2005 #10
    Doc Al, I am trying to solve it but the problem isnt that I don't know how to calculate things. I don't know how to find out the Force since I dont know how much air res. that works.

    To find the acceleration I need to know the Force, but thats where the problem comes. I can chose to go two directions. One that if I should only count 2/3 of the air res. the second is should I rather count ALL of the air res.?
     
  12. Oct 25, 2005 #11

    Doc Al

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    Staff: Mentor

    Whether you use all or just a portion of the total air resistance depends on what you are analyzing. If you treat the "car + trailer" as a single system, then use all of it. But if you treat the car by itself, then just use 2/3 of it.

    Hint: Do it both ways! Start by analyzing the entire system ("car + trailer") to find its acceleration. Then analyze the car by itself to find the forces on it. (By "analyze" I mean: Identify the forces and apply Newton's 2nd law.)
     
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