Is it easier to push a lawnmover or pull it?

  1. The answer is it is easier to pull a lawnmover than to push it. I understand it logically but how do you prove it? My teacher says it has something to do with resolution of vectors. Can someone please explain?
     
  2. jcsd
  3. robphy

    robphy 4,394
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    Start by drawing a free-body diagram [What are the forces involved?]
    ... clearly indicating the push force in one case and the pull force in the other.
     
  4. Andrew Mason

    Andrew Mason 6,856
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    I don't think it makes any difference at all if it has good wheels. The normal force is different depending on whether it is being pushed or pulled (assuming you are doing this by way of a handle inclined at an angle greater than 0 to the horizontal), but this should not affect the force required to move it, if it has proper wheels.

    AM
     
  5. robphy

    robphy 4,394
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    Is there friction in the problem?
     
  6. NateTG

    NateTG 2,537
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    Really, the question should be asked about a sled (where the difference is much more apparent).
     
  7. Well, for a manual mower, it depends on whether the blades would be moving as you push/pull the mower, and also whether grass is actually being cut (if the blades are moving). For a gas one, it's generally easier to push it since almost all of them have automatic forward motion, and it takes some amount of work to keep ahold of it.
     
  8. Chi Meson

    Chi Meson 1,772
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    The classic answer has to do with the fact that as you push the handle you are pushing down slightly, and if you pull the handle, you pull up slightly. These actions would increase and decrease the normal force respectively (this increase/decrease friction). But, as anyone who has ever cut grass should know, when pushing, you push the handle horizontally. Sometimes the push is slighly up, isn't it? Depending on the push you will introduce a torque to the system and more normal force will be on one set of wheels (usually the front).

    In other words, this is a classic problem that fails completely in the real world. Other factors completely obliterate the expected effect.
     
  9. 0rthodontist

    0rthodontist 1,253
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    You can't push the handle horizontally unless the handle is itself horizontal. You always have to push very close to the direction of the handle unless your grip is so strong you can lock your wrists against the torque the lawnmower exerts on the handle.
     
    Last edited: Jul 27, 2006
  10. Andrew Mason

    Andrew Mason 6,856
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    I see your point about the torque, but why would it increase friction when you pushed down on the mower if it has proper wheels?

    AM
     
  11. Office_Shredder

    Office_Shredder 4,499
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    If you push down on the mower, there is a greater normal force between the wheels and the shaft.

    What kind of super wheels are you using that have zero friction?
     
  12. Andrew Mason

    Andrew Mason 6,856
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    Wheels with bearings.

    AM
     
  13. Not only that, but have you noticed a tendency for one to "push" a lawnmover with both hands, yet pull it backwards with only one?
     
  14. Andrew Mason

    Andrew Mason 6,856
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    Most of us don't like walking backward. It is easier to pull it with one hand. The point here is that the difference in normal force between pushing and pulling will have virtually no effect on the frictional energy loss in horizontal motion, if the mower has proper wheels (good bearings).

    AM
     
  15. Office_Shredder

    Office_Shredder 4,499
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    Good bearings reduce, not eliminate, friction.
    And most lawnmowers are not made with great bearings (if any).
     
  16. When i push a mower (the mechanical type, where the push drives the blades) i lean into the direction of motion.
    I do the same in reverse.

    But most of these mowers have a rear roller wheel that the mower pivots on, so the blades lift off the grass when its pulled (non-cutting side of the cutting rotor) and tilt back flush to engage the grass and its pushed forward.

    So with that said, pulling is easier because the cutter is not doing any work.
     
  17. Chi Meson

    Chi Meson 1,772
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    Ideally, the increase in normal force would increase the friction, but in the real world the resulting increase in friction in the wheel bearings would be so insignificant, even if the bearings were not that great. Other factors, already mentioned, are far more significant in determining which way is easier.
     
  18. Maybe I should have rephrased the question. The lawnmover is moving on a flat ground. Lets assume there is no grass. Friction needs to be taken into account.
    Anyway here is my view on this.
    Let the force applied while pulling/pushing be F newtons. While pushing the net force acting on the lawnmover is F + mgcos where 'm' is the mass of the lawnmover. While pulling mgcos acts opposite to the pull therefore the resultant force is F - mgcos. Therefore, the work done is less. and hence, it is easier to push than to pull.

    Is this right??
     
  19. berkeman

    Staff: Mentor

    :rolleyes:
    I think the point is not so much about the friction of the wheel bearings, it's due to the softness of the grass and dirt. On a hard surface, yeah, with good wheel bearings there would be a trivial difference in effort. But on soft grass, I think there is probably a measurable difference. Well, except for the good point that mowing backwards would be obnoxious....
     
  20. Chi Meson

    Chi Meson 1,772
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    Not quite. First of all, I assume the "cos" refers to a componant of the weight of the lawnmower, where the angle is that of the handle with the horizontal. Rather than looking at componants along the handle, look at the componants that are horizontal and vertical:

    No weight of the mower acts in the horizontal direction, period.

    In such a "textbook" problem, it is assumed that the applied force acts slightly downward on the handle when pushing, thus increasing the normal force, thus increasing friction, thus making it harder to push.

    Remember, the net force on the mower must be zero if it is moving at constant speed, and the horizontal frictional force must be equal to the horizontal componant of the pushing or pulling force.
     
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