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Homework Help: Physics (Forces)

  1. Dec 28, 2004 #1
    This problem is terrible.

    A 5 g. marble is given an initial force (not constant) which causes it to accelerate up a 45º-angled ramp. At the top of the ramp, the marble has an acceleration of 8.5379 m/s^2. The marble travels 62 cm. along the hypotenuese of the ramp.
    What is the initial force (keep gravity in mind)?

    My question:
    Is this possible to find without the time given?
    If so, where do I start?

    (There is a picture of the problem attached.)
    Last edited: Dec 28, 2004
  2. jcsd
  3. Dec 28, 2004 #2

    Andrew Mason

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    I don't quite understand the question. Are we to assume that it is accelerating due to this initial force when the marble reaches the top of the ramp? If the 'initial force' is not constant, how are we supposed to find 'it'. The question is incomplete and does not have an answer.

    Your picture does not seem to be attached.

  4. Dec 29, 2004 #3
    Gravity definately has a part to play. Without using gravity, the 45degree angle is irrelevant, it would be as if it were rolling along the ground, and then you could just use F=ma which would be too easy.

    Acceleration due to gravity is 9.8m/s2 [Down] and you can use trig to find what it would be on a 45 degree incline.

    hmmm.....would you add this to the final acceleration? Because the initial force would have to allow the marble to overcome gravity and then reach the acceleration value.

    If I am correct so far, not sure if I am, then you can just use F=ma again, only using the new value of acceleration found.

    I have a feeling it is more complicated than this, because I haven't taken into account the deceleration of the marble while going up the ramp, probably because i dont know how. What grade is this?

    Hope this helps, I'd be interested in finding out if I am correct or not.
  5. Dec 29, 2004 #4
    The reason that they tell you that the force is not constant is because the you can not use the energy conservation laws to solve the problem. Remember every forces problem that you run into in the high school level are relitivly easy to solve, meaning that they do not require a great deal of calculus... The key is to set up the question appropriatly...
    Ok Step 1
    Make your plane of refrence equeal to the inclined plane, this way you do not have to deal with forces from the x and y. Since the force form gravity will be in the x, when the plane of refrence of our free body diagram is parrallel to the inclined plane.

    Step 2
    You are given what the accleration of the ball is, the mass and the inclination of the plane. This is sufficient to solve the prblem without using systems.
    Fnet= Fgsin45degrees + Fint.

    From that equation solve for Fintial and you are set!
    I hope that this has helped...
  6. Dec 29, 2004 #5
    Shouldn't Fnet = Fint - Fgsin45degrees

    I agree with AM that this question is incomplete. As well, where did you find this question?
    Last edited: Dec 29, 2004
  7. Dec 29, 2004 #6
    Hmm ... i think that derekmohammed is right.
    but what's the F in Fg sin 45??
    Last edited by a moderator: Dec 30, 2004
  8. Dec 30, 2004 #7
    Ok I admit I made a slight error in judgement, but when you substitute the value for a in fgsin45=masin45 a=g in which g is negative...

    The F in Fg is really just mg. This is the anwser. Everything is given in the question.
  9. Dec 30, 2004 #8

    Andrew Mason

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    So you are saying that the reference to the 62 cm distance and the non-constant nature of the force are useless pieces of information and that the 'initial' force is still acting on it at the top of the ramp?

  10. Dec 30, 2004 #9
    With what explanation do you have to support derekmohammed's reasoning? The problem just doesn't make any sense.

    First of all the question is referring the "initial force" to the force acted on the ball, at the bottom of the ramp.

    However, derekmohammed's method only solves for the force AT THE TOP OF THE RAMP, for the force is not constant, and thus you cannot determine the initial force. This is why i believe the question is flawed.
  11. Dec 30, 2004 #10


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    As AM and ap have stated above, this problem is just plain messed up. I third them.
  12. Dec 30, 2004 #11
    OK guys this is a simple question.

    Fnet is equeal to the force of gravity in the x axis (which is negative) and the intial force which is positvie. We are given the overall net force because we are given the acceleration of the ball and the mass of the ball. Therefore the Fintial=Fg+Fnet. There is nothing more to be said. I figured that you would rearrange the formula to solve for Fintial but I guess you did not. Fg is the force on the ball at the top of the ramp NOT Fintial.
  13. Dec 31, 2004 #12
    No... The overall net force is the instantaneous Net force at that time (top of ramp) due to the inconstant Force applied to it. Due to the time duration, the F init will no longer be F init at the top of the ramp, for it has already changed and thus we do not have enough information to solve for this F init.

    This is not a problem of just simply rearranging the formula( though i assume the person who made this question did). This problem is simply flawed... Again your method assumes the F init is constant which, noted already, is not constant. This is the main reason why you don't see your mistake.

    As well if you assume the only force acting on the object at the top of the ramp is Fg, the the net force will only be Fg sin 45, which is not correct as the acceleration of the object is already given at the top of the ramp.
  14. Jan 8, 2005 #13
    Thanks to all. This problem was part of some calculations for an experiment I had to do. I kind of realized that the initial force was more or less impossible to find, given the coniditions. By revamping the experiment, I was actually able to make it so that I knew f-initial and eliminated the ramp. (maybe you've heard of the coat-hanger cannon). Anyway thank you. I have a feeling that your suggestions may come in handy when I redo the experiment.
    Last edited: Jan 8, 2005
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