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Homework Help: Work and forces

  1. Sep 13, 2010 #1
    I have these 3 problems, and it doesnt tell me whether my answer is right or wrong right away. I choose the answer I thought was appropriate, and explained why. Can anyone take a look at the explanations and see if they're true? I'm sure some may be flawed.

    1.
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    2.
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    3.
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  3. Sep 13, 2010 #2

    PhanthomJay

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    That's a lot of questions in a single post! You should post each one separately with your relevant equations and attempt at a solution. For the first one, what are the forces acting on the mass in each case that do Work? Then use the definition of Work to get your answer. The one you checked is wrong.
     
  4. Sep 13, 2010 #3
    Sorry I didnt want to spam the forum with 3 different posts all relatively the same.

    So for the free fall case, forces acting on mass that do work are:
    -its weight, mg

    Incline:
    Since there is no friction, only force that act on its movement are mg. But I'm not really sure here: the normal force acts on it as well but 'canceled out' by the w=mg, since both of those added should equal zero, no?
    So the mg in this case is weaker than the free fall, since its not full gravity.

    Last case:
    Forces acting on the ball are mg as well as Tension.

    These kind of questions usually have the answer of 'same', which it probably is in this case, but it's not my first logical guess.

    I still dont see how my answer is wrong. Isnt the force of gravity strongest in the free fall? For incline, it would be only sin/cos of gravity, so only a portion, right?
     
  5. Sep 13, 2010 #4

    PhanthomJay

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    I understand, but when you pose several questions at once, helpers and you can get bogged down just responding back and forth to one of them, and before you know it, the post either gets several pages long, or, more likely, your other questions get lost in the abyss (black hole:wink:).
    correct; how much worK?
    No. The normal force acts perpendicular to the incline, so it doesn't cancel out the weight. But the normal force does no work...W =Fdcostheta, but theta is 90 degrees for the angle between the force and displacement, so the work done by the normal force is 0. What's the work done by gravity?
    mg is always full gravity. The component of the gravity force doen the plane is less than mg, but the distance travelled is longer. Again, what's the work done by gravity?
    again the tension always acts perpendicular to the motion, so it does no work.
    See above response
     
  6. Sep 13, 2010 #5

    PhanthomJay

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    For question 2, there is no displacement relative to the truck,but plenty relative to the ground..and also a friction force acting that keeps the block accelerating at the same rate of the truck.
     
  7. Sep 13, 2010 #6
    When you say whats the work done by gravity, are you asking for a value? Well work is force x displacement (integral, kind of) so work done by gravity for the free fall is 9.8*m*h -is that what you're asking for?

    Work done by incline is: m*h*9.8cosx
    Work done by string is: not really sure about this one :(
     
  8. Sep 13, 2010 #7
    Ahh ok..so the block IS accelerating, even though its not really moving itself?
     
  9. Sep 13, 2010 #8

    PhanthomJay

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    And regarding the third question, 'total' work and 'net' work are the same term. Since the net force is 0, the net work is___??___?
     
  10. Sep 13, 2010 #9

    PhanthomJay

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    yes, correct, now compare that to the net work done in the other 2 cases
    The work done by the normal force is 0. The work done by gravity is mgsintheta times the length (h/sin theta) of the incline. Or, the work done by gravity (or any conservative force) is always just the negative of the change in potential energy
    Use same reasoning as above.
     
  11. Sep 14, 2010 #10

    PhanthomJay

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    It is moving, with respect to the ground. It is accelerating with respect to the ground, so its speed is changing. If an object is accelerating, there must be a net force acting on it. Is work being done? (this is question 2).
     
    Last edited: Sep 14, 2010
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