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Homework Help: 2 Vector dynamics questions

  1. Oct 23, 2007 #1
    i've been having some problems in physics.... epecially in vector dynamics and i have 2 questions ..........

    1.When a freight elevator that weights 1600lbs moves there is a frictional force of 100 lbs aciting on it. what force must be exerted on the elevator by its supporting cable if the maximum upward acceleration is to be 3ft/s?

    g= -32 lb/ft^2

    the answer is T=+1850 lb but i don't know how to get it at all......

    2.an 80 lb crate is pushed across a floor from an angle of 30degrees downward with respect to the positive x-axis.... the co-efficient of friction between the crate and the floor is 0.58....what force would be required to make the crate move at a constant speed.......

    the answer for this is Fa = 80lb

    anyone know how to solve this??
  2. jcsd
  3. Oct 23, 2007 #2


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    Homework Helper

    For (1), did you draw a diagram and label all of the forces present? There is more than one. You must add them up according to Newton's second law.

    For (2), same idea as (1), draw a diagram with all of the forces on it, and add them up in both directions using Newton's second law.
  4. Oct 23, 2007 #3
    i kinda know what u mean...... but could u perhap elaborate??
    i drew diagrams and they appear to be like this...... at least i think

    Diagram for 2.

    Diagram for 1
    http://img156.imageshack.us/img156/5532/88663705ep1.th.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  5. Oct 24, 2007 #4
    1) Think of the total force as a combination of individual forces.

    F = ma

    the force on the elevator imposed by gravity is -1600lbs

    so u need +1600 lbs of force to cancel that

    T = +1600lbs + F(2) + F(3)

    where 2 and 3 are friction and the acceleration.

    mass = weight/g

    mass and weight are different. does a 50lb elevator weigh the same on mars as on earth?

    the total force needed in the wires to get the elevator moving are - countering the normal force of the elevator itself + the frictional force + 3m/s^2 (your units for acceleration are off)

    if you add all those up you get 1850 as a total force.

    2) You need to use some trig for this problem but the same laws apply. Look for the formulas in your book. You dreww out the forces at the angles specified with respect to the axis correct. Common trig functions will account for how much force is distributed in respect to angle. Follow addition of forces just like above. Remembering units are key.
    Last edited: Oct 24, 2007
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