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Homework Help: Whats the force applied for a cart traveling up a hill?

  1. Feb 28, 2016 #1
    1. The problem statement, all variables and given/known data

    A cart travels up a hill at a constant velocity in 2.5 (s). What power is developed by the cart?
    m=120 kg
    2. Relevant equations

    3. The attempt at a solution
    P= w/t= Fdcosθ/2.5= F(12)(.93)/2.5= 11,2F/2.5= 4.5F
    How do I find F?
  2. jcsd
  3. Feb 28, 2016 #2
    Tip : Using the formula P=FIIv would be an easier approach(Why?)

    The why also tells you the method of finding F.

    Hint : What forces act on the car?
    What is an object's acceleration when it travels with uniform velocity?

    Hope this helps,
  4. Feb 28, 2016 #3
    The problem I'm thinking is that F= ma and so without acceleration, how is there Force?
    Is something just flying past me here?
  5. Feb 28, 2016 #4
    Wait. I just got it.
    F=0 therefore P=0.
    I think I just wasn't expecting a trick question.
  6. Feb 28, 2016 #5
    We typically say Fnet=ma. Thus, the correct conclusion to be drawn is..?
  7. Feb 28, 2016 #6
    I'm afraid that's wrong;)
  8. Feb 28, 2016 #7
    If I'm not mistaken "net" means Σ correct?
  9. Feb 28, 2016 #8
    Yes. So what forces, or components of forces, are acting on the car, along the incline?
  10. Feb 28, 2016 #9
    F normal
    F gravity
    F applied
  11. Feb 28, 2016 #10
    Out of these, which forces affect the car along the incline?

    Hint : If you can figure out F applied somehow, you would be able to find the power developed using the formula P=F.v
  12. Feb 28, 2016 #11
    Well I know that n and g (although now that I think about it it might be g and θ) combine to find the minimal force you need to overcome gravity and inversely shows Fa, but I don't think I know how that'd work.
  13. Feb 28, 2016 #12
    Hmm..I'm gonna try squeezing in a bit more. Do you know how to draw a free-body diagram? If you do, draw one for the car.

    Now, draw the components along the incline. The net force along the incline must be zero, as we have seen earlier. Consider mg to be the force of gravity and a random variable , say F, for force exerted by the car. You will see that that F is independent of the normal.

    Can you find out the relation between mg and F, using all that has been mentioned so far?
  14. Feb 28, 2016 #13
    I'd also reccomend you reading more theory related to this stuff.
  15. Feb 28, 2016 #14
    By this I'm assuming that F= mg,
    but according to my notes, as far as I can gather, F normal must equal Fa due to gravity, but your saying it's independent.
  16. Feb 28, 2016 #15
  17. Feb 28, 2016 #16
    As I said at the start of my post, you will need to make use of a free-body diagram - your assumption is, unfortunately, wrong.

    Also, I said force that the car exerts is independent of the normal. Normal force is related to gravity, but that is not relevant here. As I said earlier, you need to construct a diagram to understand what's going on.
  18. Feb 28, 2016 #17
    I will not be able to help you there. You should post this question in the Science Education's textbooks forum, or something similar.
  19. Feb 28, 2016 #18
  20. Feb 28, 2016 #19
    So then there's no relevant relation between mg and Fn.
    Because i was hoping cos21 may subtract from mg to acquire Fn.
    Am I going about this incorrectly?

    Attached Files:

  21. Feb 28, 2016 #20
    -In this question. I hope I've cleared that satisfactorily.

    You've gotten the basic diagram correct. Take components of the forces along the incline.
  22. Feb 28, 2016 #21
    Do you mean to add the vectors?
    Or I need to account for a (-) Force down the incline so F a has acceleration... but that would still amount to Fnet and would in turn need a.
    Well i'm really getting nowhere awful fast.
  23. Feb 29, 2016 #22
    I think I may have gotten it.
    You need to use trig to make a triangle with g and Fa as sides.
    While I can't find the exact force, I can find the minimum required to keep it stationary. Its what the book is looking for, but in the real world, how could I find the exact force with what little I have?
  24. Mar 1, 2016 #23
    1. Net force zero does not imply that a body is stationary. It means net acceleration is zero; and hence, it can move with constant velocity/ be stationary.

    2. The question asks you for the Fa which gives net force zero. Anymore force, and the car would have a net acceleration upwards. So the word 'minimum' force is not correct. It is a particular value of force that gives zero acceleration, or constant velocity.
    Thta is what you have to find.
  25. Mar 1, 2016 #24
    This is a little vague.
    I'll try again. All the question requires is for you to take components of forces along the incline. Do you know how to do this?
  26. Mar 1, 2016 #25
    I believe this is something along the lines correct?

    Attached Files:

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