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Newton's Laws

  1. Oct 5, 2004 #1
    Hi everyone! I need some help.

    1.)A .2 kg rock is thrown upward with an initial speed 15 m/s. What is the force on the ball in N when it reaches its maximum height?

    Would I use kinematics to find the max height then use F=ma? If so how?
    The answer is 2

    2.) A woman whose mass as the surface of the earth is 52 kg dives off a 2 m board into a swimming pool. What is her mass in kg while she is in a freefall?

    The answer is 52.

    ~Thanks
     
    Last edited: Oct 5, 2004
  2. jcsd
  3. Oct 5, 2004 #2
    [itex]F_{net}=ma[/itex], as you know. What is the net force acting on the rock while it is in the air? Hint: the net force on the rock is constant throughout its trajectory, nelgecting friction.
     
  4. Oct 5, 2004 #3
    But how would you do this for the max height?
     
  5. Oct 5, 2004 #4

    Gokul43201

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    Shay, for a minute forget your problem and answer this question : "What are the different forces that are acting on an object that is in the air, and not in contact with any other object ?"
     
  6. Oct 5, 2004 #5
    If I just do F=ma then I need the acceleration. How do I fond the acceleration and max height with only the velocity and mass?
     
  7. Oct 5, 2004 #6

    Gokul43201

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    What is the definition of mass, and how does an object's mass change ?
     
  8. Oct 5, 2004 #7
    gravity is the only force
     
  9. Oct 5, 2004 #8

    Gokul43201

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    Forget about this for now. You'll learn how to do this only after you understand the answer to my question.
     
  10. Oct 5, 2004 #9
    Does it change when it is in freefall?
     
  11. Oct 5, 2004 #10

    Gokul43201

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    Correct. So would that not be the answer to the question ?

    How do you calculate the force due to gravity ? Does it have another name ?
     
  12. Oct 5, 2004 #11

    Gokul43201

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    Okay, I'll give you a definition, and you tell me whether or not the mass changes.

    Definition : The mass is the amount of matter contained in an object, (sort of like the number of atoms/molecules in the object).
     
  13. Oct 5, 2004 #12
    Is it 9.8 m/s^2 ?
     
  14. Oct 5, 2004 #13
    I think the mass does not change.
     
  15. Oct 5, 2004 #14

    Gokul43201

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    That's the acceleration due to gravity, represented by the symbol 'g'.

    The force due to gravity involves this g and the mass m. And there's a common name for it - begins with a "W". What is this name and formula for the force ?
     
  16. Oct 5, 2004 #15

    Gokul43201

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    Correct, you can change the mass only by removing or adding stuff to the object.
     
  17. Oct 5, 2004 #16
    Weight
    Fg = mg
     
  18. Oct 5, 2004 #17
    Ohhhhhhhhh!

    Fg = ma
    Fg= (.2)(9.8)
    Fg=1.96

    BUt why did they say "when it reaches its maximum height" and why does it talk about the initial speed? Is this just to confuse me?
     
  19. Oct 5, 2004 #18
    OK now I understand why 52 is the answer to #2. Is what I did for #1 correct?
     
  20. Oct 5, 2004 #19

    Gokul43201

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    Correct. It's a trick question. The whole thing about the velocity and the maximum height is a distraction. They have no relevance to the problem.
     
  21. Oct 5, 2004 #20
    Thanks :smile:
     
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