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Another homework of Newton's law of motion

  1. Aug 12, 2010 #1
    1. The problem statement, all variables and given/known data
    A proud angler hangs her catch from a spring balance, which is supported from the roof of an elevator.
    a.) if the elevator has an upward acceleration of 2.45m/s2 and the balance reads 50.0N, what is the true weight of the fish?
    b.) Under what circumstances will the balance read 30.0N?
    c.) What will the balance read if the cable breaks?

    a = 2.45m/s2
    g = 9.8 m/s2
    wbalance = 50.0N
    wtrue = ?

    what formulas will I use? Please help
  2. jcsd
  3. Aug 12, 2010 #2
    Just look for the forces acting on the fish. You know it's acceleration and scale reading is actually the force between the fish and the scale (spring).

    You can also solve this by using fictitious force (elevator is non-inertial reference frame).
  4. Aug 12, 2010 #3
    Sorry, Im so dumb when it comes to physics.
    Does this mean I'll use F = ma then solve weight from there?
  5. Aug 12, 2010 #4
    Gravity pulls the fish down, and the spring pulls it up. Force from the spring is greater, so the fish (along with the elevator and everything inside) accelerates upwards. So, you have


    where [itex]\vec{F}[/itex] is elastic force of the spring. When you get rid of the vectors, you have


    I assumed that upwards is the positive direction. Everything else (that points downwards) is negative - gravity in this case.
  6. Aug 12, 2010 #5
    Im so confused.... is the 50.0N a weight or a force?
    Last edited: Aug 12, 2010
  7. Aug 13, 2010 #6
    Take a look at http://answers.yahoo.com/question/i...VzdWx0BHNlYwNzcg--?qid=20061106061139AAPqm7P"

    So Neutons being weight and g being the acceleration due to gravity, the 50N = m (g + 2.45). Find the mass in kg, then solve for the new weight.

    Then go back and find out what acceleration the elevator should have to produce 30N.
    Last edited by a moderator: Apr 25, 2017
  8. Aug 13, 2010 #7


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    Both. A weight is a force. While the elevator is going upward, the net force on the spring (so the reading on the scale) is the weight of the fish, mg, plus the "ma" where m is the mass of the fish (not weight, weight is mg) and a is the acceleration of the elevator.

    While the elevator is going upward, the net force is the weight of the fish minus ma.
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