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Help: Lawn Mowing Question Physics related

  1. Dec 8, 2007 #1
    A person pushes a 18.0 kg lawn mower at constant speed with a force of 71.0 N directed along the handle, which is at an angle of = 43.0° to the horizontal .

    (a) Draw the free-body diagram showing all forces acting on the mower. (Do this on paper. Your instructor may ask you to turn in this diagram.)
    (b) Calculate the horizontal retarding force on the mower
    N
    (c) Calculate the normal force exerted vertically upward on the mower by the ground.
    N
    (d) Calculate the force the person must exert on the lawn mower to accelerate it from rest to 1.6 m/s in 2.0 seconds (assuming the same retarding force).

    i found b,c, just a little confused on which to start d
    b is 51.9 N
    c is 225 N

    there is a picture if it helps
    Please help soon!!!
     

    Attached Files:

  2. jcsd
  3. Dec 8, 2007 #2
    Hello,

    For part (d), you need to calculate the force that would need to be applied to the handle to accelerate the lawnmower.

    The original force of 71N was not enought to accelerate the lawnmower, it was just enough to keep it at a constant speed. Can you find the acceleration required to go from rest to 1.6 m/s in 2.0 seconds? If you can, then you can find the force that would need to be applied to generate this acceleration-- but you have to remember that there is already a force acting against this one, that you will need to overcome.

    What I'm trying to say (and I think I'm botching it a bit) is that if you can find the net acceleration you need, you can then find the net force you need, and from there subtract out the retarding force, leaving you only with the force you need to apply. Of course, you're going to have the same sin/cos trickery, because you have to apply the force on the handle.

    Hope this helps, let me know if you need more clarification. Personally, I don't like problems like this, because there's typically no physical reason to assume a retarding force would be equal (in this case) at different applied forces-- they typically vary according to the applied force. But in this case, they're saying to assume the same, to make the problem easier... but this goes against one's physical intuition. Bah.
     
  4. Dec 8, 2007 #3
    yes can u clarify a bit more
     
  5. Dec 8, 2007 #4
    Start with this:

    [tex] v = v_0 + at [/tex]

    and then use this:

    [tex] \sum{F} = ma_{net} \Rightarrow \frac{F_{retarding} + F_{applied}}{m} = a_{net} [/tex]

    Where, for this problem, we're only concerned with the accelerations, velocities, and everything else, in the x direction.

    They've given you what you need to find the net acceleration, that will produce an increase in velocity from 0 to 1.6 m/s in 2.0s. You can then use this acceleration, and the one force you do know, to find the other force you don't know.
     
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