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Homework Help: 3rd law question

  1. Oct 24, 2005 #1
    Your younger brother is supposed to mow the lawn using a push mower. He reasons from the Newton's Third Law that the mower will push back with the same force he exerts on the mower; therefore nothing will move and attempting to mow the lawn is pointless. What is wrong with his reasoning?

    a: The boy has not enough push force on the mower
    b: the boy forgot friction force
    c: From the 3rd law, no force on mower, but mower has initial speed therefore the mower can keep moving
    d: The boy exerts a force on the mower, but the reaction force is exerted by the mower on the boy. The two forces are acting on different objects, and thus cannot cancel.

    The answer is D but I need to know why. From what I understand, two forces acting on different objects DO constitute a 3rd law pair. The boy on the mower and the mower on the boy. The thing thats propelling them forward is the force of the surface on the boy as he pushes back on the surface. The static friction force. From my reasoning the answer should be B....
    I appreciate your help.
  2. jcsd
  3. Oct 24, 2005 #2
    D is a much more direct answer to the question. By answering (b) you are stating that the forces do cancel, which they dont.
  4. Oct 24, 2005 #3


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    You have to push to keep the mower going at constant speed only because there is friction. If there were no friction and you pushed the mower, the mower would push back on you (you feel the push in your hands) and the mower accelerates. D is the only correct answer.
  5. Oct 24, 2005 #4
    The force the mower exerts on the person has no bearing on the acceleration of the mower - only on the acceleration of the person. The two forces in this question are acting on two different bodies, so cannot possibly cancel.
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