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Homework Help: Some more help with Newton's Laws

  1. Dec 29, 2009 #1
    1. The problem statement, all variables and given/known data

    A person stands on a scale on an elevator. If the mass of the person is 96.34 kg and the elevator travels upward at a constant speed of 4.42 m/s, what is the reading on the scale?

    2. Relevant equations

    m = 96.34 kg
    V avg = 4.42 m/s

    3. The attempt at a solution

    I think what this is asking for is the weight of the person, the force exerted on the scale by the person. So if the velocity is constant, wouldn't the acceleration be 0? So F = ma = 0. I don't know if that's right?

    1. The problem statement, all variables and given/known data

    Two horizontal forces act on a 1.7 kg mass. One force has a magnitude of 9.78 N and is directed due north. The second force toward the east has a magnitude of 1.48 N. What is the magnitude of the acceleration of the mass?

    2. Relevant equations

    3. The attempt at a solution

    I don't know how to go about solving this.
  2. jcsd
  3. Dec 29, 2009 #2
    Wow your teacher likes decimals.

    If the person on the elevator is not accelerating then there is no net force as you sort of stated. If this is true, the persons weight, ma, or mg, is equal to the push up from the floor of the elevator, or the scale if the person is standing on a scale on the elevator.

    For the second problem, have you used vectors yet?
  4. Dec 30, 2009 #3
    1) a) realize that if the velocity of the elevator is constant ,then you know from newton's first law that the net force acting on the elevator is zero. If this net force is zero then the normal force exerted upward on you by the scale is _________ to the downward force you exert on the scale. The solution follows from there.

    2) initially, mass m is at rest so the net force acting on it is zero. since the two forces acting on the mass are perpendicular and since they are both positive, use dot(scalar) product to find the sum of these two vectors. the resultant vector gives the magnitude and direction of the net force acting on the mass m. To find m's acceleration just use magnitude of the aforementioned resultant vector and Newton's second law .
    Last edited: Dec 30, 2009
  5. Dec 30, 2009 #4
    Hi black_hole,

    For the second question, just in case you haven't come across vectors yet, you can find the resultant force on the mass using Pythagoras Theorem. To find the acceleration just divide this resultant force by the mass (Newton's Second Law).

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