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Homework Help: Work transfer and energy conservation

  1. Nov 2, 2004 #1
    1)A gardener pushes a 12 kg lawnmower whose handle is tilted up 37 degrees above horizontal. The lawnmower's coefficient of rolling friction is 0.15. How much power does the gardener have to supply to push the lawnmower at a constant speed of 1.2 m/s?

    not really sure where to begin with this one. so any suggestions as to how to start would help.

    The spring in the figure has a spring constant of 1200 N/m. It is compressed 15.0 cm, then launches a 210 g block. The horizontal surface is frictionless, but the block's coefficient of kinetic friction on the incline is 0.210. What distance d does the block sail through the air?

    http://s93755476.onlinehome.us/stuff/knight.Figure.11.72.jpg [Broken]

    this one seems like it has several parts. i know i could figure out the velocity that the block travels on the flat surface from the spring using like K + U = K + U for initial and final. and using 1/2kx^2 instead. and something simliar for going up the hill. but the part where its in the air is where i get lost.
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Nov 2, 2004 #2


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    1) Calculate the normal force on the mower to get the frictional force.
    The 'extra' force the gardener has to give to keep the mower going is equal and opposite to this frictional force.

    2) Calculate the the work done by the frictional force when the block slides up the hill.
    This energy is 'lost'. Now you the energy of the block at the end of the hill it becomes a trajectory problem.
  4. Mar 30, 2005 #3
    i am lost on what to do for the first question about the lawnmower..can someone help me?
  5. Mar 30, 2005 #4
    Big hints in lawnmower question:

    Constant velocity, what do you know about the net force on objects with constant velocity?
    The force of friction plays an important role here. You can find the force of fricion by the information in the problem, use that force and the hint above this to find the force applied to the lawnmower.
    Knowing the force on the lawnmower, find the force applied by the gardener, considering the angle he is pushing.

    From there you have a force and a velocity. Power = Force x Velocity
  6. Mar 30, 2005 #5


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