Newtons Law Question

  1. 1. The problem statement, all variables and given/known data
    On vacation, your 1300 kg car pulls a 540 kg trailer away from a stop light with an acceleration of 2.00 m/s2.
    (a) What is the net force exerted by the car on the trailer?
    (b) What force does the trailer exert on the car?
    (c) What is the net force acting on the car?

    2. Relevant equations
    Force= mass x accelleration


    3. The attempt at a solution

    I was able to solve for part b by applying the above equation and doing the following:

    (540)(2.00)=F so F=1080.

    This answer was correct.. however when applying the same logic to part a)

    (1300)(2.00)= 2600 ... the answer came out incorrect... im not sure what I am doing wrong?

    For part c I tried subtracting the force of the trailer from the force of the car to get the net force.. this too was incorrect. I am not sure what I am missing here.. please help?
     
  2. jcsd
  3. tiny-tim

    tiny-tim 26,041
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi laurenflakes! Welcome to PF! :smile:
    You're sort-of doing b) and c) in the wrong order …

    F = ma gives you the net force on the body (net force means total force, not the applied force).

    For the trailer, there's only one (horizontal) force, so you can't make a mistake.

    But for the car, there are two forces (the rope and the engine), so you can make a mistake …

    and you did! :wink:
     
  4. Thank you so much for replying! But I'm sorry I'm still confused (I'm very new at this so you will have to forgive me!)

    For part a) it is asking the net force exerted by the car on the trailer.. and because the car is 1300 kg and the acceleration is 2.00m/s squared then im not sure why I cannot just multiply these to get the Force of the car?

    For part c) it is asking for the net force acting on the car, so I am thinking that I would add the force of the car and the force of the trailer? (I guess the problem is I cannot get the force of the car :-/ )

    I would really really appreciate any further explanation anyone has to offer.. I can't seem to wrap my brain around this one.. Thanks!
     
  5. tiny-tim

    tiny-tim 26,041
    Science Advisor
    Homework Helper

    Hi laurenflakes! :smile:

    (I think you're confused about what forces there are, and you're ignoring the force from the car's engine … I know it isn't mentioned in the question :redface:, but sometimes they assume you'll know how cars work :wink:)

    a), the trailer, you got it right … net (total) force = ma.

    For c), the car, you should do the same thing … net (total) force = ma.

    That net force is the force from the trailer plus the force from the engine.

    But for b), the car, the force from the trailer can't be found from F = ma, but it can be found from the answer to a), using a different principle … can you see which principle? :smile:
     
  6. Hi tiny-tim!

    I really appreciate you taking the time to work with me on this.

    Im still confused... unfortunately :confused:

    In order to submit my answers to these problems I am using an online homework website, and I keep getting the answers wrong.

    For part a)
    (a) What is the net force exerted by the car on the trailer?

    I am multiplying the weight of the car by the acceleration (2.00)(1300), which gives me 2600 N... but the answer is wrong.

    For part b)
    (b) What force does the trailer exert on the car?

    I was able to get this one right by doing the following:

    Multiplying the weight of the trailer by the acceleration

    (540)(2.00)= 1080N

    This is the only part I have been able to get right so far.

    For part c)

    (c) What is the net force acting on the car?

    I tried adding the force of the trailer on the car to the force of the car on the trailer to get the net force... but seeing as I can't seem to get the correct answer for part a.. I can't get the correct answer for this part either.

    I am not sure how to calculate the force of the cars engine. Does this have something to do with the acceleration?

    When I try to add or subtract the force of the trailer from the car.. I still get the wrong answer?

    I'm sorry if I keep repeating myself.. but I really dont understand what I am doing wrong? I really appreciate your patience and contribution tiny-tim! :smile:
     
  7. tiny-tim

    tiny-tim 26,041
    Science Advisor
    Homework Helper

    Hi laurenflakes! :smile:

    ah, now I see where you've gone wrong …

    in a), you've used the mass of the car, but you want the force on the trailer, so you have to use F = ma for the trailer, and so m must be the mass of the trailer :wink:

    (F is the force on the trailer, m is the mass of the trailer,a nd a is the accleration of the trailer)

    in b), you've got the right answer by accident. :biggrin:

    Try a) again, and then see if you can see why b) was right! :smile:

    (and c) may sort itself out once you've got a) right: tell us if it doesn't)​
     
  8. Hey Tiny-Tim!

    Yay! I finally got part a) what a relief!

    Thank you for your help!

    Im still stumped on part c)... it appears to me that by adding the forces in parts a and b you would get the net force that is being asked for in part c)... but thats not the case.. Im still getting the answer wrong :frown:

    I'm so frustrated! :confused:

    I hope you dont feel like I'm asking you to do the problem for me.. I really do want to understand how to do it on my own.. any guidance?
     
  9. tiny-tim

    tiny-tim 26,041
    Science Advisor
    Homework Helper

    But c) asks for the net force on the car, while a) is a force on the trailer … so how could a) + b) possibly be the net (or any) force on the car?

    I said before: F = ma gives you the net force on the body … just use that for c).

    btw, you didn't mention b) just now …

    do you know why b) and a) are the same?​
     
  10. Yippee! Finally got part c correct. I guess I was thinking too much into it... Thanks for your help :biggrin:

    For parts a and b. They are equal because of Newton's Third law of motion which states that forces come in pairs and the forces in a pair act on different objects. So, for any object a exerting a force on object b, object b exerts an equal and opposite force on object a. This is why the force of the car on the trailer and the force of the trailer on the car are equal!


    ... right? :cool:
     
  11. tiny-tim

    tiny-tim 26,041
    Science Advisor
    Homework Helper

    :biggrin: Woohoo! :biggrin:

    (just got up :zzz: …)
     
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