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How to find acceleration. With only Force of friction?

  1. Nov 11, 2009 #1
    How to find acceleration. With only Force of friction??

    1. The problem statement, all variables and given/known data
    While traveling down between floors at a constant speed, a passenger has a weight of 800 N. During the acceleration to stop the elevator, the passenger experiences an apparent weight of 1000 N. Calculate the acceleration of the elevator.

    I am confussed because there is two forces of friction. There is no time or velocity. But the velocity is constant.


    2. Relevant equations
    I don't have an equation to find acceleration with only Force of friction.


    3. The attempt at a solution
    Gravity? 9.8 m/s ^ 2 ?
     
  2. jcsd
  3. Nov 11, 2009 #2

    berkeman

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    Staff: Mentor

    Re: How to find acceleration. With only Force of friction??

    There is no relevant force of friction in this problem. Weight is weight -- in this problem, all you have is an accleration, causing an increase in weight.

    In the first part, the velocity is constant, so his acceleration is __________.

    So from the first part, can you calculate the mass of the person?

    In the second part, he is being slowed by an acceleration (or deceleration if you like). That acceleration causes a change in weight, or an apparent change in mass. Use F=ma from here....
     
  4. Nov 11, 2009 #3
    Re: How to find acceleration. With only Force of friction??

    In the first part, the velocity is constant, so his acceleration is -9.8 m/s ^ 2?

    So from the first part, can you calculate the mass of the person? F=ma
    m=F/a
    m=800 N / -9.81 m/s ^ 2
    m= 81.55 m/s ^ 2

    In the second part, he is being slowed by an acceleration (or deceleration if you like). That acceleration causes a change in weight, or an apparent change in mass. Use F=ma from here....
    Wouldn't this be the same but with 1000 N?
    m=F/a
    m=1000 N / -9.81 m/s ^ 2
    m= 101.94 m/s ^ 2 ??

    That does not seem to make sense.
     
  5. Nov 11, 2009 #4

    berkeman

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    Staff: Mentor

    Re: How to find acceleration. With only Force of friction??

    No, no and no. First, constant velocity means no acceleration. Acceleration is the change in velocity with respect to time. Please review the equations of motion, which relate position, velocity and acceleration.

    In your equations:
    m=F/a
    m=800 N / -9.81 m/s ^ 2
    m= 81.55 m/s ^ 2

    Mass has units of kg. A Newton is a kg*m/s^2. Please review your use of units.

    For the 3rd part, you are asked to find the acceleration, not mess with the mass. You have the mass, and you have the force... How can you find the acceleration from that?
     
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