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A rock with a weight of 10 N is resting on a table.

  1. Oct 15, 2015 #1
    A rock with a weight of 10 N is resting on a table. A student makes a number of statements about this situation, are they right or wrong?

    1. "The weight of the rock is a force of 10 N by gravity in the downward direction"
    I don't see anything wrong with this.

    2. "The reaction force to this weight is a force of 10 N exerted on the rock by the table in the upward direction"
    I don't see anything wrong with this either.
    3. "The normal force exerted on the rock by the table is a force of 10 N; the reaction force to this normal force is a force of 10 N exerted on the rock by gravity in the downward direction"
    *Gravity does not exert 10 N of force on the rock. The rock is exerting a force of 10 N in the downward direction. The mass of the rock would be 10 = m (9.8) would be 1.02.
    4. "If the 10 N rock is lifted off the table by a hand that exerts a force of 12 N upward on the rock, the reaction force to this 12 N force is a force of 10 N exerted on the hand by the rock in the downward direction"
    I don't see anything wrong with this either....


    Please let me know if I'm right or missing any key concepts! Thank you so much!
     
  2. jcsd
  3. Oct 15, 2015 #2

    Geofleur

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    For 3, why do you say that gravity does not exert 10 N of force on the rock?

    For 4, what does Newton's 3rd law say about the magnitudes of action and reaction forces?
     
  4. Oct 15, 2015 #3
    For 3, now I kind of think that the statement is right.... because Gravity has an acceleration of 9.8 m/s squared, times the weight of the rock would be 10 N. So gravity in fact does exert a force of 10 N on the rock. Right?

    For 4, Every action has an equal and opposite reaction! So the force exerted on the hand by the rock should be 12! So the statement is false.
     
  5. Oct 15, 2015 #4

    Geofleur

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    Regarding 3, the rock does indeed weigh 10 N, but there is still something not right. If there is a force on A due to B, then the reaction force must be a force on B due to A. Can you see the problem for this specific case?

    Regarding 4, right!
     
  6. Oct 15, 2015 #5
    "The normal force exerted on the rock by the table is a force of 10 N; the reaction force to this normal force is a force of 10 N exerted on the rock by gravity in the downward direction"

    should the ending be..... "the reaction force to this normal force is a force of 10 N exerted on the TABLE by the ROCK in the downward direction." Gravity shouldn't even be in the sentence, correct?
     
  7. Oct 15, 2015 #6

    Geofleur

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    Right. While it is the force of gravity on the rock that causes the rock to exert a force on the table in the first place, it's the force of the rock on the table that completes the action-reaction pair.
     
  8. Oct 15, 2015 #7
    THANK YOU SO MUCH!
     
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