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Homework Help: I Need Help Quick Plz!

  1. Jan 30, 2006 #1
    im new to da whole physics ding....and i need help with it

    A person kicks a 4.0-kg door with a 48newton force causing the door to accelerate at 12 meters per second. WHat is the magnitude of the forces exerted by the door on the person? (Answer in Newtons)

    and this one too

    A 50.-newton horizontal force is needed to keep an object weighing 500. newtons moving at a constant velocity of 2.0 meters per second across a horizontal surface. The magnitude of the frictional force acting on the object is (Answer in Newtons)

    Thx guys i really need this.
  2. jcsd
  3. Jan 30, 2006 #2


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    If a force is moving something at constant velocity (speed), then there must be an opposite force of equal magnitude, otherwise there would be a net positive which would cause a mass to accelerate.

    Kicking the door is an example of impulse. The product of mass and acceleration = force, and the door imposes a force in opposition to the applied force.
  4. Jan 30, 2006 #3
    i still dun really understand....can u put it in simpler terms or sumthing? thanks
  5. Jan 31, 2006 #4


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    When one pushes on an object is resists the push, and effectively pushes back.

    Do you understand the concept of inertia, and how force, mass and acceleration are related?
  6. Jan 31, 2006 #5
    i still dun get how to answer my questions
  7. Feb 4, 2006 #6
    i still dun get it =(
  8. Feb 4, 2006 #7
    when one exerts a force on something, that thing exerts the same force but opposite in direction

    if you exert 12N upon the door, it also exerts 12N on you, but in the opposite direction. (otherwise it would crack into many pieces)

    EDIT: im not saying 12 N is the answer, i just picked a random number
  9. Feb 4, 2006 #8
    as for your second question...

    Ff (frictional force)= mass(im pretty sure this has to be in kg) x uk(coefficient of friction)
    Last edited: Feb 4, 2006
  10. Feb 4, 2006 #9


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    Is this same as Newton Second Movement Rule? If yes then i know how the solution. if No then i dunno the solution. Sorry my physics in not english but in malay
  11. Feb 4, 2006 #10
    The second question is related to Newton's second law, the first question is better suited to Newton's third law.
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